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Old 01-12-2009, 11:48 AM   #1
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furnace ignition troubleshooting


Hi 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 reacon 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 happend 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 of what my problem is?
Thank you very much for your help with my troublshooting effort.

Eugen

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Old 01-12-2009, 05:30 PM   #2
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furnace ignition troubleshooting


That furnace had several service bulletins issued. You should have a tech look at it to make sure it is up to date safety wise.

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Old 01-12-2009, 10:18 PM   #3
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Are these new bulletins?
I had techs maintain 3 and about 1.5 yrs ago. I suppose that they would have fixed any bulletin related issues. Or is there something new?

Any professional advise on my furnace issue?
I'd greatly appreciate it.
Thanks
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Old 01-12-2009, 10:33 PM   #4
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Quote:
Originally Posted by buju View Post
Are these new bulletins?
I had techs maintain 3 and about 1.5 yrs ago. I suppose that they would have fixed any bulletin related issues. Or is there something new?

Any professional advise on my furnace issue?
I'd greatly appreciate it.
Thanks
Not unless they they knew about them and were Coleman dealers.

And yes, what you describe could be one of them.

How old is the furnace

Model and serial would be helpful.
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Old 01-12-2009, 10:39 PM   #5
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So, I'd like to first try to fix it myself. If I need to call a tech I will ask about those bulletins...
The furnace worked for 9 years with just one prob 2 years ago, when we had to replace the motor.

The furnace must be 10-11 yrs old.

Would you be able to provide an opinion?
I saw good suggestions/ help in this forum.
I would think that this one is an easy one for the experts on this forum.

Thanks for any help you can offer.
We are fed up waking up frozen and with body pains..
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Old 01-12-2009, 10:44 PM   #6
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furnace ignition troubleshooting


Go t Hvac-talk and ask for Twilli. He is the resident Coleman expert.
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Old 01-12-2009, 11:37 PM   #7
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furnace ignition troubleshooting


Not sure if he will respond. I see that there is a prominent no-DIY queries in that forum...

Anyon else? Thanks folks.
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Old 01-12-2009, 11:50 PM   #8
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Old 01-13-2009, 06:30 PM   #9
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furnace ignition troubleshooting


Dear HVAC experts, could you please provide any expert opinion on my HVAC troubleshooting case?

After all this is a HE furnace and no matter what they are not that different.

From your experience I am positive that you can provide a technical word of advise.

Thank you..

Eugen
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Old 01-13-2009, 06:33 PM   #10
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They CAN be as different as that.


Model and serial number would be helpful.

Can't look up which service bulletin is applicable with out it.

90% furnaces have a lot of parts and there are subtle nuances fom model to model, and with in a given series.

With out that info we are going to be giving our best guess and you said you wanted to be absolutely sure; so do we.














You sure Twili didn't answer you he's usually pretty good about answering PM messages.
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Old 01-13-2009, 09:33 PM   #11
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All/ HVACLOVER, OK, here are more furnace component details:
  • Model number: CGU08016/ EVCON 1474-004
  • Ignition Controller: ROBERT SHAW HS780-34NL-306A

What other component details do you need?

Twilli? See: http://hvac-talk.com/vbb/member.php?u=11999
As a new member there doesn't seem to be a way to PM him on that forum.
Maybe I should look for him on other forums where I can PM him?

Thanks for help. I sense some advise will soon come

Eugen
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Old 01-13-2009, 10:19 PM   #12
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HHMMMMM

Could be a sensor that's dirty or bad, could be a weak igniter, could be a board.

Would have to take electrical and micro amp reading to be sure.

You got a volt meter?
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Old 01-13-2009, 10:48 PM   #13
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There's no thing electrical/ mechanical/ electronic that could be repaired that I did not repair in my life... I work in electronics/ telecom/ IT.

What readings do you need?

MUCH appreciated.
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Old 01-13-2009, 10:56 PM   #14
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you need at least a .5 micro amp off sensor. HSI should ohm out cold btw 75-90.

If these are good than you might have a weak roll out, weak limit, or bad board.
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Old 01-14-2009, 12:26 AM   #15
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- 120 ohms on the cold ignitor

New sequence of events:
- 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.
- at this point sometimes the ignition repeats (while the blower is still on), or the blower shuts off, and the ignition repeats.
- I believe that soon thereafter the cycle breaks completely and nothing happens till I power cycle.

Now, 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 tend to believe that it is a controller issue.

What do you think?
Thanks.

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