HEIL DC90 intermittently fails to heat

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  #1  
Old 03-25-08, 09:21 AM
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HEIL DC90 intermittently fails to heat

In January, our natural gas HEIL DC90 furnace (Model# NTGM100EHA2 Serial# L974181359) started to periodically not fully light. That is, the inducer motor would be on, the pilot would come on, and the blower would come on... but it would not be at full ignition.

Since it was rather cold (south-central Wisconsin), I was mostly concerned about getting heat to the house (and less about troubleshooting), so I tried some things to get it to restart:

- Turning the thermostat off and on fairly quickly didn't seem to do the trick (though later this has seemed to work... provided the thermostat is off for several minutes).

- Cycling power (rapid on/off) to the furnace generally (more on this below) seems to work to get it to start behaving.

After this started happening with some frequency (approximately once every couple of days to no longer than a week between incidents), I decided to do some investigation. After researching a bit on the internet, I shut the system down and opened up the combustion chamber. I was surprised to see quite a few intact leaves in there... not sure how long they have been in there or how they got in there given the apparent lack of access.

I looked at the flame sensor/HSI igniter assembly and lightly cleaned off the sensor with some 480 sandpaper.

The problem went away for about a week, then came back periodically.

Since about 3/14 (when it was getting warmer at nights here), it has been recurring in the morning (after having been off all night) and the afternoon (again... after having been off most of the day... especially if it was a sunny day). I should add that many of the occurrences prior to 3/14 were during these times as well. I would guess (though there may be a couple of outliers) that this failure-to-ignite happens only after the system has cooled for a few hours.

Anyway, since it is getting to be more repeatable, I am trying to be more vigilant... and ending up a bit more frantic.

After more research, I ensured that the condensate drain was clean and uncrimped, that the gas opening to the combustion chamber was unblocked, and that the pressure switch was functioning (went from infinte resistance to zero when the draft inducer was on).

I stumbled across the troubleshooting guide for the sv9501m 2728 Honeywell SmartValve (see p. 14 of http://www.transtaracsupply.com/pdf/sv95control.pdf) and got to the following decision point in the chart (this is after measuring 24V for the main connector):

MAIN VALVE OPENS AND
MAIN BURNER LIGHTS -> NO ->

CHECK THAT PILOT FLAME MAKES GOOD
CONTACT WITH PILOT BURNER FLAME ROD
CHECK FOR GOOD ELECTRICAL CONNECTION
THROUGH THE PILOT TUBING.
IF BOTH OF THE ABOVE ARE GOOD, REPLACE
IGNITER/FLAME ROD ASSEMBLY

and I have run into a bit of trouble understanding the checks (and whether that is a complete diagnostic).

First, I am assuming that the pilot flame makes contact with the flame rod based on:

a) removing the combustion chamber coverplate and gauging where the rod is relative to the igniter (since I can't see the rod well through the small porthole)

and

b) because the system does go to full ignition many times

Are those fair assumptions?

Second, what do they mean by "GOOD ELECTRICAL CONNECTION THROUGH THE PILOT TUBING"? Continuity from where to where? Why the tubing and not the wire?

Third, why does this automatically mean the igniter/flame rod assembly? Couldn't it also be the connection to the SmartValve?

Also, is that chart complete up to that point, or should there be other tests prior to the one I'm stuck on?

As an aside, I tried calling Honeywell support, but the homeowner line quickly seemed overwhelmed with my questions and referred me to the retailer help line... who refused to answer any questions at all. Lovely customer support, that...

So, with those questions hanging in the air, I played around with a few more things in seeing what would get the furnace to fully light once it was in the pilot-only state:

- Switching the On/Off on the SmartValve does it (at least the two times I tried it);

- Unplugging/replugging the square plug from the SmartValve does it (usually);

- Unplugging/replugging the igniter/flame sensor plug from the SmartValve does it (usually);

- Disconnecting/reconnecting the vacuum tube from the pressure sensor does it (usually);

The "usually" thing also has this novice a bit baffled.... though I think that it doesn't happen due to my replugging/reconnecting later than the SmartValve would like. That is, if I am slow to reconnect then the igniter/sensor won't be ready soon enough after the SmartValve opens... so I get a pilot but not full ignition.

At the risk of further showing my naivete, it only occurred to me last night to actually go down and watch what happened when the thermostat made its morning call.... and it was like what happened when power cycling the furnace failed (on one or two occasions) to bring about full ignition:

- draft inducer motor came on in response to thermostat call
- click from the SmartValve
- igniter slowly started to glow
- pilot eventually lit
- furnace electronic filter and blower came on

Within 15 seconds or so of the draft inducer motor coming on I unplugged/replugged the square plug to the SmartValve, and then full ignition resulted... which I was surprised at, really... since that kind of blew my pilot-warming-the-igniter theory...

I was thinking of playing around with the pilot flame height, but - really - anything having to do with the actual natural gas has me feeling like calling in a service tech.

I'm just trying to figure out if it is a board or other non-gas part that I can replace, or if it is the SmartValve giving me this grief.

I have not disconnected the intake/exhaust ports outside to see if I could snake anything out, nor have I washed the electronic air filter components (since I typically only do that once a season and we keep a pretty dust-free house)... largely because I'm thinking that these wouldn't have any impact.

I apologize for the long post and while it appears that others have had similar problems I was not always clear on the final fix.
 
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  #2  
Old 03-25-08, 03:01 PM
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Look at where wires enter the enclosed burner compartment, and follow these back and see where they plug into, and read and list for us the abbreviation letters on the board, where they plug in, and tell us the number of wires involved.
 
  #3  
Old 03-25-08, 06:12 PM
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There are three wires (2 blue and 1 black) coming from the combustion chamber that plug directly into the Honeywell SmartValve (sv9501m 2728). Are those the ones you mean?

As for the board, it is a Honeywell ST9120C 4040.
 
  #4  
Old 03-26-08, 04:23 AM
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I should add that there are two (I think) red wires coming from the chamber, but they are behind the three mentioned above and I have not traced them yet.

Interestingly, power-cycling the unit this morning did not get it to go into full ignition... twice. I ended up unplugging/plugging the square plug (not the igniter/sensor).
 

Last edited by s2zeller; 03-26-08 at 04:45 AM.
  #5  
Old 03-26-08, 03:59 PM
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Smart valves can go bad, but one wants to be certain it is not something else, as these things are pricey. Be clear on what wires go to smart valve and which go to the board and see if there are letters/abreviations where they connect up at each of these (more likely on the board).

I am not too familiar with smart valve furnaces, to envision how they are wired up. (I deal with more smart valve gas water heaters)
 
  #6  
Old 03-26-08, 06:42 PM
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The two blue and one black wires go from the combustion chamber to the SmartValve and plug into the Igniter slot on the valve.

The other two red wires start with spade connections at the side of the combustion chamber (unlike the blues/black which go through the chamber wall and connect to the igniter/sensor) and are bundled in with the other control wires on their way to the board.

On their way, one of the red wires connects (via two spade terminals) to a small board screwed to the side of the furnace (under and separate from the combustion chamber... maybe I am not using that term correctly... the chamber I am referring to is where the pilot is housed and goes to full ignition).

The board has the following on it:

36T0183 42269 J97359
L180-20F 1320-547 MEXICO

Out of curiousity, I undid the two screws holding the board in place and found the other side to have two (approximately) 3" heavily insulated wires that end attached to a small silver disc about the same circumference as a large calculator battery and twice as thick.

After the detour, the red wire joins its mate and they end up with four other wires in a 6-wire plug at the control board (ST9120C 4040).
 
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Old 03-27-08, 11:03 AM
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In review of things said:

You say the pilot always comes on and stays on, right?

And you said that there is ignition, but not complete ignition? What exactly do you mean by that? Do some or all of the buners light up? Is the flame height on the burners insufficient? Please explain about the partial ignition you talked about in your first post.

If when the pilot comes on, and you disconnect the black wire on the smart valve at the ignition wires hookup, does the pilot flame go out?

Guessing off the top of my head that the red wire going to 2 spade connectors under the burner area is for flame roll out or some other limit switch wires. Whatever they are, you should have continuity through those spade connectors and also 24 VAC through the spades, when the inducer motor runs.
 
  #8  
Old 03-27-08, 11:57 AM
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Originally Posted by ecman51` View Post
In review of things said:

You say the pilot always comes on and stays on, right?
Yes... the pilot always comes on and stays on... until, of course, I do something like unplug the control/igniter/etc.

And you said that there is ignition, but not complete ignition? What exactly do you mean by that? Do some or all of the buners light up? Is the flame height on the burners insufficient? Please explain about the partial ignition you talked about in your first post.
What I meant was it either goes (and stays) at the pilot stage, or it goes to full ignition with all burners lit. It has never done a partial, with only a few burners. Sorry to have been unclear on that.

As to flame height, I would assume it is good (when they are fully lit) since the house gets warm, but I can't completely verify that since I never really looked at them prior to the furnace acting up.

If when the pilot comes on, and you disconnect the black wire on the smart valve at the ignition wires hookup, does the pilot flame go out?.
The black wire is part of a plug with the 2 blue wires. When I unplug that from the SmartValve, the pilot goes out and the SmartValve makes a click-buzz. The same thing happens when I unplug the other plug (marked 'Control') on the SmartValve.

Guessing off the top of my head that the red wire going to 2 spade connectors under the burner area is for flame roll out or some other limit switch wires. Whatever they are, you should have continuity through those spade connectors and also 24 VAC through the spades, when the inducer motor runs.
Yes, resistance is at 0 ohms (at all times, if I remember correctly). I didn't check voltage, but I can confirm both and get back on that.
 
  #9  
Old 03-27-08, 06:15 PM
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I get 24VAC from one spade to any other piece of metal, but 0 when touching the other spade. This reading is constant regardless of whether there is no pilot, pilot, or full ignition.
 
  #10  
Old 03-27-08, 07:34 PM
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Please let me know if the attached schematic is (or not) an exact match for the wiring and components of your system. And if there are differences, where at (wire colors? labels of terminals? etc.)

 
  #11  
Old 03-27-08, 08:25 PM
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Yes, that is a slightly simplified version of the Connection Diagram pasted to the lower door of the furnace.

Missing from your diagram and present on mine are:
- LP-specific switch
- horizontal-installation-specific switch
- optional capacitor

Missing from mine and present on yours is the G coming off the C from the Furnace Control board.

Otherwise, it is identical.
 
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Old 03-28-08, 03:28 AM
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Using the schematic I posted yesterday night (post #10) as a reference...look at the 6-wire plug on the ST9120C board.
Terminal #1 (top left) and terminal #6 (bottom right) have each a red wire. These two red wires are connected to the limit switch (this is the "silver disk" you describe in post #6).

Note: I'm not sure if you have rollouts here. Pictures would really help, as a defective rollout could very well be the reason for this malfunction.

moving on:
Without disconnecting the red wires attached, put a jumper between the two spade-type electrical connectors. In other words, I want those two red wires to stay engaged (connected) with each other. And IF there are rollouts (the pics will show), what I would really like is the ends of this jumper to be at the ends of each one of these two red wires coming off terminals 1&6 on the board (in other words, bypassing the limit "and" if there are any, the rollouts as well)

Call for heat.

Tell me what the furnace does (or does not)

p.s.: as a side note: the two blue and the black wires going to the pilot burner assembly are as follows:
The two blue ones feed the 24V HSI. The black attaches to the flame rod (this is the one, I suppose, you cleaned with the sand paper)
 
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Old 03-28-08, 04:41 AM
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I was set to try this this morning and (like yesterday morning), it fired right up without a problem. It did fail yesterday afternoon and evening, so there is hope to try the experiment tonight or after.

However, there is no guarantee that it would be "correct" in that the furnace may light properly of its own accord.

So, is it safe to jumper the two red wires and leave it that way?

I am thinking that if the problem doesn't then reappear, I could then reasonably replace that limit switch... or could that limit be failing the first time around for a perfectly good reason that jumpering might aggravate?

And, yes, the black wire is connected to the flame rod that I cleaned (very, very lightly... not like sanding a dowel or something).

Thanks for the input and I'll report back when I know something.... or more likely when I don't.
 

Last edited by s2zeller; 03-28-08 at 05:13 AM.
  #14  
Old 03-28-08, 05:33 AM
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Originally Posted by s2zeller View Post
So, is it safe to jumper the two red wires and leave it that way?
No, it is not safe to leave the jumper in place. This switch is in charge of shutting the main burners OFF if an excessive temperature build-up were to occur in the heat exchanger.
Not that this is your case (an over-temperature condition), but when it comes to safeties, one can never be too safe.

The jumper is to see if the limit is the source of the trouble. I am suspicious of it b/c you report that together with the inducer blower and the pilot burner flame, the main (house) blower runs albeit not the main burners on a call for heat. A situation such as that often is caused by a hanging limit switch.

If you were able to post some pics, that would be quite a treat.
 
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Old 03-28-08, 07:18 AM
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Unfortunately, I don't have any way of getting pics up as much as I agree that that would help...

And I figured that that would be the answer WRT safety...

I take it that a failure of the Main Limit switch would have different symptoms (I ask only because that is also on the same line of red wires).
 
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Old 03-28-08, 08:46 AM
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Originally Posted by s2zeller View Post
I take it that a failure of the Main Limit switch would have different symptoms (I ask only because that is also on the same line of red wires).
How many switches are there in between the red wires?
Any one of them failing (tripping into the open position) will cut power to the main burners and kick the main blower ON.
 
  #17  
Old 03-28-08, 09:43 AM
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From your schematic (which isn't showing for me at the moment) and what I see, there are two:

- the one I described below (which interrupts one of the wires) which is the Rollover (?)

and

- the Main Limit (I presume) which both plug into via spade connectors at the combustion chamber (just behind where the igniter/flame rod wires breach the chamber wall)

I am presuming that those are the two (as the third listed on the schematic for those wires was listed as only being on some models... and I only see two possible places for a switch/sensor to be for those wires... i.e. between the combustion chamber and the control board there is only the 'interruption' at the chamber and the other one I described below).

When I removed the one switch/sensor to look at it, it would have been sitting (when installed) in a void between two vertical walls of metal grating.... I don't know if that helps with which switch that is.
 
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Old 03-28-08, 09:59 AM
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Schematics are of a general nature. Heil's NTGM line of furnaces comes in four different sizes (50, 75, 100 & 125)...yours is the 100. The smaller sizes may have 1-2 rollouts, the larger 2-3 of them. The only way to know for sure is looking at the furnace itself...the schematic may not tell you how many there are.

The important thing is to put the jumper at the end of the red wires coming from terminals 1&6 on the board. Doing so bypasses all of the safeties there (limits and rollovers), whatever number of them there may be. If this gets the furnace back on its own two feet then we'll discuss individual troubleshooting to pinpoint the defective safety.

If your unit is an upflow model, it has only one limit and likely no less than one rollover switch (which is also a limit but of the manual reset type). Downflow and horizontal models have two limits and likely one or more rollovers.

Of course there's always the possibility that the problem may be somewhere else, but first things first...we'll test the safeties.
 
  #19  
Old 03-30-08, 05:09 AM
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There are only two (main limit and rollover) switches in-line with the red wires.

I jumpered around both on two separate occasions.

The first time I set up the jumpers, it stayed at pilot on the first attempt after I jumpered. So, I thought that I may not have done it well (the clips were a bit large and were a little difficult to place).

The second time I crafted some better jumpers using spade connectors. It behaved and heated the house... until this morning's attempt (which was - I think - a clear message that these two switches are not the issue, since the mornings had actually been trouble-free for the last 2 or 3 prior).

So, that rules out those two switches?
 
  #20  
Old 03-30-08, 07:29 PM
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Yes, from the test you have performed, the limits are now ruled out as possible cause for this malfunction.

Before anything else, please check the blue wire that starts at terminal #3 on the board's 6-wire plug and ends at terminal #4 of the Smart Valve's square-shaped wire plug. Let me know how many controls are connected in series in this loop. If only one, that is the pressure switch; if two, the second is an Auxiliary Limit switch. For the explanations below, I will assume you only have one, the pressure switch.

Let's now check and see if the pressure switch is doing what's supposed to. From terminal #3 of the 6-wire plug on the board, there's a blue wire that goes to the Pressure Switch and from the switch another blue wire goes to terminal #4 on the Smart Valve's 4-wire square-shaped plug (note that the smart valve has two wire plugs, the second one being of rectangular shape).

You see, the way a pressure switch is supposed to operate in a furnace such as yours is as follows:
-Prior to the call for heat from the t-stat, the contacts of this switch are supposed to be open.
- t-stat calls for heat, inducer blower kicks ON, and right after, the pressure switch contacts are expected to close and remain closed for the remainder of the call for heat

--> If the contacts were closed prior to the call for heat from the t-stat, the burners won't ignite (the board prevents them)
--> If after the burners are up and firing the contacts of the pressure switch were to open for whatever reason (even for an instant), the board turns the burners OFF

Well...to perform this test you may have to wait until the furnace misbehaves again, but here is what I want you to do when it happens:
(a) switch the t-stat in the house to OFF
(b) unplug one of the two blue wires connected to the pressure switch
(c) put a jumper between terminals W-and-R on the board's t-stat terminal strip.....this simulates a call for heat and the inducer blower will start
(d) put a jumper between the terminals of the two blue wires of the pressure switch.....this will simulate good airflow and proper operation of the inducer blower

if the burners now start and run well, the problem is the pressure switch...or perhaps there's an onstruction on the combustion airways.

If the above 4-step procedure does not take care of the problem, without undoing anything from (a)-thru-(d) above, continue as follows:
(e) put a jumper between terminal "W" on the board's t-stat terminal board and the quick-connector of the blue wire that ends up at the smart valve (terminal-4)

Why we do "(e)" above?
...The t-stat's call for heat is a knock on the door of the board. A call for action.
...The board now acts upon it and sends a signal to the Smart Valve via the wire harness.
...If "e" above solves the problem, then there is a broken link in the communication signal; either the board is not generating it (bad board), or it is getting lost in the wires (loose/defective wires on the wire-harness), or is not being understood by the Smart Valve (defective Smart Valve)

We elimitated the limits as possible defective components. We're now trying to see if the Pressure Switch, wire-harness or board is/are the culprit(s). Later, if the problem is still not solved, we'll perform tests on the Smart Valve. It's a step-by-step approach, sorry!
 
  #21  
Old 03-31-08, 04:38 AM
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There is only the one switch, a large silver metallic disc.

Originally Posted by pflor View Post
Well...to perform this test you may have to wait until the furnace misbehaves again...
Is this necessary, or can this diagnostic be performed any time the thermostat is off (since - typically - any action to "shut off"/interrupt the furnace results in success the next time around)?

Or - by getting the misbehavior - are we potentially preserving the pressure switch in a state of badness?

I do appreciate your laying this out for me... step-by-step is exactly the kind of thing I prefer and I always want to know what is going on in any black box.
 
  #22  
Old 03-31-08, 05:54 AM
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Originally Posted by s2zeller View Post
There is only the one switch, a large silver metallic disc.
That is the pressure switch then. Your unit is likely an upflow, that is why it does not have a secondary limit

Originally Posted by s2zeller View Post
Is this necessary, or can this diagnostic be performed any time the thermostat is off (since - typically - any action to "shut off"/interrupt the furnace results in success the next time around)?
What makes your problem tough to fix is its random nature.
If you can predict with some certainty that the furnace won't fire (typically after a few hours of inactivity), have the t-stat turned to OFF for a few hours, after which go to the furnace and force a call for heat by jumping R & W. If this replicates the "main burners won't fire" situation (hopefully you'll get lucky here), proceed with the rest of the steps suggested in my previous posting.

Originally Posted by s2zeller View Post
Or - by getting the misbehavior - are we potentially preserving the pressure switch in a state of badness?
The idea is being able to pinpoint if the pressure switch is the one to blame. If that is proven (with the test), then you can replace it.

Originally Posted by s2zeller View Post
I do appreciate your laying this out for me... step-by-step is exactly the kind of thing I prefer and I always want to know what is going on in any black box.
You bet!
 
  #23  
Old 04-01-08, 04:37 AM
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Jumping the gun - testing the smart valve

If the 5-step test suggested in post #20 passes with flying colors but the original problem persists, then the limits, pressure switch and ST9120 are likely in good condition. The test that follows check for proper power feed and functioning (or lack thereof) of the smart valve system.

1. turn OFF power to the t-stat and go to the furnace
...(do not turn OFF power to furnace at service switch)
2. remove "control" plug at smart valve
3. place tips of a voltmeter between terminals 1 & 3
...(term-1 is the one with the black wire, term-3 the one with the white one)
...you should read 24+ volts between both
4. keep the tips of the meter on 1 & 3 above, call for heat from the unit by placing a jumper
...between R & W at the board's thermostat's terminal strip. The induce shall turn ON
5. you should still read 24+V at the meter
6. disconnect R-W jumper (inducer turns OFF)
7. move lead from terminal 1 at smart valve control plug and relocate to terminal 4
...(this is the one with the blue wire). voltage should read "0"
8. put back R-W jumper. After inducer is at full speed, meter should read 24+ volts
9. remove R-W jumper
10. put/connect back control plug to smart valve
11. remove igniter plug from smart valve
12. set meter to read ohms at place leads on the terminals at end of the blue wires
...(these are terminals 1 &2 on this plug and are the igniter's ends).
...You should read less than 10 ohms with igniter at room temperature.
13. connect igniter plug back to smart valve
14. connect leads of voltmeter to terminals 2 & 4 of the plug at the ST9120 electronic board
...(NOT THE SMART VALVE'S CONTROL PLUG)
15. put back R-W jumper. Inducer will kick ON and igniter will glow. As the igniter is glowing,
...notice the voltage on the meter. It should read at least 19.5V
16. remove R-W jumper; remove meter

Please report the voltage and ohm readings on the various steps above. Depending on the results reported we will be able to ascertain one (or more) of the following:
a) the trasformer in the furnace is weak
b) a break in one or more of the wires in these wire harnesses
c) a broken/defective ignitor (the glow coil)
d) a defective smart valve
e) a defective flame rod on the igniter assembly

From what I've been reading (NTGM, SV9501 & ST9120 manuals plus your reports), I believe the Igniter/flame-rod assembly is your problem. Your report on the tests above will confirm that.

The following tester from Honeywell would come in real handy:
http://customer.honeywell.com/Honeyw...ACHEHINT=Guest
 

Last edited by pflor; 04-01-08 at 06:29 AM.
  #24  
Old 04-02-08, 04:40 AM
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Originally Posted by pflor View Post
From Post 20:

Well...to perform this test you may have to wait until the furnace misbehaves again, but here is what I want you to do when it happens:
(a) switch the t-stat in the house to OFF
(b) unplug one of the two blue wires connected to the pressure switch
(c) put a jumper between terminals W-and-R on the board's t-stat terminal strip.....this simulates a call for heat and the inducer blower will start
(d) put a jumper between the terminals of the two blue wires of the pressure switch.....this will simulate good airflow and proper operation of the inducer blower
Before I reveal the results, I need to confirm a couple of things. Will this be a valid diagnostic:

1) if the t-stat *is* making a call for heat?

and

2) if the pressure switch is already jumpered before that heat call comes (i.e. (d) is done before (c))?

The reason I ask is that it is difficult to know beforehand when the furnace will fail to reach full ignition, so I left the pressure switch jumpered and was waiting for the no-heat situation to appear... which I am assuming (correctly?) to mean that I could then proceed to:

If the above 4-step procedure does not take care of the problem, without undoing anything from (a)-thru-(d) above, continue as follows:
(e) put a jumper between terminal "W" on the board's t-stat terminal board and the quick-connector of the blue wire that ends up at the smart valve (terminal-4)
(As an aside, if the heat always came on when in this jumpered state, then I wasn't completely sure that that would pinpoint the pressure switch).

In any event, with the ordering of steps (a)-(d) slightly different, I had a no-heat condition this morning and did step (e)... and still got no heat.

I see you have posted the next clue in this treasure hunt ... am I free to proceed to that at this point?
 
  #25  
Old 04-02-08, 06:59 AM
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these electronic boards have an internal self-check feature that will prevent the furnace from firing if prior to a call for heat a switch is closed when it is not supposed to.

In this case, if you jumper the pressure switch prior to R-W, the furnace will not fire. Which I presume from reading your last post is exactly what has happened.

So no, you may not have that jumper in place prior to a call for heat
 
  #26  
Old 04-02-08, 07:44 AM
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Originally Posted by pflor View Post
these electronic boards have an internal self-check feature that will prevent the furnace from firing if prior to a call for heat a switch is closed when it is not supposed to.

In this case, if you jumper the pressure switch prior to R-W, the furnace will not fire. Which I presume from reading your last post is exactly what has happened.

So no, you may not have that jumper in place prior to a call for heat
Now I am confused... well, more confused...

...because the furnace did fire with that jumper in place.

If it is not there, then I am not sure how I can have any confidence that diagnostic has worked since if the furnace fires, I cannot know if it would have anyhow (given the intermittent nature of the problem).

Is there a way to proceed so that we (temporarily) assume the pressure switch is good and can later prove that assumption true or false?
 
  #27  
Old 04-02-08, 08:09 AM
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My bad!
This board is a lot less sophisticated than the average type out there. So yes, if the pressure switch is jumpered, then as soon as R-W are jumpered too, the furnace will attenmpt ignition.

As a matter of fact, with this board, all that the call for heat does is enable an internal switch that will in turn send 24V to the smart valve thru the pressure switch.

What I should have said is that IF one of the safeties is open (limit or rollout) then jumping the pressure switch will NOT fire-up the furnace

So no, you're not going crazy.
 
  #28  
Old 04-02-08, 10:20 AM
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Originally Posted by pflor View Post
So no, you're not going crazier.
There, fixed that for ya!

So, am I able to proceed to those next instructions for the SmartValve... or is there a modified diagnostic for the pressure switch?
 
  #29  
Old 04-02-08, 11:01 AM
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You may proceed...

BTW, assuming that the safeties are O.K. (limit & rollout), which is a pretty safe assumption since you tested them...
...If you place a jumper between R & W and another jumper between the two terminals of the pressure switch AND by doing so the problem goes away for good, then the board is doing its job and the problem is on the ignition side (either the igniter/electrode assembly OR the smart valve itself).

I am assuming that the flame off the pilot burner is nice and steady and is engulfing the flame rod AND the hood above it.

I'm at work now and our firewall here does not allow me to see/add/download pics. But when I get home I'll post FYI the internal schematic of your board plus a pic of what a healthy flame off the pilot burner should look like.

A bad flame (too small, weak, unsteady or wavery, etc.) often causes problems like the one you're reporting. A bad igniter/rod assembly does that as well.
 
  #30  
Old 04-02-08, 12:51 PM
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On the pic below, the solid black lines are actual wires you can see. e.g.: blue wires connect to the pressure switch, red the limit & rollout. The broken (dotted) lines represent internal board connections...those you can't see, they are imbedded in the board. Notice the blue and red arrows, this is how power goes from the transformer, thru the board, safeties, R-W (on a call for heat) and finally thru the pressure switch prior to reaching the smart valve terminal.

the L-shaped rectangle represents the plug at the board


Pilot flame pic shown below
 

Last edited by pflor; 04-02-08 at 01:24 PM.
  #31  
Old 04-03-08, 06:00 AM
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I started on the steps from Post 23 and could only (maybe, sort of) do steps 1-13:

Originally Posted by pflor View Post
The test that follows check for proper power feed and functioning (or lack thereof) of the smart valve system.

1. turn OFF power to the t-stat and go to the furnace
...(do not turn OFF power to furnace at service switch)
2. remove "control" plug at smart valve
3. place tips of a voltmeter between terminals 1 & 3
...(term-1 is the one with the black wire, term-3 the one with the white one)
...you should read 24+ volts between both
4. keep the tips of the meter on 1 & 3 above, call for heat from the unit by placing a jumper
...between R & W at the board's thermostat's terminal strip. The induce shall turn ON
5. you should still read 24+V at the meter
6. disconnect R-W jumper (inducer turns OFF)
7. move lead from terminal 1 at smart valve control plug and relocate to terminal 4
...(this is the one with the blue wire). voltage should read "0"
8. put back R-W jumper. After inducer is at full speed, meter should read 24+ volts
9. remove R-W jumper
10. put/connect back control plug to smart valve
11. remove igniter plug from smart valve
12. set meter to read ohms at place leads on the terminals at end of the blue wires
...(these are terminals 1 &2 on this plug and are the igniter's ends).
...You should read less than 10 ohms with igniter at room temperature.
13. connect igniter plug back to smart valve
I am less than certain on whether I completed them satisfactorily because a strange thing happened round about Steps 3-5.

Way back in Post 1, I mentioned (or at least implied by my going through the SmartValve troubleshooting flowchart) that I measured voltage on the Control plug.... I mention this only because I have done it before.

So, imagine my surprise when I get 0 volts for a reading between any 2 pins on the plug... and actually between any pin and anything else (i.e. any random piece of metal on the furnace).

As I was puzzling about this, a noise started coming from the inducer which sound sort of like a cross between sparking (I saw no flashes) and a piece of paper getting stuck in a slow-moving ceiling fan.

So, I disconnected the W-R jumper and cut the inducer... then I started getting 24V on 1&3.

I reconnected the jumper and got 24 for 1&3 and 1&4... so maybe I had the switch on the meter not completely set to measuring volts?

As an aside, the inducer noise seems to be either from a warped case or other, as pushing on the circle that is the end of the motor housing (with the end of the spinning shaft as its center) got the noise to go away... as did having the furnace go to full ignition.

So, the other measurement that I was able to make was at 12... and I got 0 Ohms.

Then I ran into trouble with this step:

14. connect leads of voltmeter to terminals 2 & 4 of the plug at the ST9120 electronic board
...(NOT THE SMART VALVE'S CONTROL PLUG)
I was unclear if these were wires 2 and 4 as labelled on the plug that goes into the ST9120 board (which - to be honest - I did not see labelling for)... or the wires that come from 2 and 4 of the Control plug for the SmartValve...

Also, with regard to the pilot, you said:

I am assuming that the flame off the pilot burner is nice and steady and is engulfing the flame rod AND the hood above it.
The little porthole for viewing this doesn't give me the best view. I see the pilot and the top of the igniter/flame rod assembly... and the top seems nicely engulfed with a largely blue flame.... tho' I can't for sure say that I actually see the top of the flame rod there.
 
  #32  
Old 04-03-08, 01:13 PM
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Way back in Post 1, I mentioned (or at least implied by my going through the SmartValve troubleshooting flowchart) that I measured voltage on the Control plug.... I mention this only because I have done it before.

So, imagine my surprise when I get 0 volts for a reading between any 2 pins on the plug... and actually between any pin and anything else (i.e. any random piece of metal on the furnace).

As I was puzzling about this, a noise started coming from the inducer which sound sort of like a cross between sparking (I saw no flashes) and a piece of paper getting stuck in a slow-moving ceiling fan.

So, I disconnected the W-R jumper and cut the inducer... then I started getting 24V on 1&3.

I reconnected the jumper and got 24 for 1&3 and 1&4... so maybe I had the switch on the meter not completely set to measuring volts?

As an aside, the inducer noise seems to be either from a warped case or other, as pushing on the circle that is the end of the motor housing (with the end of the spinning shaft as its center) got the noise to go away... as did having the furnace go to full ignition.


When you were reading "0V", the thing that was stuck inside your inducer fan housing was probably preventing the motor to reach full speed. lower speed means lower airflow, which in turn could cause the pressure switch to remain with its contacts open. In other words, if there is an obstruction to the airflow, the contacts of the pressure switch may not close, and then you have the reason for the "0V". after removing the jumper in R-W and reconnecting, the obstruction probably cleared and you had full flow once again, so the pressure switch contacts closed and hence the proper voltage readings

So, the other measurement that I was able to make was at 12... and I got 0 Ohms.
This reading is likely a MISREADING. "0-ohms" would mean a shorted igniter. If anything I would have expected a resistance greater than 10-ohms here.


Then I ran into trouble with this step:

Quote:
14. connect leads of voltmeter to terminals 2 & 4 of the plug at the ST9120 electronic board
...(NOT THE SMART VALVE'S CONTROL PLUG)

I was unclear if these were wires 2 and 4 as labelled on the plug that goes into the ST9120 board (which - to be honest - I did not see labelling for)... or the wires that come from 2 and 4 of the Control plug for the SmartValve...



The labels are NOT on the unit (board or elsewhere) but they ARE on the schematic I attached to post #10


And how about the voltage on step-15?

At any rate, from your last post it is clear that you should consider cleaning-up the passageways (pipes) of the combustion air. You may even have an inducer blower that is on its way out (from the noise you report it makes)

I'm becoming more and more convinced that your main problem here with the burners erratic behavior is due to a "sometimes" inadequate electrical signal going from the electrode in front of the pilot burner to the metal hood on top of it (the ground electrode). This electrical current is very tiny, in the order of the micro-amperes. A weak or wavery flame or a sooted/rusted flame-rod or ground hood could cause that the electrical current be below the minimum value required for the smart-valve to authorize the main burners to fire (increase gas pressure to pilot burner). A rusted, or loose, or too long a pilot tubing, could cause that too. As could as well a dirty (plugged) pilot burner orifice.


You should have an electrical signal on the flame-rod circuit NO LESSER than 1.3 micro-amps. Anything less than that will cause the problem you're having. But if you do not have a meter capable of taking micro-amp readings, you won't be able to know for sure. About 4 micro-amps is that is needed here for the smart valve to do its job consistently.
 
  #33  
Old 04-03-08, 07:24 PM
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I'm wondering if I tossed you a whole bunch of red herrings... or at least described them as such.

The 0V readings were for any of the pins. Wouldn't I have gotten 24V on pin 1&3 (so long as the system was powered on)? I'm honestly thinking that (at 6 a.m. after a not-so-great sleep) the meter was low on batteries or mis-set... or maybe it was just me.

I'm not sure what was happen with the inducer fan. I've never heard it sound that way before (or tonight) even tho' it has been on for longer times with the system going to full ignition. I'll keep an ear out for it, but I'm wondering if it was some flukey thing...

The 0 Ohms reading wasn't so much as misreading as a mis-estimating. My lowly meter only has one Ohms setting (and it has x1000 by it)... so it was likely something greater than 0 but I just couldn't tell what reliably.

I am (hopefully tomorrow) going to be borrowing a meter that does micro-amps and Ohms (without being x1000) so I should be able to get much better readings.

Which brings me to the question of:

How do I measure the microamps when there is only one wire coming from the flame rod?

As far as measuing the voltage at the board... I guess I better get those glasses I think I'm needing. I had a copy of your schematic in front of me and still didn't see the wire numbers this morning... so no reading for Step 15 yet. Thank you for re-pointing that out to me!

At any rate, from your last post it is clear that you should consider cleaning-up the passageways (pipes) of the combustion air.
Can you elaborate on this... both why you think so and how I would do it (or is it something for an expert)?
 
  #34  
Old 04-03-08, 08:16 PM
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Originally Posted by s2zeller View Post
I'm wondering if I tossed you a whole bunch of red herrings... or at least described them as such.
I got a chuckle. Not having been raised around here but rather way, way south, I don't have a clue what this means.

Originally Posted by s2zeller View Post
The 0V readings were for any of the pins. Wouldn't I have gotten 24V on pin 1&3 (so long as the system was powered on)? I'm honestly thinking that (at 6 a.m. after a not-so-great sleep) the meter was low on batteries or mis-set... or maybe it was just me.
maybe the meter or a bad connection. Especially not being a digital one (or so it seems from what you describe).

Originally Posted by s2zeller View Post
The 0 Ohms reading wasn't so much as misreading as a mis-estimating. My lowly meter only has one Ohms setting (and it has x1000 by it)... so it was likely something greater than 0 but I just couldn't tell what reliably.
A precise instrument is a must nowadays with the new equipment. There's no other way, unfortunately

Originally Posted by s2zeller View Post
How do I measure the microamps when there is only one wire coming from the flame rod?
Connecting the leads of the meter in series with the flame-rod...but you will need to cut the black wire...a "non-invasive way of doing it is by getting an adaptor which Honeywell makes for this purpose


If I were is your shoes, I would buy a new pilot-burner/flame-rod assembly. They should be cheap, and will save you the trouble of all this testing, which I'm sure must be wearing you out
I foresee three most likely reasons for this malfunction: (1) a defective pilot-burner/flame-rod assembly (replace), (2) a lousy pilot burner flame (clean pilot burner orifice and/or increase gas pressure on pilot line), or (3) a defective smart valve.

But you may want to keep your ears wide open for funny noises off the inducer motor. You may have some work there that needs to be taken care of in the near future.
 
  #35  
Old 04-04-08, 04:53 AM
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"Red herring" would be something that sets one off on the wrong path. I follow them all of the time!

Having come this far, I am going to press on. My father-in-law has a good meter, so I will complete those tests and see where we stand at that point.

... and I don't mind interrupting the black wire from the flame rod (I've got a number of other splices in place from diagnostics and repairs in some other kitchen appliances).

Again, I appreciate your time and I'll let you know.
 
  #36  
Old 04-04-08, 03:30 PM
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12. set meter to read ohms at place leads on the terminals at end of the blue wires
...(these are terminals 1 &2 on this plug and are the igniter's ends).
...You should read less than 10 ohms with igniter at room temperature.
13. connect igniter plug back to smart valve
14. connect leads of voltmeter to terminals 2 & 4 of the plug at the ST9120 electronic board
...(NOT THE SMART VALVE'S CONTROL PLUG)
15. put back R-W jumper. Inducer will kick ON and igniter will glow. As the igniter is glowing,
...notice the voltage on the meter. It should read at least 19.5V
16. remove R-W jumper; remove meter
So, backing up a little bit, I re-took the resistance measurement for Step 12 and got 3.7 Ohms.

I again got to Step 14 and had some problems knowing exactly how this measurement should be made. If I unplugged the plug at the board, then the blower kicked on.

If I left the plug in, I got 24V without jumpering W-R.

I measured the micro-amperage for the flame rod in the meantime, and (of course) had a bit of trouble (as the probes allowed the current to flow and things went to full ignition... so I had to be quick to see what the reading was during pilot-only).

After several tries (and it seemed to lock-out and provide no reading and the igniter remained unpowered... not sure if that was the meter or the smartvalve), it *seems* like the reading was in the neighborhood of 2.8 microamps or so. It was hard to see at just the right time as it always went to full ignition fairly quickly after the pilot came on.
 

Last edited by s2zeller; 04-04-08 at 04:18 PM.
  #37  
Old 04-04-08, 04:53 PM
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Originally Posted by s2zeller View Post
So, backing up a little bit, I re-took the resistance measurement for Step 12 and got 3.7 Ohms.
3.7 Ohms --> ignitor is good

Originally Posted by s2zeller View Post
I again got to Step 14 and had some problems knowing exactly how this measurement should be made. If I unplugged the plug at the board, then the blower kicked on.

If I left the plug in, I got 24V without jumpering W-R.
good! that's the expected reading, means that the smart valve is getting the power it needs to perform its duties.

Originally Posted by s2zeller View Post
I measured the micro-amperage for the flame rod in the meantime, and (of course) had a bit of trouble (as the probes allowed the current to flow and things went to full ignition... so I had to be quick to see what the reading was during pilot-only).

After several tries (and it seemed to lock-out and provide no reading and the igniter remained unpowered... not sure if that was the meter or the smartvalve), it *seems* like the reading was in the neighborhood of 2.8 microamps or so. It was hard to see at just the right time as it always went to full ignition fairly quickly after the pilot came on.
Shortly after the pilot flame is lit, the micro-amp signal will start and should continue for the length of the heating cycle. 2.8 micro-amps is a good reading...the real test is to see if the signal remains steady while the burners are on full flame. If the pilot flame is unsteady, the micro-amp signal will fluctuate, and a drop below 1.3 would trigger the problem you have been reporting.

From the readings you've reported so far, the board, wiring, smart valve and ignitor/rod assembly all seem to be doing their job the way it is expected of them. You should leave the micro-amp meter connected next time you run this test again to see how steady is the micro-amp signal remains (if at all) and if it drops below 1.3 at some point. If something blows the pilot flame out or makes it waver so that it no longer engulfes BOTH the flame-rod and hood above (which is the ground rod) AT THE SAME TIME, then the micro-amps drop and the smart valve shuts the main burners OFF.

From the above, it looks like your problem here is likely a less than ideal pilot flame (small size?, flickery? weak?)...which would point to either low gas pressure in the pilot line, a dirty orifice (do you know what is it?), a wavery flame resulting from drafts, or a combination of all three.
 
  #38  
Old 04-04-08, 05:09 PM
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Increasing pilot burner flame size

If the flame is weak, the recommended thing to do is to clean the pilot burner orifice, and to do that you first have to shut the gas OFF The pic below shows you the inside of a pilot burner...the orifice looks like a very tiny cone (like an ice-cream cone), but metallic and with a tiny orifice on it. If it gets dirty, the flame becomes small, wavery or both.


But even after you clean it up, you may need to increase the pilot burner gas pressure. to do so, find in the picture below where it says "pilot adjustment under cap-screw". Locate it in your gas valve...remove cap screw, get a small flat-head screwdriver, insert, turn it counterclockwise (I think) to increase the pilot gas pressure and flame size. Needless to say, the unit should be "in full bloom" (firing) for you to be able to perform this adjustment.
 
  #39  
Old 04-04-08, 06:10 PM
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Originally Posted by pflor View Post
Shortly after the pilot flame is lit, the micro-amp signal will start and should continue for the length of the heating cycle. 2.8 micro-amps is a good reading...the real test is to see if the signal remains steady while the burners are on full flame. If the pilot flame is unsteady, the micro-amp signal will fluctuate, and a drop below 1.3 would trigger the problem you have been reporting.

From the readings you've reported so far, the board, wiring, smart valve and ignitor/rod assembly all seem to be doing their job the way it is expected of them. You should leave the micro-amp meter connected next time you run this test again to see how steady is the micro-amp signal remains (if at all) and if it drops below 1.3 at some point. If something blows the pilot flame out or makes it waver so that it no longer engulfes BOTH the flame-rod and hood above (which is the ground rod) AT THE SAME TIME, then the micro-amps drop and the smart valve shuts the main burners OFF.
I actually did keep watch of the readings once full ignition started. They seemed to vary in the 6.8-7.4 range, and didn't ever dip down.... which reflects just the eyeball observation of the pilot seeming fairly consistent in size and color once it has lit.

Originally Posted by pflor View Post
From the above, it looks like your problem here is likely a less than ideal pilot flame (small size?, flickery? weak?)...which would point to either low gas pressure in the pilot line, a dirty orifice (do you know what is it?), a wavery flame resulting from drafts, or a combination of all three.
With regard to drafts... I did notice that the puddy that surrounds the igniter/flame rod wiring as it leaves the combustion chamber is a bit loose. Not gaping, but just seems like it could stick better and be more snug.

I did notice the screw adjustment for the pilot flame on the smartvalve, but it was very tight and - at the time - I didn't want to force it and break something.

Am I correct in that the coupling that gives me access to the orifice doesn't require any special teflon tape or other sealant when it gets reassembled?
 
  #40  
Old 04-04-08, 07:38 PM
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6.8-7.4 micro-amps is a very good value of current.

I'm not sure I know what "puddy" is. Do you perhaps mean "putty"? are you maybe referring to the ceramic cover that surrounds the flame rod an isolates it from its holding bracket? Even a minute crack there could cause this tiny electrical current to leak to ground instead of going to the ground electrode (the hood) through the flame...something that would happen in an unpredictable manner (like the failure of your furnace to ignite, which seems not to have a definite pattern)

I'd love to know what happens with the micro-amp reading when the main burners fail to ignite. I think that's the ultimate clue needed to solve this mystery. But you'll need to leave that meter in place until the problem appears...and your in-law may not be to thrilled to leave his toy for too long with you I suppose

You won't break anything by putting a bit more force than usual in loosening the cap-screw that would allow you access to the inner screw for pilot-gas pressure adjustment

No sealants of any kind on the threads of the pilot burner and couplings when accessing the orifice. The connections there are strictly "pressure" fit...a small adjustable wrench is all that is needed there.
 
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