Is a heat pump just a bidirectional AC?

[Sermil]

So, I'm reading explainers on the new hotness (pun intended) which is heat pumps.

And... it just sounds like an AC system? You compress a substance so it gets hot, let it radiate until it reaches equilibrium with the environment that you want to make warm, and then expand the substance and let it absorb heat from the other environment which you want to make cool. Isn't that how an AC works too?

Is the only difference that heat pumps can work in either direction? They can basically "AC the outdoors" to make the house hotter in winter?

If so, how do they manage the reversal trick? They can't be mechanically compressing because sometimes you want the house side to be the compressed side, and sometimes you want the house side to be the expanded side.

The explainers I've seen are all so busy explaining the basic compression / expansion cycle that they don't explain what makes heat pumps different than AC units.

[gomipile]

They have a reversing valve so that the heat exchangers that act as the condenser and evaporator switch roles.

With newer heat pumps, especially air source heat pumps, there's some engineering magic to make them more efficient at lower outside temperatures.

Some are ground source, though. They use water to exchange heat with the ground below the frost line, so they don't ever have to deal with temperatures below freezing.

Also, a central heat pump installation will usually include resistive heating elements as emergency heat. These kick in if the outside temperature is too low or if something in the heat pump system fails.

Those are the main differences I can think of immediately.

[Gnoman]

This guy is good at explaining things.

[Ninja_Prawn]

Grumpy mechanical engineer voice Technically, it's the other way around. An 'Air Conditioning' unit is really a special type of heat pump. 'Heat pump' is the older and more generic term.

It should be noted that heat pumps designed for the domestic heating market may or may not be reversible, and you'll definitely pay more for a reversible unit.