Emergency Heat vs heat pump in very cold weather

[Mikey]

I'm renting a small apartment for a couple of months with an AC/heatpump problem. I't a relatively new Carrier system. When it's cold, say 50° or less, in normal mode the compressor runs all the time, and the coils seem to be frozen most of the time I look. The incoming air feels cool, and it's all the system can do to maintain the inside temperature in the mid 60s. If I switch the system to EH (Emergency Heat) mode, the compressor never runs, inside temps are within a couple of degrees of the set point, and the system cycles on and off as I would expect. I suspect that it's costing less in EH mode as well, since the compressor is off. Is this symptomatic of a common problem I could advise the landlord to have fixed, or is it a basic capacity issue? My home system (also recent Carrier, but a lot bigger) doesn't have the problem.

[nukeman]

The system may just need to be charged. The other possibility would be that it was way undersized, but the temperatures in your area are mild shouldn't be low enough where the performance of the HP really drops off. He should have an HVAC guy come out and take a look at it. If it is low on Freon, it also won't cool properly when summer comes around. During summer, you would see the indoor coil ice up instead of the outdoor one.

For cost, EH will always cost more. If it was very cold (say below 0), then you may want to just run on EH to save the compressor some work. Some newer models still produce heat (from the compressor) about half the cost of resistance heat at these low temperatures. However, the capacity also drops dramatically, so a bigger fraction of the heat comes from the resistance heaters as the outside temperature falls. In central FL, you should not have to worry about this.

[jadnashua]

A heat pump, working correctly, can usually maintain until the outside temp approaches 40-degrees. If yours can't keep up at 50 and is icing over, I think there's a problem. The heat pump give you a multiplier efffect - IOW, it can move heat easier than it can be generated by the emergency backup, so running the compressor would be less expensive than the emergency heat. Problem is, when the outside air gets around that 40-degree mark, there isn't enough heat in it to be cheaper to run - there's just less heat in the air to move. Air-source heat pumps just aren't much good below that point. A ground source heat pump normally has much more heat available, but installation generally is more complex (often, lots of tubing burried in the ground or multiple wells, or a nearby lake or pond). You may not be happy when you see the electric bill!

[Mikey]

Thanks, guys. I'll suggest the landlord have the system checked; his dealer may offer a seasonal checkup at reasonable cost. I know they've mentioned outrageous bills during the cooling season; maybe this severe-cold winter will make them equally unhappy.

[Lightwave]

When it's cold, say 50° or less, in normal mode the compressor runs all the time, and the coils seem to be frozen most of the time I look. The incoming air feels cool, and it's all the system can do to maintain the inside temperature in the mid 60s. If I switch the system to EH (Emergency Heat) mode, the compressor never runs, inside temps are within a couple of degrees of the set point, and the system cycles on and off as I would expect.

At minimum, something's wrong with the system control logic. The auxiliary heat source (whatever turns on when you enable EH) should come on automatically when outdoor temperatures drop far enough that the heat pump can't maintain the indoor temperature on its own.

The temperature at which the heatpump gives way to auxiliary heat depends on how the heat pump is sized relative to the heat loss of the heated space. Switching over at 50F/10C is not necessarily indicative of a problem--particularly in an air conditioning dominated climate--but auxiliary heat should automatically be used once the design balance point has been reached.

There should never be solid ice on a heat pump. Frost, yes, but solid ice on the coils indicates a defrost problem.

In short, your heat pump needs a service visit to fix a problem with auxiliary heat activation and to look for possible problems with defrost control and possibly other issues with the compressor.

[SteveW]

Where I live, our local power company mandates that HVAC techs set the balance point of newly installed heat pumps at 20 degrees F. Mine has no trouble extracting enough heat from outside air at that temperature.

To us in the Midwest, it's funny to hear temps of 50 being described as "cold."