[lupire]

Efficiency is a misleading word for heating, because the units don't match; they are both energy, but different "type". The denominator is fuel energy lost, but the numerator is thermal energy change within an area of interest. A heat pump moves thermal energy from outside, changing unimportant thermal energy into good thermal energy.

"Coefficient of performance" is a better term.

https://en.m.wikipedia.org/wiki/Coefficient_of_performance

[stormbrew]

This just seems like unnecessary hair splitting to me. Obviously the efficiency of something is subject to the important inputs and outputs involved.

If someone asks: "how efficient is this heat pump at heating my house?" And you start digressing about how that's the wrong question to ask you'll be giving them an impression opposite reality, which is for most people: it will use less electric energy than heat energy it puts into your house, almost all the time.

[hedora]

There are theoretical upper bounds on the COP though. As heat pumps get better, it may some day make sense to say things like "this heat pump is 95% efficient, so there is no point in replacing it".

[ketzo]

Huh, I've always heard the "heat pump efficiency > 100%" but never really understood what that meant. Thanks for the explanation.

So the ">100%" comes from the fact that you're spending less thermal energy than you are moving?

[dghughes]

No OP but yeah heat pumps are very efficient some >300% efficient since you're just "pumping heat" from one place to another not generating it.

Even electric heat alone is 100% efficient no incomplete combustion or degradation over time. Efficient bu much more expensive than just moving heat already in the air.

Ground-source are better for now since they are moving heat from a relatively consistent source the Earth. It's about 15C to 25C one meter down where the ground-source heat pump lines are run.

Generating vs moving heat I think is misunderstood by people or really more likely they just don't care. As long as the bill is low!

[based69]

If you could really move heat 300% more efficiently than using a resistive heater to generate heat, couldn’t you just move a bunch of heat to power a steam turbine and get a perpetual motion machine?

[pbhjpbhj]

It's not entirely clear what you mean. But, you say "if you could really ..." as if ground-source/air-source heat pumps don't do this exact thing; do you think they don't work?

Running a steam turbine by pushing water down a borehole to come into contact with a high-temp (>100°C) source has been done; it's not perpetual motion. Energy pumping the water is << energy output from the steam produced. It's solar energy that's being used, ultimately.

[dghughes]

No, first of all perpetual motion machines don't exist, will never exist.

The 300% efficiency means if you had 1 Watt you could use it to power a fan and coolant lines to move heat. Or use 1 Watt to generate heat and then once made to move that heat.

The moving of heat already existing in the air (or in the ground) is more efficient than generating the heat and then moving it.

[PaulDavisThe1st]

Slightly more precisely: you're using less energy (in the form of electricity used to run the heat pump) than you are moving into/out of the heated/cooled space.

[Gibbon1]

Yeah take the energy you pump from Reservoir A to Reservoir B divided by the energy requires to do that and that's COP. Which you can think of as an efficiency.

[thedougd]

Exactly.