> Unless you heat with a heat pump. But even then, pumping heat for heating operates on less favourable temperature differences than pumping heat for AC

Modern split-system air conditioning units are just heat pumps, and often they are actually more efficient at heating than cooling. For example, Mitsubishi Electric units give out 4-5x the amount of heat vs amount of electricity put in. They will comfortably work at this efficiency down to around -15c (below that the efficiency reduces, but it's still higher than an electric radiator) so are suitable for most climates.

http://library.mitsubishielectric.co.uk/pdf/download_full/37...

I'm rather surprised they are't more common here in Northern Europe, where a lot of residential heating is powered by electricity. Compared to alternatives like storage heaters, they aren't that much more expensive. And you can use them in the summer for the occasional heat wave :)

Here in England we heat by pumping fairly hot water through metal radiators, heated by gas or oil. A heat pump produces only lukewarm water at a slow but efficient rate. It is really hard extract the heat out of lukewarm water into a room, often the typical UK room loses heat faster than you can get it out of the water. (Hot water heats the room much more easily). As such the early adopters of heat pumps have expensive items that barely heat their houses and when they try to get more heat out of them in a hurry they become very inefficient.

There is almost no domestic air conditioning industry. On the rare occasions it gets too hot we open a window and thank the Gods. Thus there are very few cheap and reliable domestic suppliers of air conditioners for heating. And the air heating ones are tarnished by our experience with the unsuitable water heating ones.

So we burn oil/gas or electric storage heaters... Achieving a 300% efficiency and actually being able to heat a poorly insulated UK room just sounds too good to be true.

In northern Europe where ground source heat pumps are increasingly popular, houses built with those typically heat via coils installed in the floors. As you say, with only lukewarm water radiators by the windows (the traditional solution for oil/wood/etc. heating) aren't enough.

Again it sounds great but I know people whose underfloor coils started to leak at which point the system is written off... Retrofitting anything to an existing solid floor is a nightmare as well.

Sorry to be so negative. But that`s why so few of us have heat pumps... Its a shame, the air to air ones sound great on paper, much better than a £600 storage heater but I just can`t bring myself to trust them.

What? Can we start with an example such as: 1) Take a room starting at 20C, how much energy to get the room to 25C? 2) Conversely take a room at 25C, how much energy to get the room to 20C?

Intuitively heating should be cheaper, since loses are heat. Whereas with cooling you'll have inefficiency (e.g. in the pump) in the form of heat, therefore more energy to remove that additional heat.

Yes, look up the Coefficient of Performance: https://en.wikipedia.org/wiki/Heat_pump#Performance_consider...

A typical AC can take 1 kW of input power and remove ~3kW of heat energy from the room. The outdoor part of this system is heated by 4 kW, because you put the high-power stuff outside, where it's unimportant to performance. You can't thermondynamically make an AC work if it's purely in the room.

With resistive electric heating, you put in 1 kW of heat and the room temperature increases by 1 kW of heat energy.

With a heat pump in heating mode, you can put the inefficiency to use, using a 1 kW input to extract 3 kW of energy from, say, the ground, and putting 4 kW of energy into the room.

Two main problems with the argument: Cooling, as you say, is typically 30->25. Heating is often 0-25, requiring 5 times more energy (though we should be doing this in Kelvin...) Second, heating is super easy to do thermodynamically by just burning dirt-cheap natural gas. For a Michigan anecdote, I paid about $150/mo to keep my house cool in July, and it's typically $100 to keep my house warm in January.

> You can't thermondynamically make an AC work if it's purely in the room.

If you do keep it in the same room, it's called a dehumidifier. :)

(And yes, that's why air put out by a dehumidifier is warmer.)

> You can't thermondynamically make an AC work if it's purely in the room.

Could an AC that created matter from energy work?

> Cooling, as you say, is typically 30->25. Heating is often 0-25, requiring 5 times more energy

This argument seems flawed. You don't cut the heat off every night and let things cool back to ambient. You maintain a stable temperature.

The air outside right now where I am is 0.7 degrees, I have my heat pump set to 21 degrees.

What the parent comment neglected to mention is the working temperature of the gas / fluid in the heat pump.

R410a boils at -48.5 degrees[1]. So based on the size of the system and some maths I couldn’t explain so I’ll hand wave away, there’s your answer. Sorry, that’s the best I can do, you’d have to read some Wikipedia entries to get a better understanding.

Modern heat pumps / AC can be up to 1:4.5 thermal efficiency, but it depends on the the temperature difference, the size of the system, and what you’ve got it set to.

Other places I’ve lived it gets to 48 degrees outside and you set the AC to 17 degreee because Australians are really bad at insulating their homes, so a 31 degree temperature difference, so in that sense the parent comment is flawed.

1. https://en.m.wikipedia.org/wiki/R-410A

The rate of heat transfer through walls and windows also depends on the temperature differential.

Sure, but modern insulation is pretty darn good.

Also the sun will be adding some amount of heat during the day, which helps in the cold but makes the situation even worse in the heat.

> You can't thermondynamically make an AC work if it's purely in the room.

We have two AC units in our home that are entirely inside, there is just a hose connecting the unit to the window and the hot air is blown through the hose.

"Purely in the room" in this context means a closed system - in your house you are still dumping the excess heat outside your system

By "purely in the room" I imagine they meant within a closed system. By having a hose connecting your units to the outside world your room is no longer a closed system (ignoring things like drafts and imperfect insulation etc that already prevented it from being considered closed).

The way modern heat pumps work, I can't answer your question yet. What's the outside temperature? Let's say that taking your room from 15C to 20C takes 1.0 energy unit. If it's 15C outside, then I can use a heat pump to gain a bunch of efficiency, say only using 0.2 units of electricity to heat up the inside by one unit. If it's 25C outside, I can do the same in reverse, but I do have to also cool the A/C. We'll say taking the room from 25C to 20C when it's 25C outside takes 0.25 units.

The problem happens when there is a big difference in outside temperature. All of the gains of heat pumps disappear. So cooling from 25C inside to 20C inside when it's 30C outside might take 0.5 units of energy, which is a pretty common case. Heaters generally have to deal with much bigger swings, though. It's not uncommon to heat from 15C inside to 20C inside while the outside is at -10C, which takes (asymptotically approaching) 1.0 energy unit.

So the inefficiency of the A/C producing heat does matter, but not as much as the difference between the desired temperature and the outside temperature. The rate of heat transfer of the house also depends on the difference in temperature. Keeping a house 10C cooler than ambient takes less energy than keeping a house 20C hotter than ambient.

Considering that temperatures on Earth vary from about -90C to 50C, and humans like to keep indoor temps around 20C, cooling is generally more efficient than heating.

Heat pumps can move more than one unit of energy from A to B for every unit of energy they consume.

You can use heat pumps for heating as well.

So you move the heat and then... magic cold air comes instead? We assume the ambient temp. is "hot", not just the system to be cooled.

You move the heat and some air becomes colder (inside) and some air becomes hotter (outside). Any sufficiently advanced technology is indistinguishable from magic.

The key feature I've missed is that "moving heat" does not imply moving the hot air itself.

When you compress a gas, it releases energy as heat. When the gas expands, it absorbs energy by getting cold.

This gas is the refrigerant that the article refers to. Basically, what air conditioners, refrigerators, heat pumps, ect, do, is compress and expand the gas in a closed loop.

The whole theory behind a heat pump is that you get more heat, per unit of input energy, than if you used the input energy directly for heat. The consequence is that you end up blowing cold air outside.