Heat pump starts initially but will not come back on to maintain temps

Master of Cold · 10-13-2011, 08:36 PM

You can use just about any board on any unit, as long as you have the proper freeze thermostat.

johnniebravo · 10-13-2011, 08:41 PM

Thats the dilemma, the existing defrost thermostat looks nothing like the ones offered online. The ones i have seen look like they clip on to a part of the coiling while the one i have has an enclosure with a small nut holding it on. I suppose i could change it out to a different type but i am unsure where to place it. In the photos i posted earlier you can see my defrost stat.

Master of Cold · 10-13-2011, 08:46 PM

Place it next to the old one.

johnniebravo · 10-14-2011, 06:36 PM

How critical is it to match up the thermostat exactly? The number i pulled off the thermostat comes up on parts sites but it looks different from the one i have.

I was able to see, on the thermostat, 37TU31. I could see there was a HH carrier/bryant number but couldn't read it.

Is there anything that would prevent me from moving to an ICP defrost thermostat?
http://www.hvacpartsoutlet.com/Defro...ostat-ICP.aspx

Lastly, I removed the stat and placed it in a bowl of ice water for 5 minutes and it tested ok using my multimeter. So, I don't believe i NEED one at the moment but i may get one just in case as a spare.

johnniebravo · 10-16-2011, 08:48 AM

Had the opportunity to run the heat pump last night. Turned it on at about 7:30pm and it ran fine and shut off when target temp was reached. About 90 minutes later it came back on as it was supposed to do. I though everything was ok. Woke up at about 3am this morning and it was back to its old ways, inside blower fan running and thermostat calling for heat but pump not running outside. Got up at 8am to re-check the outside unit and it was frosted over just like before (same as pics above).

While it was frozen the pump was still calling for heat (y, 26v), red was still at 26v, and defrost thermostat was beeping using ohms with tone on multimeter. I measured voltage between the contactor coil posts and it was at zero on both sets (they are side by side). I pressed down on the contactor and the compressor did run when it was down and immediately stopped when i let up; i did this twice. Well after pressing the contactor for a second it looked as if the ice started to melt off of the frozen part. Not sure.

I then went into the house to see if i switched over to AC if the compressor would start. I switched to AC, changed the temp, and after 3-5 minutes, the compressor and fan kick on. I let it run for a minute, turned it off and switched back to heat. After the short 3-5 minute delay the heat pump kicked back on and is heating as requested. EDIT: The heat lasted only a few minutes and the TVX area frosted over again and shut down the pump.

Also, when i switched over from heat to cool inside, i could hear what sounded like the reversing valve doing something.

It appears that the system is not defrosting just that small part of the outside coiling and the ice is causing it to shut down. Could the defrost thermostat be in an improper place causing this symptom? Or could the defrost thermostat be broken but appear to be ok?

Marty S. · 10-16-2011, 11:38 AM

The defrost sensor is just a switch that closes around 28 degrees and starts the timer on the defrost board. A pressure switch lock out would show 24V at Y but keep it from going to the contactor.
None of that explains why the thermostat or duel fuel control are not bringing on the aux heat when the heat pump can't keep up but it works on E-heat. Sounds like a fun one to work on in person but over the internet, not so much.

johnniebravo · 10-16-2011, 11:57 AM

Quote:

Originally Posted by Marty S.

The defrost sensor is just a switch that closes around 28 degrees and starts the timer on the defrost board. A pressure switch lock out would show 24V at Y but keep it from going to the contactor.
None of that explains why the thermostat or duel fuel control are not bringing on the aux heat when the heat pump can't keep up but it works on E-heat. Sounds like a fun one to work on in person but over the internet, not so much.

Maybe it is a pressure switch lockout, it seems that as soon as the frost appears the system shuts off and the coil connectors do not show 24v. Seems like I am running out of options with a new board and a working defrost thermostat.

johnniebravo · 10-19-2011, 06:57 AM

Had a local company send a tech out to look at the system. I explained to him all the checks I had already done and all of the maintenance items/replacement parts I had already added (capacitors, defrost board, contactor, indoor blower motor). He diagnosed the system as having a bad TXV valve at the outside unit. He said it was not opening and after the unit ran for 20ish minutes it would develop low pressure and the sensor would shut everything down until the pressure equalized. As it was explained my system has a TXV both outside and inside (cannot see the inside one) and the outside TXV is not used in cooling which is why the problem only manifests in heat mode.

I also spoke with another competing tech and he had the same diagnosis. Actually, over the phone he described my problem so well that if I didn't know better I would have thought he had been to my house when it was acting up. AND, get this, he explained that i needed to weigh the cost of a repair versus the savings....is it cheaper to just run gas versus spending a large sum of money that i may only recover after 5-6 years of running the heat pump. Overall I was very pleased with his perspective and advice. He is an independent contractor.

A big thanks to everyone here that gave ANY advice. I have a much much greater understanding of how my HVAC system works now. I pretty much had it narrowed down to the problem he diagnosed based on what you all suggested. I haven't scheduled the repair yet, but I will. The price is a bit steep at $800ish, but as I understand it is what the going rate is here to get the job done right (labor, TXV, drier, and system evac and recharge). If I had not replaced my own capacitors, contactors, and indoor blower motor I probably would have paid much more and probably would have been advised to get a new system.

Also, i asked about how it was wired and he mentioned that my e-heat would not kick in until outside temps reached about 40 degrees or less.

Just out of curiosity, I checked into the local HVACR program at my community college and it would only take 3 semesters/50ish credits to complete their degree program and get certified (shorter because i would transfer credits for all classes not HVACR related from my other degrees). I might look into it, my MGIB would pay for the classes and I am certain it would all pay for itself somewhere down line just repairing my own equipment.

I will post an update with more details for closure and searching once the repair is done and i have an itemized receipet.

johnniebravo · 11-22-2011, 08:48 PM

Had the system repaired last week, the txv was the problem. Evening else was ok, system is working great now....the best since I have lived in this house. Thanks for all the suggestions and help.