My AC works in both directions, in winter it moves more cold outside than the power it consumes. Not sure what the factor is exactly, but I think same as for cooling.

Thermodynamics unfortunately disagree. As your temperature deltas get smaller efficiency goes down.

"Thermodynamics" is singular :) As for the numbers, my AC's manual shows COP of 3.71 for heating and 3.13 for cooling.

So you are spot on, in winter temperature deltas are larger, and efficiency goes up.

> "Thermodynamics" is singular :)

> plural in form but singular or plural in construction

(https://www.merriam-webster.com/dictionary/thermodynamics)

I think American and British English treat words like this differently.

Those high COPs are probably for relatively small temperature deltas. Heat pumps get _less_ efficient when the temperature deltas are larger. See page 18 of the manual linked below for an example. As the temperature gets lower, the heating COP gets lower. The same should be the case with cooling (higher outdoor temperatures lead to lower COPs), but the data is not presented in the same way.

https://backend.daikincomfort.com/docs/default-source/produc...

You are saying that heat pumps get less efficient when deltas are larger, and the parent post says they get more efficient when deltas are larger. In a sense, you're both correct.

There are multiple relevant temperatures for a heat pump, and the pump is more efficient when some of those are higher and some lower. A heat pump has two heat exchangers, one on the inside of the building and one outside. Each of those heat exchangers has two temperatures: the refrigerant loop temperature at that point, and the ambient temperature (air for air source heat pumps, ground for ground source heat pumps). There's also a fifth relevant temperature that has indirect influence: the setpoint (the desired indoor ambient temperature).

Efficiency increases when the temperature delta between the refrigerant and ambient temperatures is higher (both indoor and outdoor). But those temperature deltas vary inversely with the delta between the indoor and outdoor ambient temperatures.

So, in summary:

- Heat pumps get less efficient when the temperature delta between indoor and outdoor temperature is higher.

- They get more efficient when the temperature delta between refrigerant and ambient temperature is higher.

The net effect of this is that heat pumps become less efficient as the temperature becomes hotter outside in the summer and colder outside in the winter.

Correct!

You can also think about it as far as actually moving heat. Cold is the absence of heat, and so when the air is colder, there is less heat moved for the same effort and you have to work harder -- less efficiently -- for the same amount of head to get moved.

I see, the previous commenter stated the opposite :). Anyway, both numbers are > 1.

> My furnace produces 0.9W of heat for every 1W of energy input.

I assume you mean that 10% of the energy immediately escapes your house?

Yes, last 10% goes up the chimney, so only 0.9W goes into the house.

There’s a reason I say living space, I.e. a space with people living in it.

A living space will naturally heat itself with zero furnaces or electric heaters. Because the living things inside it will always produce heat (at least until they cease to be living). On the other hand, you’ll have a hard time getting living things to cool any space they occupy.

> On the other hand my air conditioner moves 3.5W of heat outside for every 1W of energy input.

Heat pumps work both ways, and it’s still easier to heat a space with a heat pump than cool it. Sure your AC can move 3.5W of heat for 1W of energy input. But that means 1W of energy allows you to remove 3.5W of heat from a space. But if you used the heat pump to heat the space, you would get 4.5W of additional heat, because that 1W of energy used to power the heat pump becomes waste heat that can be trivially captured and used to heat the space.