5 ms·

Also, over similar temperature differences, AC is more energy efficient than heating. With AC you only move the heat, so spending 1 unit of energy you can move

by sampo 8y ago

Also, over similar temperature differences, AC is more energy efficient than heating. With AC you only move the heat, so spending 1 unit of energy you can move 2 or 3 units of heat. More, if the system is highly optimized. With heating, you need to spend 1 unit of energy to create 1 unit of heat. (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.)

miketery 8y ago
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.
- adrianN 8y ago
  Heat pumps can move more than one unit of energy from A to B for every unit of energy they consume.
- timerol 8y ago
  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.
- LeifCarrotson 8y ago
  Yes, look up the Coefficient of Performance: https://en.wikipedia.org/wiki/Heat_pump#Performance_considerations 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.
  - pc86 8y ago
    > 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.
    - roland35 8y ago
      "Purely in the room" in this context means a closed system - in your house you are still dumping the excess heat outside your system
    - roganartu 8y ago
      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).
  - TylerE 8y ago
adrianN 8y ago
You can use heat pumps for heating as well.
fyfy18 8y ago
> 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/3767 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 :)
- gandalfian 8y ago
  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.
  - jabl 8y ago
    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.
avip 8y ago
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.
- kgwgk 8y ago
  You move the heat and some air becomes colder (inside) and some air becomes hotter (outside). Any sufficiently advanced technology is indistinguishable from magic.
  - avip 8y ago
    The key feature I've missed is that "moving heat" does not imply moving the hot air itself.
    - gwbas1c 8y ago
      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.

by sampo 8y ago

miketery 8y ago

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.

adrianN 8y ago
Heat pumps can move more than one unit of energy from A to B for every unit of energy they consume.
timerol 8y ago
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.
LeifCarrotson 8y ago
Yes, look up the Coefficient of Performance: https://en.wikipedia.org/wiki/Heat_pump#Performance_considerations 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.
- pc86 8y ago
  > 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.
  - roland35 8y ago
    "Purely in the room" in this context means a closed system - in your house you are still dumping the excess heat outside your system
  - roganartu 8y ago
    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).
- TylerE 8y ago

adrianN 8y ago

You can use heat pumps for heating as well.

fyfy18 8y ago

> 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/3767 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 :)

gandalfian 8y ago
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.
- jabl 8y ago
  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.

avip 8y ago

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.

kgwgk 8y ago
You move the heat and some air becomes colder (inside) and some air becomes hotter (outside). Any sufficiently advanced technology is indistinguishable from magic.
- avip 8y ago
  The key feature I've missed is that "moving heat" does not imply moving the hot air itself.
  - gwbas1c 8y ago
    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.