heat pump vs resistive heater

[LarryFine]

Each time the temperature drops below 30°F?

Yes. When it's blowing cold air, it's below 30F and jumper time.

Red to green for the fan and red to yellow for the compressor.

Fortunately, the reversing valve sets to heat when de-energized.

 

[GoldDigger]

Regardless of how low the temperature drops, you are likely to get most of the waste heat from the motor into the conditioned room unless there is outside ductwork that is not sufficiently insulated. Mini-splits would not have that problem.

 

[kwired]

Yes. When it's blowing cold air, it's below 30F and jumper time.

Red to green for the fan and red to yellow for the compressor.

Fortunately, the reversing valve sets to heat when de-energized.

Even older heat pump should be able to function with reasonable efficiency down to at least 10F. Not sure why yours is locking out at 30. Can you adjust this lockout device or is it fixed setting? Next question might be is it opening at the correct temperature or is it malfunctioning and opening at 30?

 

[kwired]

Yes, I am sure they design these things to just self destruct under certain conditions. . I have heard older units would shut down at a certain temp, but newer units typically don't, they just don't put out full output.

They all have less output as outdoor temperature starts to drop.

Newer units just have been adapted to handle those conditions even more efficiently like with variable speed compressors and/or better methods of controlling refrigerant flow into the evaporator. Overall output is still lesser ability when it is trying to pull heat out of colder air but they have gained ground on getting what they can more efficiently from input energy perspective.

When heat demand is higher than the unit can deliver you are going to need aux heat if you want to keep up with the demand.

 

[gadfly56]

Regardless of how low the temperature drops, you are likely to get most of the waste heat from the motor into the conditioned room unless there is outside ductwork that is not sufficiently insulated. Mini-splits would not have that problem.

If it has any claim to high efficiency, that will be a pittance. The internal fan motors seem to be all under 100 watts. You'd be better off inviting a friend over to help heat the room by merely being there.

 

[electrofelon]

They all have less output as outdoor temperature starts to drop.

Newer units just have been adapted to handle those conditions even more efficiently like with variable speed compressors and/or better methods of controlling refrigerant flow into the evaporator. Overall output is still lesser ability when it is trying to pull heat out of colder air but they have gained ground on getting what they can more efficiently from input energy perspective.

When heat demand is higher than the unit can deliver you are going to need aux heat if you want to keep up with the demand.

Thanks for the 3rd grade physics lesson, I had no idea .

When they give an output rating, it is at a certain design temp. For the low temp mini splits I think it's around -5°, maybe -10, I haven't looked in a while. I am saying, from what I have seen and heard, they will still put out heat below those temps but less than nameplate. I am not sure if they put out more than nameplate at higher temperatures. Of course the COP changes with temperature but that is a separate issue.

 

[kwired]

Thanks for the 3rd grade physics lesson, I had no idea .

When they give an output rating, it is at a certain design temp. For the low temp mini splits I think it's around -5°, maybe -10, I haven't looked in a while. I am saying, from what I have seen and heard, they will still put out heat below those temps but less than nameplate. I am not sure if they put out more than nameplate at higher temperatures. Of course the COP changes with temperature but that is a separate issue.

IDK. I do know even a single speed compressor doesn't draw nameplate amps at -5F.

I do know they generally draw more amps when in operating in cooling mode than when in heating mode - I presume because they are transferring more heat- or "doing more work" simply because there is more heat available to be moved.

My water source heat pump draws less when heating than when cooling as a general rule, the water source is a pretty constant temperature either mode.

 

[winnie]

As the evaporation temperature drops, the evaporation pressure and vapor density drops. For a constant speed constant displacement compressor this means that the mass flow rate drops and compressor amps will drop.

I'm pretty sure that the nominal 'rating temperature' for heat pumps is something pretty useless like an outdoor temperature of 47F or the like.

Good manufacturers will provide tables of capacity vs temperature.

Interesting that these units are designed for cooling with heating as an addition.

I'd be curious what the design would look like if heating were the primary goal. Would the compressor for a heating only unit be inside the house? What about putting the heat pump evaporator coil in an attic space to take advantage of solar heat gain? Would lower boiling refrigerant be used?

Jon

 

[LarryFine]

Even older heat pump should be able to function with reasonable efficiency down to at least 10F. Not sure why yours is locking out at 30. Can you adjust this lockout device or is it fixed setting? Next question might be is it opening at the correct temperature or is it malfunctioning and opening at 30?

Since jumping at the T-stat base keeps it running, I have to blame it on my NEST T-stat.

 

[mtnelect]

Don't forget about the "Defrost" cycle. The cycle will run about 5 minutes, then you are back in operation.

 

[drcampbell]

... I'd be curious what the design would look like if heating were the primary goal. Would the compressor for a heating only unit be inside the house? What about putting the heat pump evaporator coil in an attic space to take advantage of solar heat gain? Would lower boiling refrigerant be used? ...

The compressor might or might not be indoors. The main path of heat transfer (in the hermetic compressors most-commonly used for residential HVAC) is the refrigerant flowing through the compressor case, not ambient air flowing over it. People concerned with noise will be reluctant to locate it indoors.

There'd be little point in putting the evaporator in the attic; there's negligible solar heat gain in wintertime, especially with snow on the roof.

"Lower boiling point" sorta misses the point of refrigeration. The boiling and condensing temperatures are dependent on pressure, which is how a refrigeration cycle works in the first place: compress the refrigerant to a high pressure so that it condenses and dissipates heat at a (relatively) high temperature, the reduce the pressure so that it evaporates and absorbs heat at a low temperature.
There are some refrigerants commonly known as "low-temperature" refrigerants, that offer higher pressures, and more-importantly, higher gas densities, but refrigerant selection is a compromise of a myriad of different characteristics, and the difference between "high" summer temperatures and "low" winter temperatures isn't enough to drive the selection by itself.

Ammonia is commonly used for large low-temperature refrigeration. Frozen-food processing plants, for example, where the evaporator temperature is -40º. It offers the best efficiency, which in large systems is important enough to offset its drawbacks. (toxic, noxious, flammable, explosive and aggressively corrosive to copper and its alloys) But you'd never tolerate that for residential work.
(and by "large", I'm referring to systems that are significantly bigger than hockey rinks, few of which use ammonia)

 

[kwired]

Since jumping at the T-stat base keeps it running, I have to blame it on my NEST T-stat.

I would hope there is a parameter that you can change there.

AFAIK the outdoor temp it uses isn't the temp at the location of the unit as there is no outdoor temp sensor installed but rather a temp it collects from some weather service on the web from either estimates or periodic reporting by some nearby source. Nearby meaning it could still be some point that is 50 to 100 miles away from some installations.