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My air handler was sometimes staying on after the unit outside

Customer Question
would shut off and sometimes...
My air handler was sometimes staying on after the unit outside would shut off and sometimes it would not come on at all when the unit started up. I could turn the breaker off for a couple hours and it would come back on and run fine for a couple weeks and then do the same thing. Now the air handler won't come on at all. Trane XE1000 heat pump. air handler is under house. Service tech said he couldn't advise me unless he was actually there when it was acting up. Any thoughts?
Submitted: 2 years ago.Category: HVAC
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Answered in 29 minutes by:
5/30/2015
HVAC Technician: Phil, Mechanical Engineer replied 2 years ago
Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6,877
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
Verified
Welcome!
The technician was absolutely correct... and honest. Some technicians would have resorted to selling parts until the problem was resolved.
The problem is most likely in the thermostat, or the printed circuit card inside the air handler that the thermostat wires end up being attached to.
I have developed a heat stress test that works about 70% of the time on the printed circuit cards.
Use a hair drier to slowly and gently warm the card to 120F (warm to the touch but not hot and not over 140F)... if you get any change in behavior that means the card is bad...
A micro crack has opened in the printed circuit at a solder joint, or inside its CPU in that case .... or a short circuit has developed as the card expanded when you heated it.
Constant expansion and contraction as these electronics heat and during their duty cycles over the years is what wears these cards out.
It is also a good idea to replug all of the connections in the unit, that burnishes any oxidized terminal connections and will sometimes solve the problem, especially at the printed circuit card and in any sensor molex connectors. Do that with the power off to the unit of course.
These cards are also damaged by Electrical Spikes caused by power failures and lightning. These high voltage spikes can literally explode the fine 'wire' connections inside the chip. There is also a shunt type device, often printed on to the card that allows a high voltage spike to arc across to ground harmlessly, some of these can wear out after a few times and allow the high voltage spike into the rest of the card and the CPU micro-circuits. The high end printed circuit cards use a large capacitor like device, or several in parallel, to capture and release such surges, these do not wear out.
We can go from there.
.
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Customer reply replied 2 years ago
Ok so if I replace the old thermostat with the new Honeywell T3210D how do I wire it?The old baystat has wires on the following terminals:R-red wire
T-black wire
o-orange wire
G-green wire
Y-Yellow wire
X2-blue wire
B-black wire
W-white wireThe terminals on the 3210 are as follows:E
Aux
Y
G
O
L
R
B
C
HVAC Technician: Phil, Mechanical Engineer replied 2 years ago
Hello again, replacing parts prior to diagnosis can end badly, especially prior to discussing it with the expert first... these thermostats especially mixing Honeywell and Trane parts can be complex and introduce new problems, so that you can end up with an unfathomable mess that can take hours to resolve, and of course that does not fit our ultra low price structure.
I will opt out, hopefully other of our experts here will want to get involved.
.
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Customer reply replied 2 years ago
You stated the problem was most likely in the thermostat and then you come out with this? Wow. If the other "experts" are as good as you I think I will pass.
HVAC Technician: gotoman for ac, HVAC Technician replied 2 years ago
gotoman for ac
gotoman for ac, HVAC Technician
Category: HVAC
Satisfied Customers: 5,256
Experience: 51 years of experience install design and repair of all makes and models of AC
Verified
Russ here. Welcome. I am here to help. Please do not rate until you are satisfied with answer. Always available for follow up questions.
On that age Trane unit the problem is going to be the fan relay. The fan relay has piggyback time delay board on it. This board goes bad and gives the same systems you have noted. . You can remove the time delay really and just use fan relay. Russ Trane Dealer
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Phil
Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6,877
6,877 Satisfied Customers
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial

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