I live in North Florida and the temperature seems to drop into the 30's maybe 10-15 days out of the year and the 20's maybe 2-5 times a year. We had a few days where the highs were in the 30's this past week. I noticed that my first floor seemed cooler and that the thermostat was showing 62-65 a few times while the target was 68. The second floor was fine 67/67. We were out of town the previous weekend and I had set the first floor to 55 because it was not very cold while we were gone. Actual temp on first flow was 63 when we got back. I'm not sure if it struggled the next few days to get to 68 because it was already down at 63 when extreme cold hit or if there is a bigger issue. I also notice a lot of condensation on the inside of our double pane windows. Does all of this sound normal? The first floor system is a Carrier from circa 1996/1997. I think it is a 10 SEER. I think the same is true upstairs, but it is a smaller unit. I believe both are heat pumps. Appreciate any insight.
Update: My wife just texted me a picture and the outside of our air compressor is completely frosted with ice. It was not like this earlier in the week AFAIK.
Best Answer
Your heating system is sized for your house and climate. At some outside temperature, it will reach a point where ANY SYSTEM, running all the time, cannot input enough heat to compensate for the heat exiting via the roof, walls and windows.
The rate of heat flow equation is analogous to Ohms law I = V/R Take I a rate of heat loss, V as the Temperature difference on either side of the wall and R as the Thermal Resistance of the wall. The greater the temperature difference, the faster you lose heat. When you lose heat, the rate slows down and you reach an equilibrium.
A heating system able to maintain 65 degrees in the coldest weather you experience is probably adequate, but barely so.
Keep in mind that the ground itself also gets cold. In prolonged cold snaps, such as we've had this winter, it is not surprising that you have a cold floor. The ground might even have frozen underneath it. This is not expected behaviour in Florida.
The fact that you're using heat pumps also comes into play. We don't use them very much up here in Canada, because our winter temperatures are too low for them to work efficiently. A heat pump is trying to suck the heat out of the outside air and dump it in the house (like a refrigerator running backwards). The colder the outside temperature, the less efficient this is.