[New] furnace consistently kicking off on very cold mornings

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Old 02-06-12, 09:30 AM
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[New] furnace consistently kicking off on very cold mornings

I have a Goodman furnace Model GMH95 which was installed in my new construction in August 2010. This is a two-stage model.

It's had(s) a consistent problem...

If the day before is reasonable (40+), but the following morning is below freezing (well usually under 30), then the furnace does not come on like it is supposed to (per schedule set by my thermostat).

The thermostat that was installed is a Honeywell FocusPro 6000.

This would work(s) like clockwork and the only way to fix it is to turn off the heat and then switch it back on. This would work for a while, but ultimately you would have to wait it out. After about 3 days, it would be running normally and bringing the house up to temperature.

When the furnace failed, the house temperature was 50-degrees consistently. If the furnace was switched on/off, it would consistently turn on for about 14 minutes and then kick off completely. Eventually it would start again, but it could be 30 minutes or two hours -- no way of knowing.

What I have tried:

> Replaced thermostat with the one (same model) from upstairs that works fine - no change

> Original installer put in a new board - ran ok for a day or two - same issue but temperature now goes above 50 (up to about 57 only) - ultimately no change

Information:

> Seems the 14(ish) minutes correlates to going from one stage to another. Installer checked voltage and found power completely "disappears" when the furnace kicks off

> Seems the pressure switch is fine, but has not been replaced

> The system does not blink at all to signal that there is a problem or to provide an error code

> I've not yet tried a new thermostat and that is next on my list of diagnosing

Burrrrr it's cold in the morning... need to figure this out. Man is it ever annoying to wake up or go home to a cold house after you've paid the gas bill.

Any suggestions??
 
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Old 02-06-12, 10:27 AM
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The furnace manual is at http://www.goodmanmfg.com/Portals/0/pdf/SS/SS-GMH95.pdf


I'd review page 13 and understand how your furnace is set up to operate between the two stages.

And I'd use a multimeter to determine whether the thermostat is keeping the furnace turned on or not. If the thermostat is calling for heat you will find about 24 VAC between the C and W terminals.

Something is presumably turning the furnace off. Did the repairman not observe this problem when they were there?
 
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Old 02-06-12, 11:52 AM
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The contractor did indeed have the cover off and observed that when the problem occurs the power just "disappears."

As to what he actually has the contacts on, I do not exactly know, however it was clear that the furnace was cutting out because power stopped.

There is no problems with the breakers or anything like that, so he chaulked it up to a faulty board and simply replaced it. From his observations, he did not see that it was the pressure sensor.

I'm going to purchase a new 2-stage thermostat and see if that fixes the problem. If it does then it's a problem with the thermostat.

I've been trying to figure out if the "learning" properties of the thermostat or furnace could have anything to do with the problem. Both the thermostat and furnace have fuzzy logic that "learns" about your habits and implements them.

Thanks for your assistance.
 
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Old 02-06-12, 12:17 PM
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The furnace is designed to use a single stage thermostat only.

If you have a two stage thermostat, perhaps that's the problem.
 
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Old 02-06-12, 03:39 PM
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Thermostat is single stage heat/cool Honeywell FocusPro 6000 model th6110d1021, so I guess it's not that.
 
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Old 02-07-12, 05:57 AM
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Furnace came on this morning at 7am as the thermostat called for. It ran for exactly 12 minutes, shut off, and has not turned back on.

I'm convinced that the time is a symptom of the furnace's inability to jump stages, but I have no idea what the actual problem is.

Any suggestions anyone?
 
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Old 02-07-12, 07:38 AM
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You probably need to review page 13 of the manual carefully and consider how your furnace is set up to run.

And you need to follow directions given earlier to determine if the thermostat or something else is shutting off the furnace.

Wringing your hands is unlikely to produce identify the problem. You need to get in there and start identifying what is shutting off the furnace.
 
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Old 02-07-12, 07:48 AM
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Thanks for your response.

What happened this morning:

> 6:53AM - furnace turned on (thermostat set for 7AM)
> 7:05AM - furnace turned off (56-degrees in home)

> 8:07AM - furnace turned on
> 8:20AM - furnace turned off (58-degrees)

> 8:23AM - furnace turned on
> 8:30AM - furnace turned off (61-degrees)

Did not turn as of when I left (9:15AM).

I've located an individual that has very similar problems to mine and it was the pressure switch (which is what I thought it was to begin with based on the symptoms).

I'm going to call the contractor and tell him to come replace it. Fact of the matter is, if he cannot figure out what it is, there's no harm in replacing what it could be.

Here's the post: Goodman GMH95 Intermittent Problem - HVAC - DIY Chatroom - DIY Home Improvement Forum

Thanks again.
 
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Old 02-07-12, 10:51 AM
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98% of the time when a pressure switch fails to close, the reason is a defect in the venting system or drainage system of the furnace, not a defect in the pressure switch.

So by replacing the pressure switch you have a 2% chance of fixing the problem.

I would concentrate on identifying the actual problem rather than guessing and supposing that the pressure switch is the problem.

Indeed, it appears that you haven't actually checked to see if the pressure switch is opening and shutting off the furnace --- you are guessing it is because someone else had a similar problem.

Guessing and supposing is a poor diagnostic method.
 
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Old 02-08-12, 03:34 PM
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power completely "disappears" when the furnace kicks off

What does that mean? The power to the furnace from the 24v transformer? The power from the relay to the blower? Where was he measuring the power?
 
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