Heat Pump Performance When Cold Outside ???
 

I need an expert out there to tell me if my heat pump is working properly. Outside temp is 27 degrees. Indoor ambient air temp (read from both thermostats) is 66 degrees. Thermostat is set to 68 degrees. Heat pump seems to run and run for long periods, in some cases hours on end with very little increase in temperature. In fact, vent temp reading with my laser thermometer is 69 degrees, no wonder it's not improving much.

If I bump the thermostat to anything 3 degrees or more above the current ambient air temp (in this case setting it to 69 degrees), a little "aux" symbol appears, which I presume to mean there is a toaster grid somewhere heating up (this of course means $$$$). When aux is indicated, the vent temps shoot up to 89 degrees within a minute or so. At this point, within 20 minutes the ambient air temp will rise about 1 degree, which then shuts the aux feature off, and the cycle continues with the fan blowing for the next hour or so until the temp again drops enough to invoke the aux feature.

Is this behavior normal? My rather crude understanding of a heat pump is that it "reverses" function in order to remove heat from the air outside the house and release this heat into the inside of the house via the furnace blower. Sort of like a reverse air conditioner. Am I right? If so, logic would dictate that efficiency would drop off sharply as the temperature outside drops, since there is very little heat to remove from the outside air. But this seems a little out of the ordinary. In fact, I don't remember it behaving this way last year. But then again, I don't think 27 degrees is normal for North Texas (Chicago transplant, still figuring out the weather here).

Does this type of operation sound normal, or should I schedule an appointment with an HVAC guy? Are there some simple things I can check myself before doing so?

Thanks in advance!

By the way, we are using those oil-filled radiator type electric heaters on a low setting in the bedroom overnight. They are great for taking the chill out of the room. But I have no idea what they are doing to my electric bill. In fact, they worked so well last night that I tried shutting off the central heat pump, since it seemed to be running and running with no end in sight and very little heat output. Big mistake - it was 58 degrees this morning when we all got out of our rooms. But I worry that the heat pump might run all night and just incur some ungodly electric bill!

Any thoughts?

It's an interesting topic, wish I had good answers for you.

I've been looking at and thinking about absorption cooling system for a while now, that's the process that propane refrigerators use. Getting your head around the thermodynamics of it is surprisingly tricky.

I'm guessing that a heat pump in the cycle you're talking about sends coolant out at say, 0 degrees F and has it returned at 27 F. If the pressure in the system is right which by design, it would be, at that temp. some condensation would take place, which would (should) give off heat.

I can't quite get my head around it. I can figure it the other direction, but in reverse it clouds up.

Howstuffworks claims that in climates that spend time below freezing, a heat pump is not necessarily more efficient than other types:

http://home.howstuffworks.com/how-to-troubleshoot-a-heat-pump.htm

Make sure it does a defrost cycle every now and then. In certain conditions, the coils outside can get iced up - there is a cycle that is supposed to melt that stuff off. Good airflow is important too, so it can suck every last bit of heat out of the surrounding area.

But..once it gets that cold, it's struggles to give you heat. The heatstrips ought to come on, once the pump can't hack it. (besides when you turn up the thermostat past 2-3 degrees above current room temp). Having the heatstrips do their thing for 20 mins. might use up around the same amount of electricity as the pump running for an hour and 20 mins.

It is 39 here right now, and it heated my house up a degree (70 ->71) in about 15 mins.

I used to know the temperature below which an air to air heat pump is no longer effective. It may be 32 degrees faranheit. After that you need to use the toaster coils to get heat. Yes, they are expensive to operate. The space heaters you have are the same effeciency as the strip heaters but have the advantage that you can just warm the areas where you actually are....so they will consume less energy than heating the entire house to the comfort zone.

Good luck....at least in Texas it is not likely to last long.

Tom W

The efficiency of a heat-pump ends below exterior ambient temperatures at or below 32'. Your interior furnace's infra-red coils do the heating below that point. Above 32' they're excellent. Your latitude is not bad as a heat-pump heating source. Further south is even better.

It sounds like your aux heat isn't kicking in automatically when the outside temperatre drops very low. I don't think most heat pumps are designed to operate effectively when ambient temps are at, or below, freezing.
20 years ago, my foks installed a 'dual-fuel' heatpump system in their Virginia Beach home, which uses an oil furnace as the aux heat, instead of a resistance heater. When the outside temp drops to about 36 F, the system automatically switches to oil heat. Also, if it's above about 36 F, where it would normally be running on the heatpump, but one wants to boost the inside temp a few degrees by nudging the thermostat, the system switches to the oil heat temporarily to quckly raise the temperature. (Sounds like this part of your system is still working, except for the difference in aux systems) Once the higher setting is reached the heatpump takes over until the outside temp drops below about 36 F.
There's also an 'emergency heat' switch on our thermostat. If the heatpump fails, it allows heating with just the oil furnace.
Sounds like possibly a temp sensor/circuit in your outside compressior-condenser unit isn't working. If you have an aux heat setting on your themostat, you may have to monitor the outside temp and switch it on accordingly until someone can check out the heatpump.

Happy Motoring, Mark

A good wiki link on Heat Pump operations Clicky

The electric auxiliary heater in your heat pump has a Coefficient Of Performance of 1.0. Thus, as continues below...

So, as the wiki explains, your heat pump will have to run harder and longer to produce heat at 32 °F outdoor temperature than at 42 °F, but it may still be performing better than the resistance heating.

The manufacturer should have all of this built into the temperature sensors, and it should bring on the 'toaster' only when needed.

The only way to tell would be to put a kilowatt-hour meter on the unit, and compare hourly consumption with a steady outdoor temperature.

Heat pumps suck! The point at which they no longer warm the air is when it gets cold . . .

Water to air heat pumps do a lot better. Instead of trying to take heat from the air which is at 27 f they will be taking heat from the ground which if deep enough will be as warm as 50 f.

The most effecient type is the open loop type which takes water from a well and discharges it into a stream or lake, or into a second well so that it does not deplete the water table.

It sounds as if the OP's heat pump is working normally.

Tom W

Except that, before the outside temp drops below freezing, OP's system should have automatically switched over to the resistance (aux) heating.

Happy Motoring, Mark

I seem to remember him saying that he was deliberately setting the stat so that it never kicked on. See his middle paragraph.

Tom W

Doesn't matter where he sets the thermostat. So long as he requires heat, there's a separate temp sensor circuit on the outside unit. Before the ambient outside temp drops to freezing, the system should have automatically switched to the aux/resistance heating. The heatpump shouldn't come back on until the ambient temp is somewhere above freezing. In the case of my folks heatpump, that's around 36 F.

Happy Motoring, Mark

I thought the Aux only came on if the ambient temp excedes the setpoint by +/- 2 or 3 degrees. Where the heat pump should come on at +/- 1 degree.

Anyway, get an electric blanket and shut the heat off. Or like I do set it at around 62 - 65 degrees. I have one of those oil filled heaters and they work great. Using electric heat it takes the same amount of BTU's to heat a room if your using a radiator or forced air.
Just heating the bedroom with electric heat is less costly that heating the whole house by any means.

Danny

AS i stated before, there is a situation that triggers the switch to aux heat, so long as the OUTSIDE ambient temp is warm enough to allow efficient heatpump operation. Lets say you want a significant increase in temp, so you bump the thermostat setting from 64 to 68. The aux heat should kick in, as it will warm up the house much faster than the heatpump can. Once the new INSIDE temp setting has been reached, the heatpump should take over to maintain the new setting, at least until the OUTSIDE abient temp drops near freezing. Then a separate circuit switches the system back over to aux heat, usually a resistanc heater, because the air to air heatpump doesn't work effectively in freezing temps. As long as outside temps are at or below freezing, the aux heat should stay on. Otherwise the heatpump would be running constantly and barely heat the house. That's why heatpumps haven't been popular in colder climates.

Happy Motoring Mark