No heat after replace thermostat.

Recently, A customer asked if I can replace a Nest Smart Programmable Wifi Thermostat, after replaced it, tested and found hp and heat strips working. Week later when weather got colder, I have a called back said it not heating and vents blowing cool air, went back and found compressor sometime come on sometime won’t, and heat strips completely won’t come on, w won’t energize. Replace with Honeywell t4, doing the same thing. Looked up the info about the unit but found non, attached is the wires connection of old thermostat and outdoor info plate. Thanks for your help.

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So it works in cool mode with new thermostat , but it does not work in heat mode right?

Did you replace the NEST or install it. If you installed a NEST, you need to replace it. If the customer supplied it, they pay to replace it.

In the photo, B & O get energized?

Originally Posted by TechmanTery

In the photo, B & O get energized?

I thought b was com on old tranes.

If I am not mistaking, You’re shorting out when the unit is going to heat mode. I dealt with this before. Find the common coming from the unit and fix it there.

B is common, T is not used on the new stat but then you create a problem with defrost if memory serves.

T comes from the defrost board and energizes a heat/cool anticipator in the stat. I forget what happens if it is not connected. I remember if you put in a new defrost board without T the thermostat will not control correctly, temps way off. Can’t remember the particulars on the defrost side though so maybe that was just a dream.

Originally Posted by BNME8EZ

B is common, T is not used on the new stat but then you create a problem with defrost if memory serves.

T comes from the defrost board and energizes a heat/cool anticipator in the stat. I forget what happens if it is not connected. I remember if you put in a new defrost board without T the thermostat will not control correctly, temps way off. Can’t remember the particulars on the defrost side though so maybe that was just a dream.

I think that is the problem, the temperature sensor at defrost board will close and energize the heat strips, but the old one was mercury and it’s gone, anyone know how to get it up running again? The funny part was it’s worked right after I replaced the new thermostat though. Weird

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Originally Posted by amirgh

If I am not mistaking, You’re shorting out when the unit is going to heat mode. I dealt with this before. Find the common coming from the unit and fix it there.

B is the common that go to the a/h

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Originally Posted by ksefan

Did you replace the NEST or install it. If you installed a NEST, you need to replace it. If the customer supplied it, they pay to replace it.

Well, customer supplied the thermostat, it’s the cheap version nest, not the learning kind.

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Originally Posted by amirgh

So it works in cool mode with new thermostat , but it does not work in heat mode right?

Yes

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