# Is it normal for actual temperature to be below target temperature during cold weather?

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.

• Added an update. Jan 30, 2014 at 19:45
• Based upon some research, I am watching the unit for 90 minutes to determine if it is defrosting. Will post results. Jan 30, 2014 at 22:04
• My pump has been frosted for about 5 hours now. I did not see any signs of it defrosting. I wonder if that is the culprit. It's odd though, I didn't notice that earlier in the week when the pump had trouble keeping the temperature about 63-65. Jan 31, 2014 at 1:05

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.

This is more common than you would think.

A heat pump works by sucking heat out of the outside air, and releasing the heat into the air inside. Once the outside temperature drops below a certain point (usually 40 degrees, although some systems are rated for lower), a heat pump can no longer find enough heat outside to move inside. Your air handler will also have standard resistive heating coils.

Generally the heat pump will be sized to keep the house comfortable down to the coldest possible operating temperature. On the other hand, the heating coils are sized to maintain a certain temperature differential, which I think is 45 degrees for southern Florida. (Someone please correct me if I'm wrong.)

• OP is in North Florida :) In any case, the winter design temperature in Miami is 35F, Jacksonville is 25F. So the Delta T of 45 is about right anyway. The design temperatures are usually somewhat higher than record lows, so there will be times a properly sized system cannot keep up. Jan 30, 2014 at 21:30
• Does that mean the system starts losing efficiency or the ability to compensate for heat loss when the outside temperature drops below 45? Jan 30, 2014 at 22:06
• No, you've conflated two different things. First, as the temperature drops, a heat pump does lose a little bit of efficiency. But at a certain temperature, it stops being able to work at all. This is the 40 degree temperature I quoted you. Jan 30, 2014 at 23:04
• Second, heating and cooling systems provide temperature differential, not absolute temperatures. For example, I said that coils in units installed in southern Florida (I'm thinking Tampa Bay Area) are usually rated to provide 45 degrees of temperature differential. This means they can heat a house up to 45 degrees warmer than then outside. If you tho about it, it would be impossible to build something that could guarantee a specific temperature regardless of the outside temperature. Jan 30, 2014 at 23:06
• So if it is 20 outside, the best I could expect out of my pump is 65? I realize there are other factors, but all other things being equal. Jan 31, 2014 at 1:03