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Old 03-06-2009, 07:55 AM   #1
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Help Request: Signs of a faulty thermostat?


We have a 15-year old (Carrier) heat pump attic system, and the fan will occasionally start for ~1-2 seconds seconds, then stop again, and repeat that start/stop cycle a few times, and usually, if not always, ends up staying off.

Until recently, I had observed that the room temperature (let's say 70 deg.) matched the set temperature on the thermostat (i.e. also 70 deg.). But if I then set the temp to 74 deg, the unit would turn on, and stay on, until the room temp was also 74.

I first suspected the thermostat, but on reading through a number of posts here and elsewhere, found general agreement that this was a common novice mis-diagnosis.

The unit has since been serviced by the pros, who found nothing evidently wrong with it (and as tends to happen in these intermittent case, the unit was working just fine when qualified help was available.) He inquired about any changes to filter type, blocked vents, etc, and other common causes of this, but in the end there was no definitive answer. He did mention that the thermostat might be the cause.

Since his visit, I have noted one instance when the room temp was 70, and the heat was set to go on at 67, and I heard the fan switch on/off as described.

My sense is that this supports the notion that the thermostat should be replaced, but I still have some uncertainly because:
  • What I don't know about HVAC systems could fill canyons
  • The unit is 15 years old
  • In the past we have had battles with flying squirrels in our attic, and I don't know the extent to which they may have chewed any of the wires (though that's probably something to look into ASAP)
Does this sound like as strong an argument for a bad thermostat as I think it is, or are there other likely causes?

With many thanks,
- Dave
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Old 03-06-2009, 08:47 PM   #2
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Help Request: Signs of a faulty thermostat?


while I'am not fimilar with you'er unit. I wouldn't say it's the t.stat... Possibly a relay or fan control board.. If you are talking about the blower in the air handler
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Old 03-07-2009, 08:46 AM   #3
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Help Request: Signs of a faulty thermostat?


Sounds like the thermostat may have lost its setting with the temp diff. Sound like it may have be set at .5 deg

1st stage should be set at either 1 or 2 deg

and

2nd stage should be set at 3 or 4 deg

However, with the age of 15 yr it could be several things
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Old 03-08-2009, 06:36 PM   #4
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Help Request: Signs of a faulty thermostat?


Thanks all - The Unit's a Carrier 38AYC, and

Since I was going to replace the stat with a programmable one anyway, I went ahead and did that, but didn't solve this - and it sounds like I shouldn't be surprised.

Before I replaced the stat, it started happening even when I set the system to "off". Still does this now that the stat's been replaced.
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Old 03-09-2009, 08:48 PM   #5
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Help Request: Signs of a faulty thermostat?


Does it have a pilot or not?
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Old 03-10-2009, 06:03 AM   #6
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Help Request: Signs of a faulty thermostat?


Re: kenmac's note - Yes - The blower in the air handler is what is starting and stopping.
kgphoto - No pilot; I don't think the unit uses any gas.

Thanks again,
Dave
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Old 03-10-2009, 01:20 PM   #7
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Help Request: Signs of a faulty thermostat?


I didn't realize that this was an electric only heater.
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Old 03-11-2009, 05:48 AM   #8
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Help Request: Signs of a faulty thermostat?


Quote:
Originally Posted by kgphoto View Post
I didn't realize that this was an electric only heater.
Sorry about that - I probably didn't make that too clear (see note about what I know about HVAC systems ).
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Old 03-11-2009, 05:52 AM   #9
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Help Request: Signs of a faulty thermostat?


Quote:
Originally Posted by kenmac View Post
while I'am not fimilar with you'er unit. I wouldn't say it's the t.stat... Possibly a relay or fan control board.. If you are talking about the blower in the air handler
Would (should?) the tech that came out the first time have checked these? I can now reproduce the start/stop behavior consistently, so perhaps it's worth having the pros take another look...
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Old 04-28-2009, 08:45 AM   #10
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Help Request: Signs of a faulty thermostat?


Thanks, All - kenmac - you called it.

In the end, it was the control board that went bad, so even when the thermostat wasn't calling, the blower in the air handler on and off.
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