Intermittent ignition on EVCON THE90


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Old 01-13-09, 08:32 PM
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Intermittent ignition on EVCON THE90

Model number: CGU08016/ EVCON 1474-004
Ignition Controller: ROBERT SHAW HS780-34NL-306A

Hi guys,

I hope I can get some help here.
Here's what my issue is ... and my family wakes up in cold... We need a solution to this intermittent issue.
-----------------------------------------------------

i guys, I hope that the HVAC experts can help so that I can ensure that I am replacing the right part, if a replacement is needed at all..

EQUIPMENT: EVCON THE90 FURNACE
ISSUE: very frequent cycles and locked cycles where there's no ignition

My furnace is cycling a lot. I suppose that that is a good reason why my electric bills are high....

Anyway, the main thing right now is that the furnace will not come on at night and we're getting sick... while the draft inducer may soon burn out as it's on so often.

Here's what happened when the furnace does not ignite:
- The draft inducer comes on immediately when the Honeywell thermostat asks it to.
- The pressure switch closes and stays closed for the entire duration when the the draft inducer runs.
- The HSI does not light and the ignition process stops right there - while the draft inducer keeps running, the pressure switch is closed.
- After a reset by turning the power or sometimes the thermostat on and off. the correct ignition cycle takes place and the furnace runs fine.
- Normally the ignition issues seem to be more pronounced overnight.
- NOTE: even this "reset" works: while the draft inducer runs I take the pressure tube out and plug it back in. The pressure switch obliges and opens and then closes back. The board does the job properly, the HSI lights out, etc -- all ok.

What gives? I would think that the issue is the Robertshaw HS780-34NL-306A ignition control module and that I would have to replace that, but since it is pretty costly I would like to be sure before I buy.

Is there any way to be absolutely sure if the controller is the issue?
I am positive that the pressure switch and the inducer close the circuit right.
Is there any OTHER input that the controller looks at before deciding to turn on the HSI?

Thank you very much.

Eugen
 
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Old 01-13-09, 09:33 PM
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That's a very professional write up, Eugen, the best I've seen on this board. I'm supposing you are an engineer?

Your question asking about other inputs suggests a professional analysis of the problem --- if all the inputs to the ignition module are there, it should either work properly of it's defective and needs to be replaced.


I agree that you have all the inputs present and that the control module is working erratically. I'd replace it.
 
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Old 01-13-09, 11:17 PM
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Yes, I am an engineer...

Here's some new food for thought....
I'd appreciate another opinion, please.

I have a NEW behavior that is even more urgent... it looks like the burner won't stay on as the GV is released.

Here's a description of the new behavior. I'd appreciate an expert advise, before I (really) rush to buy the controller.
Thanks

NEW BEHAVIOR:
- the thermostat calls for heat
- the inducer and APS do their job, while the LS and ROS are closed. As a matter of fact the 4 above are closed at all times (after TS calls for heat). Just small variations (likely due to amp/consumption variations when HSI lights, and then when the GV opens).
- the voltage on the inducer is ALWAYS around 120V. The reason I am highlighting the above is that it is supposed to drop to 24V after the gas is burning, transforming itself in a flame sensor.
- (Most likely) as a result of the above the GV closes after a few mins of gas burning, while the blower remains ON.
- while the blower is still on the ignition-to-GV shutoff cycle happens TWICE
- After the 2nd GV shutoff the blower is shutdown. The inducer stays on, the LS and ROS stay on. The 110V on HSI is on. No trace of 24V on the hybrid HSI/IGN at any time.

I am not sure if this new behavior is related to the previous one, related at the beginning, but regardless... this seems to be even more urgent, as the furnace seems to struggle to stay on.

The main issue seems to be the IGN voltage remaining at pre-ign/ ign level = 120V. After IGN it is supposed to drop to 24V, which it does not. The drop is just of half a V or so.

So, what's up with this dual mode IGN/Flame Sensor thing? Is it defective (or dirty) OR is the controller to blame? I believe that it is a controller issue.

What do you think? THANK YOU
Thanks.
 
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Old 01-13-09, 11:57 PM
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Originally Posted by nochaos
- the voltage on the inducer is ALWAYS around 120V. The reason I am highlighting the above is that it is supposed to drop to 24V after the gas is burning, transforming itself in a flame sensor.

I'm presuming you meant that the voltage on the ignitor is remaining at 120 VAC?

Does the HSI remain red or white hot after the burner lights?


Not too many furnaces use the ignitor as a flame sensor, and frankly I wasn't aware that the voltage was supposed to switch to 24 volts after the ignitor high voltage switched off.

Sounds like the ignition module would have a SPDT or DPDT relay that switches on to heat up the ignitor and after the period for the ignitor to heat the relay would then change state and apply the 24 volts you say should happen. If so that relay doesn't sound like it's working reliably.

Going back to your first post, the problem appeared to be that the 120V was not being switched on to the ignitor. Perhaps that was the same sticky/defective relay but instead of not applying 120V perhaps it was only applying the 24V.

--I don't suppose you tested the voltage being applied to the ignitor on that occasion?

So my assessment is that both the problems you have observed might well be caused by the same sticking/defective relay.

Often a way to test or confirm such a theory is to tap on the ignition module to make it easier for the relay to change state. The handle of a screwdriver makes a good tool for that.
 
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Old 01-14-09, 12:07 AM
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Heh, heh! "nochaos"!

Is that a prayer or are you bragging?
 
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Old 01-14-09, 12:27 AM
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Just when I was about to order the controller....

I decided to think about this a bit more...

1) The 120 to 24V switchover for this hybrid HSI/ flame sensor is written on the schematics I found inside the furnace housing.

Now, my opinion on the 24V disappearing act seems to be erroneous as I realized that the relay inside the controller would trip when I placed the (crapy) multimeter terminal on the IGN connector. If the relay is so touchy-feely it must be either an issue of a messed up relay - about to crack, inside the controller, or (I guess) a matter of unstable flame sensor - maybe even dirty.

I am holding back from purchasing the controller till I:
- a) check the HSI/sensor and clean it up - what is a safe and effective way to do that?
- b) do the knocking test on the controller when the relay won't switch to 24V - it will be a matter of timing and listening...

2) I am tiny bit less desperate as I seem to have a way for keeping us warm by ensuring that I do not give the controller "ideas" - i.e hiking up the temp on the thermostat high enough to have the furnace work continuously.
When it will get very hot we will switch off and that should take us through the night.
I will sure have to wake up early, or at night to turn it back on....

But I should now be very close to solving this mystery.

So, is it the HSI/sensor, or is it the controller? I should be able to tell by tomorrow, after I return home from work...

Thanks for your help.
I will update again tomorrow,
Eugen
 
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Old 01-14-09, 01:05 AM
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Personally, I wouldn't try cleaning the HSI. They are very fragile and the risk of breaking it would be high.

At a minimum, I'd have a spare ignitor on hand, which is a good thing to have in any case.

I'd be very surprised if it's the HSI and not the ignition module, but life is full of surprises.
 
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Old 01-14-09, 10:22 AM
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The HSI/ flame sensor has 120 ohms across, cold

Kind of out of the normal range of up to 80, so I hear.
It may even be that one caused the wearing off of the other?..

Anyway, I have a MAJOR issue with the furnace design, and the way the HSI is buried in the guts of the furnace. It's beyond me what convoluted brain could design something like this.
The gas supply pipe goes in from the front - the GV takes the supply from left side and the output goes into the furnace at a 90 degrees angle (measured across the input line).

In the tunnel of "design stupidity" goes the gas pipe and a metal bracket/ collar, that has the HSI mounted on with a screw, at the FAR end, far inside.

I will look again, but last night I did not see any obvious way of taking the HSI out without taking out the gas supply assembly (line + GV).

I really hope that I am wrong...

Gee... OK, finished venting.
'later
 
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Old 01-14-09, 11:09 AM
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I don't recall ever working on that furnace, so I can't be of any help, I'm afraid.


It may be that the entire burner assembly is designed to be removed as a unit for maintenance and repair. That can be a worthwhile design.
 
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Old 01-14-09, 08:37 PM
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Sure can't take out the HSI....

Gees, that's one assembly and there's no way to take it out without taking out the gas line...

3 questions:

1) I will likely order the controller now... If that does not work I will do the HSI thing. I would like to stay away from dismantling gas piping.

Question, what do you think about this replacement controller: 50E47-843 White Rodgers UNIVERSAL IGNITION MODULE **Replaces many others... see replacement cross-reference here** - HVAC Products Any experience with it? Would it work ok, as they say that it is?

Is it pin-to-pin compatible with the Robert Shaw I am replacing: HS780-34NL-306A?

Just in case that you know.

2) If/ when I want to reach the HSI to replace or whatever - can you tell me how the piping is normally screwed?
I have the pipe connected like this:

elbow-pipe-connector-pipe-GV (that surely cannot turn)
How would I disconnect? My quess is that one of the pipe screws or the connector screw in in the opposite direction.
Sorry I am a total rookie at screw-in piping...
I want to make sure that I am turning the right piece and not crack/ force anything.

3) I know how to check for gas leaks. What do you recommend to use for connection sealing? I suppose that the tape used for water pipes is not the best for this purpose.

Thanks a lot.
 
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Old 01-14-09, 10:12 PM
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The universal replacement modules of the kind you describe are usually quite good, I used them as a matter of routine because one could be made to work on many different furnaces.

But their very flexibility requires some judgement to use them.

I would expect you would solve any of those problems, but you might have to puzzel through one or two.

Figuring out how to change the connections from the old to the new module would be among those challenges. Most are going to be fairly straightforward (such as connections to the hot surface ignitor) but they may be labeled somewhat differently.

As I said, I think you would figure those out without too much difficulty.


It sounds like the burner assembly is designed to be pulled out as a unit, but that the installer didn't leave a union in the gas line to make that practical to do.

That's the mark of a carless installer. It's easier to just start piping away from the furnace, and it's unlikely anyone is going to catch that carelessness and make him repair it.

The way I'd correct that is to shut off the gas to the furnace, at the furnace shutoff valve if possible or the gas meter.

Hacksaw through the gas supply line near where it threads into the furnace burner assembly. Unthread the remaining stub to the burner assembly and the other stub going to the first fitting toward the gas meter.

Then connect a coupling to the burner assembly and a pipe nipple to the supply side, of suitable length so that you can install a union to connect the two pipe nipples. Hopefully the furnace shutoff valve is upstream of the union or I'd consider installing another valve so that you can shut off the gas while you are at the furnace, open up the union and pull out the burner assembly easily.

Is that clear?

---gas supply pipe--- shutoff valve--- union---burner assembly


And yes, you can use teflon tape to seal the pipe threads.


You can use water and dishwashing liquid to soap out the fitting with a small paintbrush, but use a lot of detergent, which increases the sensitivity by making it easier for the solution to bubble.


The only other mystery would be the union. Buy the size you need (probably 1/2" Iron Pipe) and look it over to figure it out.

The basic idea is that it allows you to connect two iron pipes without turning either one. You'll figure out why that is an advantage.

The only other thing I'd recommend is using two wrenches when installing iron pipe fittings, one to back up the pipe you don't need to turn and the other to turn the fitting you do want to turn.

Good luck!
 
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Old 01-16-09, 07:20 AM
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Thanks for the instructions

Quite helpful

I do have a shutoff valve on the vertical supply line.

The total length of the piping that I described is just about 10 inches, if I recall it right (I am at the office now).
Within that I have the: Pipe--Coupler--Pipe--Assembly.
On the vertical (just outside the furnace enclosure) I have: Elbow--pipe--valve--pipe--reduction coupler/ adapter to supply copper line. The supply line goes up for about 5-6 feet.

I will surely have to hack saw the pipe at the location you mentioned, unscrew the coupler from the left hand side pipe, and replace the coupler and the sawed off pipe with new pipe + union.


Now, there is another bummer... I would not know what HSI model I have till I take it out! That means that by doing the above I will have to take out the assembly and the HSI, check out the HSI, replace the HSI and the assembly, connect the union and the pipe, run like that till I receive the new HSI, and redo the steps above. Double the work. Great...

Not to mention that in order to take out the burner assembly I have to unscrew like 4-6 screws. Antique design, with no clips of any kind. To top everything off - yes, the installer did a number on the old folks that I bought the house from 9 years ago... Or it looks like the number is on me now...

I would almost want to just replace the controller and see if that solves the problem... But then the HSI has 120 ohms and that likely means that it's worn off and has to be replaced. My biggest fear is not that I'd have to place a follow up order for the HSI, but that after replacing the controller the whole thing may be solved for a while, after which the whole shows will restart. That can happen if somehow the HSI resistance puts some sort of strain on the 120V/24V IGN to sensing switching cycle and the respective relay.

I suppose I better take the safest, yet not cheapest way of replacing both and be done with it..

Thanks.

Thanks,

Eugen
 
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Old 01-16-09, 09:20 AM
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Your ignition module uses a phenomena called flame rectification to verify that there is a flame present or it turns off the main burner gas.

If an AC voltage is applied to a flame, the flame will rectify a small DC current on the order of a few micro amps (uamps, not milliamps). The ignition control then can detect the presence of that small DC current and leave the main burner gas on if that is detected.

Measuring that flame rectification current is the proper test for determining whether the ignition module is working properly or the flame sensor is bad --- in your case the flame sensor is the ignitor.

Typically, to measure the flame sensor current where the HSI is the flame sensor, you'd measure the DC uamps between the chassis ground and the ground pin on the module.

Again typically, a good figure for the micro amps would be 4-5 or so. 2-4 will usually provide reliable operation of the furnace. 1-2 may provide intermittent operation of the furnace, similar to what you are experiencing.

Less than one and the furnace tends to shut the burner off rather than leave it running.

If you are getting "good" micro amps and the furnace is shutting off on a flame failure like yours is, that suggests the ignition control is bad.

I am not able to find the installation manual for that series of ignition modules, so I can't verify the testing procedure or the specifications for the current flow that I've described. But that tends to be typical across a wide variety of different ignition modules.

So you might want to try that test and see if that offers you a better idea on what to do.
 
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Old 01-16-09, 11:10 AM
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I sure will do this test.

I sure will test this. VERY educational info.

Question:
Originally Posted by SeattlePioneer
Typically, to measure the flame sensor current where the HSI is the flame sensor, you'd measure the DC uamps between the chassis ground and the ground pin on the module.
Are you referring to the control module, that supplies the AC to the HSI/sensor?
What if that is connected to the chassis ground and hence there will be no current between the chassis ground and the ground pin on this module?

Thanks for your clarification.
 
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Old 01-16-09, 12:23 PM
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Originally Posted by nochaos

Are you referring to the control module, that supplies the AC to the HSI/sensor?
What if that is connected to the chassis ground and hence there will be no current between the chassis ground and the ground pin on this module?

Thanks for your clarification.

Yes. The current flow is from the 24 volts supplied to the HSI by the control module to whatever piece of metal (ground) the flame may strike, and then through the furnace chassis back to the module through the module ground connection.

It may be that the module is grounded in more than one place, but the flame rectification circuit typically measures the current from that pin to make that measurement possible and also to encourage installers to take care to ground that pin effectively.

That's the way it usually works anyway. As I noted earlier, I haven't found the manual on your ignition module, so the above is describing the common industry practice.
 
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Old 01-16-09, 12:36 PM
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The module has 2 IGN leads that are connected to the HSI/sensor

I understand that the 120V (ignition) and 24V (flame detection) are supplied through these 2 leads.
Wouldn't the current you mention be between the 2 leads, as opposed to the ground?

Sorry, I just want to make sure that I understand the electrical circuit here..
Further more... measuring amps between ground and 120V AC may not be a pleasant surprise??

Please clarify. Thanks.
 

Last edited by nochaos; 01-16-09 at 12:58 PM.
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Old 01-16-09, 01:46 PM
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With a separate flame sensor, the flame rectification current is typically measured along the wire going from the flame sensor to the circuit board. That's not the practice when using the HSI as a flame sensor, because of the 120VAC voltage that's present when the ignitor is heated.


That's why I'm suggesting measuring the DC current flow from the chassis of the furnace to the "G" (for ground) pin on the ignition module. Both are at near zero voltage, but the flame rectification DC current will still flow from the furnace chassis to the G pin on the module --- and through your multimeter when you have it connected up.

So again--- whenyou have the 24 VAC applied to the ignitor after the burner is lit, you get a small DC current flow through the flame to the chassis of the furnace, and then back to the G pin on the ignition module.

At least that's the way it typically works with other such modules that can use the ignitor as a flame sensor.
 
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Old 01-16-09, 05:57 PM
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Definitely sub-0.2uA

There is zero voltage, zero resistance, virtually zero Amps.
I am not surprised at all - between the 2 points there are 2 straight wires...

If there's no other way to test the HSI/ sensor I will replace both. But I'd like to order the HSI/ sensor before doing the pipe job and for that I will have to ID the HSI/ sensor.

I will have to contact Coleman and ask them for the HSI/sensor model #.

Thanks.
 
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Old 01-17-09, 12:14 AM
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Keep in mind that current is perfectly capable of flowing with near zero voltage if there is negligible resistance in the circuit.
E= I/R. Copper wire provides negligible resistance so you will find negligible voltage, but the current can still be high, and in this case you only need a few millionths of an amp.


Consider the typical 120 volt circuit, which has a "hot side" at 120 VAC and a neutral wire running back to ground potential at the breaker box. A load across the 120 VAC to the neutral wire will have a 120 vac potential across the load, but if you measure the potential to the neutral wire it will be at or near zero volts. But if you measure the amperage flowing through the neutral wire it can still be many amps.

Same deal with measuring current from the furnace chassis to the ground terminal on the ignition module.

Then again, perhaps you need to check to see if you have the 24 VAC being applied across the ignitor terminals. No 24 VAC no flame rectification current, which would also explain why you found no DC current.

Incidentally, is the ignitor staying turned on all the time?

There have been somewhat varying reports on these conditions over the course of this thread.

I still suspect that the problem is with the relay that I'm supposing should be switching the voltage to the ignitor between 120 and 24. It's not switching reliably and not making good electrical connections, which would explain all the problems you've described, from what I can understand.

And again, tapping on the ignition module might get it to do more of what it should be doing, which might tend to confirm that the module is the problem.
 
 

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