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Main burners won't light, but pilot is on, whats wrong with Luxaire gas furnace

Main burners won't light, but pilot is on, whats wrong with Luxaire gas furnace


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Old 02-02-09, 10:33 PM
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Main burners won't light, but pilot is on, whats wrong with Luxaire gas furnace

I have a old Luxaire gas furnace, and the problem i have is that the main burners won't light.

The standing pilot light is burning. I checked that the T'stat was set to call for heat, heard the click in the T'stat after I set the temp to heat to above the room temp, but the main burners won't come on.

This furnace was working fine earlier. I believe it might be a millivolt system.

I tried putting a jump wire across the limit, thermopile, and thermostat terminals on the gas valve, hoping I could 'jump' it to work, but to no luck.

Does anyone here have ideas about what might be wrong, and how I could test?

Thanks
 
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Old 02-03-09, 06:27 AM
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Take a look at the pilot burner. Usually a thermocouple system will have a thermocouple that is a thin rod about 1/8" in diameter sitting in the pilot flame. A thermocouple usually has a coppery tubing running from the thermocouple to the gas valve. A pilot generator for a millivolt system is about 1/3" in diameter. Which do you have?

And I'd do a basic check to see that you have 120 volt power to the furnace.

Do you have a multimeter that measures AC voltage and resistance and understand how tro use it?
 
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Old 02-03-09, 07:14 AM
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SeattlePioneer,

I will have to go back to the house to look at the furnace, this is not my furnace, but a friends.

From what I remember, there were two lead wires coming off the flame sensor, and those 2 wires were attached to screws in the top of the gas valve assembly. Does that indicate it is a pilot generator for a milivolt system, or do thermocouples also have such leads?

I put a small jump wire across the leads of the t'stat, and saw an occasional small spark there. (that means I should have power there?)

Yes I have a multimeter, and have some familiarity with using it, mostly to check for AC power at outlets or receptacles. Resistance is OHMS measurement?

Where do I test for 120 volt power?

Also, since I have to leave my house (this pc/forum) to drive to the furnace, can you tell me what further test I can do once I am in front of the furnace?

Is it logical that the problem is either existing in the pilot sensor, or the gas valve?
 
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Old 02-03-09, 07:17 AM
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Originally Posted by mnalep View Post
SeattlePioneer,

From what I remember, there were two lead wires coming off the flame sensor, and those 2 wires were attached to screws in the top of the gas valve assembly. Does that indicate it is a pilot generator for a milivolt system, or do thermocouples also have such leads?
That describes a millivolt system.

A good place to start would be disassembling the pilot burner and cleaning the pilot orifice. That's a common reason for millivolt systems not to work and probably needs to be done in any case.
 
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Old 02-03-09, 07:33 AM
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When you say pilot orifice, are you talking about the opening for the flame in the pilot burner itself, or perhaps where gas in the main burner is supposed to contact the pilot flame?

I'm a little confused - why clean the pilot burner - since I do have a pilot flame?

Should I clean the thermopile with steel wool?

Does my idea of putting a jump wire across the leads on the gas valve have any merit?
 
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Old 02-03-09, 03:24 PM
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I took the thermopile assembly apart, cleaned it with steel wool, and reinstalled.


My thermo-pile looks like the one pictured, lower on the web page, here:
Identify the components of your gas furnace

I am getting about 1/2 volt, or just a little more, on the contact leads on the gas valve. (Interestingly on either the AC or DC settings on the multimeter. Is this supposed to be AC or DC voltage?)

Main burners did not turn on after a few minutes. I then tapped on the gas valve and what I think is a regulator a few times, and the main burners came on. I did not hear any valve open, but the flames were strong.

The pilot light burns blue and strong the whole time. I guess I could try a new thermo-pile? Or does it sound more like a stuck gas valve. How would I know? Is 1/2 volt or so correct on thermo-pile?

The main burners cycled off and on a few times, and I decided to leave it for now. (The main burners cycle on for 3-4 minutes, off for 20 seconds or so, then back on)
 
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Old 02-03-09, 03:32 PM
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Usually a millivolt gas valve has three connections, labeled TH, Th/TP, and TP.

The pilot generator/thermopile should be connected to TH/TP and TP

You need a multimeter that measure DC millivolts (1 volt=
1000 Millivolts)

With the pilot light lit for several minutes and the thermostat off, what is the millivolts between TH/TP and TP? I presume that's going to be the 1/2 volt you were talking about.

With the thermostat turned on, what are the millivolts between TH/TP and TH?

Also with the thermostat on, what are the millivolts between TH and TP?
 
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Old 02-03-09, 05:55 PM
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SeattlePioneer,

I took a pic of the milivolt gas valve, but can only read the label on the connections from the Thermostat. These 2 leads are labeled TH1 and TH2. (I'd post the pic, but I can't see how I would fdo that here).

There are 2 other connections, one from the THERMO-PILE, and one from the LIMIT switch, but I did not read what the labels on those were. (I can check later.)

When i read the multimeter, I tried it on the 5 Volt DCV setting, and also on the 10 Volt ACV setting. The needle read the same location both times, about 1/2 Volt - or I guess more accurately 500 MilliVolts?

This was read with the pilot light on for a while, and across the 2 leads for the THERMO-PILE connections (which I guess might be the TH/TP and TP leads as you said), and with the thermostat set to call for heat.

I don't think I measured the THERMO-PILE leads with the thermostat dialed down.

The other readings I tried were of the leads from the thermostat, and the limit switch.

What I read when i set the thermostat up, was like 0 volts, but when I set the thermostat down, I read about 1 Volt (which seemed the opposite of what I expected to see?)

I also read the leads on the LIMIT switch, and saw that when the main burners went off, I read about 1.5 volts, but when the Main burners came on I saw like 0 volts.

Unless the other 2 measurements I mentioned above were across the TH and TP lables, I'm not sure I did that.

What does TH, Th/TP, and TP stand for?
 
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Old 02-03-09, 06:10 PM
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You need a more sensitive meter if you are going to do work on a millivolt system.
 
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Old 02-03-09, 07:29 PM
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SeattlePioneer,

I'll shop for it tomorrow. Any recommendations as to brand or were to get one that's not too pricey?

What millivolt measurements should I expect when I test the THERMO-PILE leads?
 
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Old 02-03-09, 08:00 PM
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SeattlePioneer,

I'll shop for it tomorrow. Any recommendations as to brand or were to get one that's not too pricey?

What millivolt measurements should I expect when I test the THERMO-PILE leads?
 
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Old 02-03-09, 08:06 PM
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One that reads to a sensitivity of 5 milliamps or so should be fine. Those are pretty readily available for, oh, something like $50-100 I would suppose, although I haven't shopped for an inexpensive multimeter in years.

Probably you'll get one more sensitive that that.
 
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Old 02-03-09, 10:09 PM
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I found this link to a robertshaw thermopile gas valve, and it appears to have 3 screw terminals, labeled as you asked me about earlier:

http://www.robertshawtstats.com/spaw...oltSystems.pdf

Mine actually has 6 screws, is made by Honeywell, model VS87A9X4X2, and the two terminals I did see labeled were TH1 and TH2.

I also found this replacement part, with a picture, but it also does not clearly show the terminal screws, or ow many are present...

Millivolt Gas Valve 1/2" Gas Pipe Inlet 3/4" Outlet (Honeywell): American HVAC Parts

How do I translate the testing you wrote about (with 3 terminal screws?), to testing mine (with 6 terminal screws on it)?

I wish I could upload my picture - but my profile says "you may not post attachments"
 
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Old 02-03-09, 10:44 PM
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The Honeywell gas valve specifications for which you provide a link describe the electrical connections as follows:

Terminal block has 3 combination screw and 1/4 in. (6 mm) quick-connect terminals.
Electrical Connections (in.) Terminal block has 3 combination screw and 1/4 in. quick connect terminals.


This has the three connections I described earlier. Each one has a screw terminal and a spade connection. The spade terminal and the screw connection are connected electrically, making three terminals each with two choices of how you connect a wire.

You describe your gas valve having six screws, something I don't recall ever seeing in a millivolt gas valve.


I'm finding your descriptions very hard to understand.
 
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Old 02-04-09, 07:42 AM
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It sounds like the later versions of the gas valve have 3 screws and 3 quick connects, for a total of 6 connections?

Where mine just has 6 screw on connections. (there are still 6 wires from the thermocouple, thermostat, and limit switch to be attached on either version of this gas valve, right?).

Problem seems to be that the labels on the newer version are just for 3 pairs. I assume the pair they call TH, relate to mine that are labled TH1 and TH2.

So when they say put a meter test lead on TH on the newer version, I don't know if they mean TH1 or TH2 in my case?

Same for the other leads marked TP or TH/TP. (I will look at mine later, but I'm guessing they will be TP1 and TP2 and TH/TP-1 and TH/TP-2 or something like that.)

Do you have email, where I could mail you the pic I have that might make my description clearer?
 
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Old 02-04-09, 07:44 AM
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Sorry. I'm not familiar with the kind of arrangement you describe. That being tha case, I can't offer potential mistakes to you under the guise that it might be a help.
 
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Old 02-04-09, 08:05 AM
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I'd rather have your help than be on my own.
 
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Old 02-05-09, 12:56 PM
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SeattlePioneer,

I got a Millivolt meter, and got some readings off the gas valve. Maybe you could comment on the thermo-pile measures I got?

The leads coming from the thermo-pile generator read at anywhere from 728 millivolts to 746 mv with the main burners off/pilot on. It drops to around 378 mv to 381 mv when the main burners are running/pilot on.

I think the thermo-pile is supposed to generate 750 mv, so I'm thinking the therm-pile is ok? The furnace has been cycling on and of for 2 days now.


In case you might comment on some other measurements I did - I got the labels for all 6 leads on the gas valve, and measured voltages there also...

The 6 leads on the gas valve were:
Thermostat (TH1, TH2)
Limit Switch (L1, L2)
Thermo-pile (RED, WHITE)

With PILOT on, Main Burners OFF, Thermostat OFF:
740 mv - TH1/WHITE
740 mv - TH2/RED
724 mv - TH1/TH2

With PILOT on, Main Burners ON, Thermostat ON:
381 mv - TH1/RED
119 mv - TH2/WHITE
120 mv - TH1/TH2

Thanks.
 
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Old 02-05-09, 01:44 PM
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I'm presuming that L1 and L2 are actually wired to a limit switch, is that correct?

Are any on the six connection points you describe wire to each other in addition to the Th, Tp or Limit?

If so, please describe which of the six terminals are wired to each other.
 
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Old 02-05-09, 01:56 PM
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L1 and L2 are the LIMIT SWITCH connections.

There are 6 individual screws, basically 3 pairs. I do not see any additional connections from any pair, to any other pair.
 
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Old 02-05-09, 04:24 PM
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Approximately out of how many call for heat cycles does the main burner(s) not come on?

If it comes on more often than not, and you are testing when the burners are able to fire up, that will not help. You need to test when the main burner does not come on when it should be coming on.

If that is the case, and your tests are when main burner does not come on, with call for heat, if you disconnect stat wires from gas valve and ohms test the two wires, do you get 0 ohms(no resistance), as you should?

And if you disconnect the L1 and L2 limit wires from the gas valve and ohms test them, do you also get 0 ohms (no resistance)?

If stat and limit both show good, and you are getting MV generation, and the pilot stays on, but the main burners do not come on, then I'd have to guess a faulty gas valve -or, gas is not coming out of the line between burner and pilot area - or not coming out the burner, in area of the pilot.
 
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Old 02-05-09, 04:39 PM
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Originally Posted by mnalep View Post
L1 and L2 are the LIMIT SWITCH connections.

There are 6 individual screws, basically 3 pairs. I do not see any additional connections from any pair, to any other pair.

Are these pairs of screws electrically connected to each other?

Based on your earlier description, I'm supposing the answer is no. You appear to describe one pair of terminals as L1 and L2, connected to the limit switch and so on.

There must be internal connections that complete the circuit to turn on the equipment, but without a description of that I can't really help you.
 
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Old 02-05-09, 05:52 PM
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One thing is apparent --- the 120 millivolt voltage drop across the thermostat that you report (Th1-Th2) is way too high, and could be the cause of the problem.

A thermostat should be like a switch, and a voltage drop of 5-10 millivolts might be typical.

Check the electrical connections and make sure they are tight.
You might have a thermostat designed for 24 VAC operation, which would be the wrong type of thermostat.

Take the cover off the thermostat and look to see if the thermostat has a heat anticipator -- a finely wound coil of wire with an adjustable pointer cross it. If so, you have the wrong thermostat and need to install one designed for use with millivolt controls.


You can also try using a jumper wire across Th1 and Th2 --- that might cause the furnace burner to turn on.



Seattle Pioneer
 
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Old 02-06-09, 12:16 PM
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SeattlePioneer

I checked the thermostat booklet, and it is designed to handle a millvolt system.

One thing I did notice today with the thermostat, when I set it up manually using the temporary override, it did drop back once, which I did not expect. It is a programmable stat, so I had set it up using the increase temp button, which I thought was supposed to hold until the next programmble period (which has not arrived yet).

Could the batteries be the cause of the drop on the stat?
 
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Old 02-06-09, 12:26 PM
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ecman51

The main burners have been coming on reliably, as far as I know, since I cleaned the thermo-pile.

I will try the ohms test. I am guessing that I can do that test when I dial the stat down (not call fo heat) also?

There is an additional tube, that I guess bleeds gas from a smaller 1/4" gas line right at the pilot light (not the other 1/4" tube that goes to the pilot light itself. I did blow thru that tube when I took the thermo-pile apart to clean it. But that was only 1/2 of the length of the tube going to the gas valve. perhaps I should disconnect the other half of that line and blow thru it also.

Can I take the gas valve itself off (after closing the main gas lever) and look inside and see if there is any blockage, or if the valve is sticky? (I have no idea what is inside of that valve, or if this question is legitimate.)
 
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Old 02-06-09, 12:28 PM
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Originally Posted by mnalep View Post
Could the batteries be the cause of the drop on the stat?


Probably not. Usually with digital thermostats the furnace is turned on using a relay.

The millivolt loss is still too high. Try jumpering out the wires to TH1 and Th2 at the furnace gas valve and see if that turns on the furnace.

If you don't find a wiring connection that causes the milliovolt drop you describe to lower sharply, replace the thermostat. It's not making a good electrical connection.


You can also measure the millivolt drop across the limit switch as well --- your L1 and L2. That should also be only a few millivolts since that is just a switch as well. Poor electrical connections through limit switches are also a common problem with millivolt systems.
 
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Old 02-06-09, 08:21 PM
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Originally Posted by SeattlePioneer View Post
Sorry. I'm not familiar with the kind of arrangement you describe. That being tha case, I can't offer potential mistakes to you under the guise that it might be a help.
General Controls did indeed make a 6 terminal B-60 valve. Typically used for forced air furnaces to include the limit circuit. However most of the time the 2 terminals marked "aux" were not used and came with a factory installed jumper. The limit was usually wired into the t'stat circuit. I'm wondering if the original valve had a plugged bleed tube as the only problem. That would be the 1/8" coming of the valve and going to the pilot. If it is plugged the gas will not pass, however you may still hear the operator click.
 

Last edited by mbk3; 02-06-09 at 09:01 PM.
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Old 02-06-09, 10:19 PM
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Hello mbk3---

I'm not familiar enough with this valve to offer further advice. Perhaps you can.

I'm supposing that there must be some internal connections for those screw terminals in order to connect the external parts (limit, thermostat and pilot generator) together.

If I were there I would soon figure that out, but laboriously trying to deduce that remotely doesn't seem worthwhile to me, and might produce bad guesses.

"A man's gotta know his limitations" I think that was a Clint Eastwood character!
 
 

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