Comments by "Terje Oseberg" (@terjeoseberg990) on "Technology Connections"
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@Redtooth75 , So basically you work on systems like this daily and don’t understand how they work? Well, mechanics don’t design cars or engine, yet they fix them all the time. I’m sure it’s exactly the same for HVAC technicians. The people who understand how their work are physicists and engineers, not HVAC repair technicians.
If you dump 1 kWh of electricity into any device, no matter what the device is, due to the conservation of energy, 100% of that 1 kWh will be converted to heat. The question is whether that heat is put inside or outside your home. In heater/heat pump mode, the heat pump would obviously be designed so that 100% of that energy would be put inside your home. The only question then, is how much heat the pump is able to pump from outside to inside. If that’s 0, then the heat pump becomes exactly as efficient as a resistive heater. If the heat pump is able to pump any heat at all, it becomes more efficient than a resistive heater. This is just a fact.
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@Redtooth75 , OK, I found this explanation on Quora. He says that if it’s extremely cold outside, the heat pump can start pumping heat in the wrong direction, and he explains how.
“If you had some horribly inappropriate refrigerant in a loop between the indoor and outdoor air handlers, then, yes, you could be moving enough from indoors to outdoors for the COP to be below 1 - not even releasing as much heat indoors as the electricity consumed.
Consider a loop of liquid water, being pumped around the loop. With minimal refrigerant effect through the expansion value, it would mostly absorb heat energy inside the house and release it to the outdoors in the outdoor heat exchanger. While the electrical energy used would mostly be heating the water, it would dump most of that heat outdoors, not indoors. In that case, the “heat removed” (from the outdoors) by the refrigeration loop is negative - heat is being added to the outdoors, so the equation still works, but COP refrigerator is negative and COP heat pump is therefore below 1.
Every heat pump has some lower limit below which it is less efficient than electric resistance heater. In the newest, most efficient units, that can be -15F / -26C, but at some point, it should switch to electric-resistance mode for a COP of 1. If it doesn’t, and operates as a heat pump at extremely low temperatures, its COP of would be less than 1.”
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@Redtooth75 , Read my original question, “Can a heat pump in heating mode ever pump heat in the wrong direction?” You obviously failed to answer this question, and I therefore had to find the answer myself. That answer is “yes”, and I just explained how this could happen. It happens when it’s too cold outside for the refrigerant to evaporate.
In my first reply to you I claimed, “A heat pump consuming 1 kWh of electricity will always produce exactly 1 kWh of heat, and then it either will or won’t pump heat from outside to inside, …” this is only incorrect because I didn’t have the answer to my initial question. Now that I have that answer, I understand that the heat pump can generate 1 kWh of heat while depositing that heat inside and transporting a portion of that heat outside since it’s too cold outside to evaporate the refrigerant.
The heat pump is still producing 1 kWh of heat, but it’s unfortunately sending some of that heat outside instead of inside. In the Quora answer he mentions the heat pump switching to “resistive heating mode”. I have no idea how this “resistive heating mode” works, but it certainly seems like a good feature to have in locations where it becomes extremely cold.
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