Thermostat for heatpump fails three times and counting

I have a heat pump on the roof. It is a Carrier from 1984. I have owned the house since 1993 and have replaced the contact breaker once and the condenser fan once. It is serviced annually. It has been super reliable regardless of age and therefor is somewhat of a freak. Location is Scottsdale AZ. Three weeks ago my tenant called to report the system was not blowing any air and the compressor was not turning on. Nothing, zip, nada. I called my tech of 18 years and he diagnosed a bad thermostat which makes sense. The system returned without any other requirements and ran like it always does without issue. Four or five days later the same symptoms return, the tech replaces the thermostat and the system returns without further issue. OK, maybe we got a bad thermostat, I guess it happens. Ten day pass since the replacement stat was replaced which brings us to today, Sunday at 6pm, and the tenant tells me we have the same symptoms again. Tomorrow the tech will be on the site to evaluate. Does anyone have any ideas should the stat be found out to be the culprit again? I mean, how unlikely is that! I cannot begin to imagine that the heat pump has anything to do with it. I am not a tech but I fully grasp the theory and mechanicals of how systems function. Thank you in advance.

 

I am writing to update yesterday's post. Today the quick remedy was to replace a 45amp fuse for the system. When systems go silent, the usual terrors include a capacitor, one or both main fuses, or the thermostat. I should have hopped up on that roof myself and put my meter on those fuses, but instead I paid $110 to have it troubleshot and fixed for now. Hope this helps someone. Peace.

 

Never seen a fuse reset it self. Have seen fuses still have power but voltage would go away with a load. 27 year old unit might have other problems.

 

FWIW, there are some devices that can auto reset and are fuse replacements. https://www.amazon.com/Maxi-Blade-C...elf-resetting+fuse&qid=1595566837&s=hi&sr=1-7 is an example...

 

Thermostat? They all work on 24vac except digital units that work on a battery for the electronics,but the 24v is still used for controls. New wifi uses 24v ac, no batteries. If your air handler is also on the roof and losing one leg, the 24v transformer probably had no power. What is strange is why did a new one work after the replacement? With a defective fuse (non open but with high resistance it would have cause a lot of internal heat) might have been low voltage therefore the transformer was barely putting out enough voltage for the thermostats to work. Very strange that it would toast thermostats.

The root cause for all your issues could be the compressor is going and/or the start and run caps are the wrong size. The compressor could be taking huge slugs of current to get started and it will cause a temporary voltage drop. After many cycles or so it's taking a toll on the electrical components. Sure new units are expensive but it probably cut the electric bill by a third and more comfort. Todays units need to be a minimum of SEER 14. SEER 16 is better for a little more. You old unit you be lucky if it is SEER 8 and that was when before the energy guides where in place. Check you electric company if they have any rebate programs.