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I have a trane XL20i heatpump (2 ton). every winter when it

Customer Question
gets below 25 degrees F...
I have a trane XL20i heatpump (2 ton). every winter when it gets below 25 degrees F, I get an error code 79 on my Thermostat. I I shut power off and then on, it clears the code and runs for awhile then another 79. The colder the temp the more often I get error. My HVAC company is great and have checked pressures, coolant, etc. Trade has really not been any help. Trying to figure out what to do before next winter. Summer operation is fine.
JA: Just to clarify, do you think this is a larger HVAC problem, or something specific to the thermostat?
Customer: Good question, really don't know. The therm is a trane xl850
JA: Do you plan on doing the work yourself?
Customer: no
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the HVAC Technician about your situation and then connect you two.
Submitted: 1 year ago.Category: HVAC
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Answered in 32 minutes by:
7/14/2016
HVAC Technician: Phil, Mechanical Engineer replied 1 year ago
Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6,022
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
Verified

Hello, I have 55 years in the business, much of it with Trane equipment.. If Trane cannot help and your local qualified Trane contractor cannot help, I will not be able to solve the problem from here without heroic efforts and a lot of time spent... hours at the least.

Let me know if you would like me to work with you on this for a while with no promises or not... if you do want me to spend the time I would need a positive rating so that I do not end up spending hours without compensation,

OR... I can opt out. let me know, we can go from there. So far I think the problem is with the electronics, possibly a mismatch between components.

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Customer reply replied 1 year ago
Phil,Not sure what you mean by mismatch of components. I have two Xl20i systems. One 2 ton I described and one 3 ton for another level. Both installed in 2009 by same HVAC contractor who I trust. Both were brand new systems with new thermostats and indoor units replaced. 3 ton works fine. 2 ton works fine except the last two years when it gets to 25 degrees or lower, I begin to get error code 79. It gets worse as temp goes down. All the obvious pressures, freon, connections, etc have been checked and are fine. I am looking for next obvious component that is likely failing and have it replaced. Obviously can't reproduce easily since it is temperature related.
HVAC Technician: Phil, Mechanical Engineer replied 1 year ago

Components can be mismatched from the factory or clear back to the component level, in this case the printed circuit board....OR.. there can be defects from the factory that made the printed circuit card (mostly in china these days)...there are ways of simulating the outdoor 25F condition (use of a resistor to replace the thermistor sensor that is sending the 25F signal).. this will take me hours however. ..and with no guarantee at all from me.

Let me know if you like to insure that I do not spend those hours at my own expense and I will dive into this for you without a limit. Meantime I will opt out and we can see if anyone else here wants to take the chance.

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Phil
Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6,022
6,022 Satisfied Customers
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial

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