I am in Florida with a 4 year old heat pump system. The Hunter programmable thermostat is set for 80 degrees 24/7. The air cycles on and off periodically as usual and the air temp coming through the registers is cold. The problem is that as the heat of the day increases, the preset temp is not maintained. It can read (and be) 88 degrees in the room during the afternoon hours while the preset temperature on the thermostat is still set for 80. At night, we are able to maintain the 80 degrees. I know from this forum that Hunter thermostats are junk, but how can I be sure that replacing it will be the solution before calling for service? Any info will be much appreciated.

P.S. My husband insists this started after a bad thunderstorm.

Hubby could be right about that. When I lived in Florida I went through 3 central a/c's due to bad storms and the unit shorting out during a power surge during the storm. I also had numerous VCR's get fried as well. Best to listen to hubby as I think he's right about this one. Get a new thermostat and this should solve your problems.

Since the thermostat works during the night and not well during the day I do not believe it is a thermostat problem.

During the night it is cooler with NO direct sun. During the day you have higher temperatures and the direct heat of the sun to deal with.

I believe your system is more at fault then the thermostat.

To start with you keep the A/C set on 80. That is great but it leaves very little for comfort swing during the day with what was mentioned above.

To test the theory set the temp at 75 during the night to pull the house down and leave it set for 75 during the day also. Lets see if the differential of the two settings is the same or different. You seem to have a 8 degree differential so lets see if it stays the same.
Post back with findings.

YES Hunter t-stats are junk but usually it will not cause the problem you are having. So do not replace the thermostat yet.

How do you remove the heat from your attic? If its with power vents make sure they are working. Heat build up in your attic will make the A/C work over time in the heat of the day. Check your condenser coil on the A/C unit to see if its dirty. A dirty coil = high head pressure and in the heat of the day = not cooling. Good luck.

Thanks to everyone for their responses. After reading them, I first rinsed the condenser coils from the inside out with a garden hose, but I honestly didn't see that they were very dirty.

The other thing I did was turn the thermostat down to 75 as was suggested. That afternoon was very cloudy and rainy, but the temp still never got below 82. Overnight, it got down to 76. The next 2 days, again the temp only got down to 82 and today 85 when it was sunnier.

Regarding heat in the attic. That has been a concern of mine. I have a hip roof and no power vents. Only a ridge vent and intake vents. I investigated the idea of a power vent this spring, but never followed through. I think you might be right about needing one. I can feel hot air coming down from the soffit vents when I stand next to the house.

The only question that leaves me with is why haven't I had this problem with my AC in the preceding 4 summers since the AC was installed if it's inadequate attic ventilation?

Please reply with whether you think I need the power vent in the long run. And if in the short run you think it's the thermostat or if I should call the AC repairman.

Thanks again to everyone.
Nina in Florida

The only question that leaves me with is why haven't I had this problem with my AC in the preceding 4 summers since the AC was installed if it's inadequate attic ventilation

A good attic vent power vent never hurts if installed correctly.

At this point I would call for ON SITE professional service.

If it worked for 4 years it should still work now unless some problem developed.

I wanted to give a follow up. I called some A/C guys. One thought it was the condenser fan overheating. But when they finally came out, it turned out to be the automatic shut off that is triggered when the condensation drain is clogged. They vacuumed the water out with a shop vac and put some type of dissolver in the line. I know now that I need to put a little bleach and water in there periodically. It's a little annoying that it was something I could have fixed myself. But I'm overall glad that it didn't cost an arm and a leg. Thanks again...

Good deal glad you got it running again.