heat pump vs resistive heater

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When the COP (coefficient of performance) of the heat pump is 1. The manual may have a graph of COP vs temp. Id probably switch to pure resistance at a COP of 1.3. You also need an air handler that will slow down as the heat output falls. Otherwise, having 80F air blowing out the ducts doesnt feel very comfortable.
 
The answer depends on the specific heat pump being considered and what sort of cost comparison you are running.

At the simplest level the equivalent point occurs when the COP of the heat pump drops to 1. But many heat pumps simply stop being able to function at all before that point. Once the heat pump is below its allowed operating temperature, then you simply cannot use it.

For a minisplit that I just installed, its lowest rated operating temperature is -5F. At this point its rated COP is 2.1. This means that it is more efficient to use the heat pump down to -5F, but below this temperature it cannot be used at all.

-Jon
 
thomasjohnschulte said:
At what temperature Farenheit wiil the two different electical methods become cost equal -- if ever?
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Is this a textbook question or reality question?

In reality:
If by cost you mean only the amount of electricity used, they will never be equal as long as the heat pump can function. See Jon's post. The heat pump will always use less electricity until it can't be used anymore. So the answer to the question, more or less, is when the temperature Farenheit is below the heat pump's specified lowest temp.

If you mean the total cost to install and use, say for new construction, that would require a economic model with inputs of cost, performance, and forecast usage and temperatures. You could spend a whole career developing that, and it would still provide only best guesses.
 
jaggedben said:
Is this a textbook question or reality question?

In reality:
If by cost you mean only the amount of electricity used, they will never be equal as long as the heat pump can function. See Jon's post. The heat pump will always use less electricity until it can't be used anymore. So the answer to the question, more or less, is when the temperature Farenheit is below the heat pump's specified lowest temp.

If you mean the total cost to install and use, say for new construction, that would require a economic model with inputs of cost, performance, and forecast usage and temperatures. You could spend a whole career developing that, and it would still provide only best guesses.
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For that consideration, you should model heat pump versus gas heat if you have access to reasonably priced gas. A 95% efficient condensing furnace will beat a heat pump for equivalent BTU output if the COP drops much below 3, so your climate zone matters.
 
I recently was told that there is an "inverter heat pump" (the term used by the electrical contractor who told me about it) that can operate at much lower ambient temperatures than heat pumps that do not use that tech. And that is 100% of what I know about them.
 
ggunn said:
I recently was told that there is an "inverter heat pump" (the term used by the electrical contractor who told me about it) that can operate at much lower ambient temperatures than heat pumps that do not use that tech. And that is 100% of what I know about them.
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I took a quick peak at Mitsubishi's web site, and the concept is VFD for heat pumps. Instead of bang-bang control, it's PID so the equipment is running all the time.
 
Inverter heat pumps are _very_ common for minisplits. I just installed 4 of them.

I don't know if the inverter is enabling for low temperature operation, but as noted above the units have a rated COP of 2 at -5F outdoor temperature.

What I don't like is that you are forced to use the PID loops built into the system, and the temperature sensor is attached to the indoor unit. I'd really like to be able to command the unit 'operate at this % power' and let my home automation system adjust the call for heat as needed.

-Jon
 
ggunn said:
I recently was told that there is an "inverter heat pump" (the term used by the electrical contractor who told me about it) that can operate at much lower ambient temperatures than heat pumps that do not use that tech. And that is 100% of what I know about them.
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"Inverter heat pumps" have been available for decades in the higher-end market segments.

They may be able to operate at much lower ambient temperatures, but it will be a result of their overall system design, (including lubricant selection) not simply because they contain VFDs. Read the specs for each individual unit.

They are generally more efficient than the more-primitive on-off machines. By running only at the capacity needed, heat transfer can be more effective, lowering the compression ratio and reducing the work required to deliver (or remove) each unit of heat.

An added benefit is summertime humidity control. After installing variable-speed control on the compressor and fans, you can run the machine at precisely the sensible-heat/total-heat ratio that delivers the desired humidity. With on-off control, humidity control is possible only with added hardware such as bypass dampers or reheat coils.

"Sensible heat": That which a thermometer can sense
"Total heat": Includes the latent heat of water vapor, which doesn't show up as temperature
 
electrofelon said:
I think many/most/all mini splits don't shut down when their "lowest" outdoor temp is reached, rather they will no longer produce their full nameplate heat output
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That seems like an opportunity for something vital to go "sproing!"
 
electrofelon said:
I don't see why, there's just a bit less heat in the air to suck out. I mean we all know you hate anything that doesn't run on gas 😉
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Do you understand how HVAC compressors work? If there isn't sufficient heat to vaporize the refrigerant on the expansion side, you run the risk of slugging the compressor with liquid refrigerant. Any liquid is incompressible at HVAC pressures, and you'll bust the compressor, or if you're lucky pop some overpressure device which will still shut down the unit.
 
'Inverter' minisplit systems usually have suction accumulators before the compressor to prevent liquid slugging. This is because the units often operate at near 0 superheat for efficiency. ('Superheat' is the measure of refrigerant vapor temperature - boiling temperature at the local system pressure. A superheat of 0 means that liquid is possible in equilibrium.)

For my minisplits I can't find anything in the manual that says they have a low temperature cut-off. They published data simply provides rated capacity down to -5F.

According to post 22 on this thread at GBA the similar 'hyper heat' units are rated down to -15F but keep running below this. https://www.greenbuildingadvisor.co...e-reports-from-minisplit-owners#comment-47975

Relevant to the OP, my unit is nominally a 1 ton system (12000 BTU per hour). At 65F inside, 32F outside its maximum capacity is 16200 BTU/hr with 1.82 kW electrical input. (COP 2.6). At 65F inside, -5 outside the numbers change to 13800 BTU/hr and 2.0 kW (COP 2). Not specified anywhere that I can find is the COP at partial load; all the tables provide information for the unit running full tilt.

-Jon
 
gadfly56 said:
Do you understand how HVAC compressors work? If there isn't sufficient heat to vaporize the refrigerant on the expansion side, you run the risk of slugging the compressor with liquid refrigerant. Any liquid is incompressible at HVAC pressures, and you'll bust the compressor, or if you're lucky pop some overpressure device which will still shut down the unit.
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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.
 
My heat pump turns itself off around 30F, so I remove the T-stat and jumper from red to yellow.
 
gadfly56 said:
Do you understand how HVAC compressors work? If there isn't sufficient heat to vaporize the refrigerant on the expansion side, you run the risk of slugging the compressor with liquid refrigerant. Any liquid is incompressible at HVAC pressures, and you'll bust the compressor, or if you're lucky pop some overpressure device which will still shut down the unit.
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It's not necessary to fill the compressor with liquid to damage it, or to hit it with a large slug of liquid.

Refrigerant passes through the narrowest part of the intake valve at Mach 1. Even a small amount of small liquid droplets will behave like high-speed schrapnel. This is not a subtle effect; compressors begin to sound like ball bearings in a tin can.

Suction-side accumulators do exist, but like everything else, have their imitations. One of the reasons for a minimum operating temperature specification might be the temperature at which the accumulator becomes overwhelmed and allows liquid to pass through.

(a more-likely reason is the lubricating oil becoming too viscous to circulate through the system effectively -- compressor longevity falls like a piano from an airplane when all the lubricating oil pools up in the bottom of the accumulator)

LarryFine said:
My heat pump turns itself off around 30F, so I remove the T-stat and jumper from red to yellow.
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Each time the temperature drops below 30°F?
 
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