Heat loss through a surface is proportional to the difference in temperatures from one side to the other.

So if you are cooling a house to 70 degrees in 90 degree weather, you are fighting a 20 degree difference.

If you are heating a house to 70 degrees in 30 degree weather, you are fighting a 40 degree difference.

In a severe heatwave, you might spend weeks with much of the day at 100 degrees; during a cold snap last winter, there was a span where I live where the temperature was usually around 0, and never higher than 10 degrees.

Meanwhile, a furnace works by directly converting energy to heat; for simplicity, let's assume a heater generates 1 unit of heat for 1 unit of energy (it's 100% efficient).

An air-conditioner, however, moves heat, moving heat from inside to outside. An air conditioner doesn't use 1 unit of energy to move 1 unit of heat; an air conditioner might move 3 units of heat using 1 unit of energy.

So putting the two together, it takes roughly 6 times as much energy to heat my house on the coldest days as it does to cool my house on the hottest days.

The full answer is a lot more complicated than that, because other things in houses are generating waste heat and you need to look at the deviations above and below your target temperature range over the full year and across many different places. Someone else has already posted a link to US energy usage showing that total energy spent on air conditioning is a fraction of that spent on heating, but from the above you should be able to get an intuitive feel for why that's so.

> an air conditioner might move 3 units of heat using 1 unit of energy.

So run that system backwards for heating, and you would at only use twice the energy for heating. That said, you can typically cut your heating bill in half using ASHP, with it losing efficiency as the outside temperature decreases. It rarely goes below a COP of 1, so it makes sense to use in place of resistance heating.

However that assumes that you must use electricity for heating. Using wood for heating, you are using a renewable resource that is cheaper for every unit of heat you get.

The downside of electric heat pumps is that they simply don't work when it gets too cold outside. They can't push the thermal gradient hard enough and end up switching to inefficient resistive heat. Go far enough north and even the resistive heat doesn't work very well and everybody uses gas.

When the correct gas is used, they are more efficient than resistive heating down to -20-30 C. Very few places in the world get that cold. That said, and as I mentioned, using wood or district heating, is far more common in those parts of the world. At least in Europe.

Very few places, like all of Canada, parts of the northern US, Alaska, most of Russia, etc...

Even then you have to be willing to keep the thermostat set at 55F/13C or the resistive heaters are going to kick in.

I'm unsure if you know how cold -20C actually is. At -20C outside you can still get an efficiency of 2.0. The majority of Canada does not reach that temperature outside of a few cold spells. Only two handful of cities in Canada have nights with less than -30C. In other words, the majority of time it will be more efficient than resistive heating. The north or interior of Alaska is typical mainland climate and very cold, and same goes for Russia. In both countries a non trivial part are not that cold. The majority of the world are simply not so cold that resistive heating is better than an ASHP system.

That is not to say gas isn't cheaper.

> An air-conditioner, however, moves heat, moving heat from inside to outside. An air conditioner doesn't use 1 unit of energy to move 1 unit of heat; an air conditioner might move 3 units of heat using 1 unit of energy.

Be careful, you're angering the thermo gods! If that was true, I would simply use an air-conditioner to heat my house. Cool, the outside and move the heat inside. In reality, you have the efficiencies backwards. Cooling is more energy intensive than heating.

> If that was true, I would simply use an air-conditioner to heat my house.

It's true, and people do. Install a heat pump, and save your traditional heat source for when it's sufficiently cold outside.

Heating using resistance is inefficient and gets you at most 1 unit of heat energy per unit of electrical energy. Heating and cooling using heat pumps can move more than one unit of heat energy per unit of electrical energy.

Heating via electrical resistance is 100% efficient. All energy goes into generating heat. There is no loss.

Gas heating is something like 90% efficient now.

People, stop. Downvoting doesn't change facts.

> Heating via electrical resistance is 100% efficient.

No, this is quite wrong. You're not taking into account generation and transmission losses. Gas heat is actually much more efficient than electrical resistance heating.

Plus, as others are saying, using energy to move heat from one place to another allows you to move more heat than you could generate just by converting the energy to heat.

Your pedantry only works in a closed loop where all externalities are accounted for.

There is no real world scenario in a place that has a beating season where the cost to the consumer is better with electricity versus gas or oil. Electric costs about 4x for whole house heating in my neck of the woods, and houses quote electric only heat sell at a discount.

My guess is that your evaluation of electric externalities doesn’t include the health impacts of burning coal, ecosystem damage due to hydroelectric or long term storage and other capital costs incurred by nuclear generation.

> There is no real world scenario in a place that has a heating season where the cost to the consumer is better with electricity versus gas or oil. Electric costs about 4x for whole house heating...

I guess I live an unreal life. All of my heating and cooling is electric. My bills are lower than anyone I've compared with and in every case I'm heating and cooling more square feet. The magic? Ground source heat pump. Several people tried to talk me out of it and I'm glad I didn't listen.

That’s awesome — it tends to be a capital cost that people cannot afford to retrofit.

In my area, you need to drill deep wells for heat pumps to work. Our local branch library did it and it cost about $40k.

That's great — really, it is; if I ever get to build a custom house, I want to use a ground source heat pump — but we were specifically arguing over electric resistance heating.

I guess I missed that. Yes, electric resistance heating is horrible on the wallet.

I have electric resistance as a backup to my heat pump, only because they wouldn't install it without it. It would cost so much to run I have the breakers turned off just so it can't be shocked by my bill.

Yes, 100% is inefficient because you can get higher ratios if you use heat pumps. The loss is that you are using more energy compared to how much energy you would use if you had a heat pump instead.

That's not efficiency. That's an efficiency ratio, which is different.

Heat pumps have a higher EER, but also higher parasitic losses, which is the source of confusion on the whole thread.

It's like claiming that rockets are "more efficient" than cars because they move you faster.

The reason you are being downvoted is because your argument basically boils down to having an unreasonably narrow definition of what “efficient” means. In context, it’s obvious that “efficiency” here is the ratio between the amount of heat provided and the amount of energy spent. You might call it EER, I might call it COP, but it’s irrelevant because in context the word “efficient” is unambiguous.

Narrow interpretations of definitions to support a point don’t actually provide much useful insight to the topic being discussed, and are seen as distracting. So these non-useful comments are being downvoted to make them less visible. It is not an issue of whether they are technically correct.

You could probably make an interesting point about the electrical efficiency of heat pumps, or you could make a comment about terminology and explain that “technically” the correct term is X, but instead you chose to interpret a term in a way that contradicts the contextually obvious meaning and then base an argument on top of that.

My account is rate limited. Responding here, then going away.

"arrow interpretations of definitions to support a point don’t actually provide much useful insight to the topic being discussed, and are seen as distracting. So these non-useful comments are being downvoted to make them less visible. It is not an issue of whether they are technically correct."

Except, I'm not arguing for or against a "point", only that the terminology was wrong. But it does matter, because it leads to confusion, and if you stop to think about it for longer than it takes to jab at the downvote button, the reason why it matters is interesting.

It's not that people here are thinking about the subject carefully, they're just disagreeing reflexively.

If your goal was to argue that the terminology is wrong, then you’ve made some errors in formulating that argument—both logical errors and social errors. We all like a good pedant but you have to be upfront about it and explain why using the correct terminology is important. You failed to do either, instead, your arguments appeared to be arguments about conclusions rather than arguments about definitions:

> Heating via electrical resistance is 100% efficient. All energy goes into generating heat. There is no loss.

This sounds a bit coy, doesn’t it? It’s a bit suspicious that heat pumps are completely omitted from your comment, and it’s also a bit suspicious that explicit discussion about terminology is omitted, and instead the definitions are included as a presumption. Putting the main point of your argument into a presumption of the actual text of your argument is nothing short of poor form (i.e. rude) if done purposefully, and the rest of the argument built on top of these presuppositions is boring and uninformative.

If you can imagine a better approach, consider discussing the terminology and definitions directly, and then explaining why sloppy terminology can cause confusion. You have still not explained why the terminology is so important here. Instead, you’ve made the following statements:

> …if you stop to think about it for longer than it takes to jab at the downvote button, the reason why it matters is interesting.

> It's not that people here are thinking about the subject carefully, they're just disagreeing reflexively.

Consider whether these statements make claims about the internal thought processes of people who disagree with you, and whether making claims about how other people think will further whatever goals you have commenting on this site.

> Except, I'm not arguing for or against a "point", only that the terminology was wrong.

The problem is, the terminology is only wrong if you take an extremely narrow approach of "heating" being "generating heat". But it's quite obvious that we are discussing heating of an enclosed space. And it doesn't actually matter at all whether that heat being placed into the closed space was generated from raw energy within the enclosed space, or moved from externally into the enclosed space. And if there is no difference between these two options, you can achieve greater than 100% efficiency between energy-spent and heat-provided by moving the heat.

> It's not that people here are thinking about the subject carefully, they're just disagreeing reflexively.

Voting isn't specifically for correctness/incorrectness, but encompasses many things. In this specific case, I think the voting reasoning or on your comments can be likely be summarized as "is the comment contributing usefully to the discussion, or providing useful or insightful information in some other way". I think people aren't viewing your comments in this discussion as providing that, and some people may believe they are actually the opposite.

The only reason behind that difference is because it's not generating heat, but moving heat. The difference to users is unimportant. As long as BTUs are pumped into / out of their closed environment, their needs are being met.

And the bottom line is that for the same unit of energy used to generate 1 BTU from resistance heating, we can move several BTUs.

EDIT: I'd also like to note that heat pump systems which include resistance heating systems will typically mark them as "emergency heating". Because they are less efficient, unless it's so brutally cold outside that the heat pump has dropped below 1 EER or can't even operate properly.

EDIT2: I forgot that the units in SEER are not the same on both sides of the ratio. (That is, it's not actually a ratio, since those are supposed to be unitless.) So my original "13+ BTUs" was high. Changed it to a non-descript weasel-word instead.

This one I don't agree with. Heat pumps are typically a few multiples as efficient in terms of KW in to heat BTUs out. I'm not sure what useful point you are trying to make with this particular comment.

I often find that it surprised people to realize that a heat pump is effectively >100% efficient.

Ok, it might be better than heating via burning gas or coal. It's still less efficient than a good air conditioner.

I'm upvoting you, because it always irritates me when people refer to resistance heating as "inefficient". It is true in specific ways and its useful to understand those ways.

It is nice to know that the conversion from electricity to heat is effectively 100% efficient and we are really making a cost comparison instead.

This is backwards; while it's true that a resistor is less mechanically complicated than a compressor-expander-condenser heat pump like an air conditioner, the coefficient of performance for a resistor is limited to 1 (and is very nearly 1, say 0.9999), while the coefficient of performance for such heat pumps is 2–6. That is, for every joule of energy you burn up in the compressor, you transfer 2–6 joules out of your living space into the outdoors.

I know, that sounds crazy, and sometimes the CoP is given as an "efficiency", which makes it sound even crazier — a CoP of 2.0 is "200% efficient"! But it's not science fiction; it's a simple consequence of the Carnot cycle being reversible and the Carnot efficiency of heat engines at ordinary temperatures being quite low.

Actually, compressors use very little energy if you keep them running.

They use a lot of energy when they turn on initially, after that, its very little . Ive been working on smart thermostats that take advantage of this phenomenon to lower HVAC costs.

So you are saying that it's better to run a modern AC (inverter, all that) continuously on a low power than every hour or so? I'm thinking in context of cold leak (through windows, ...)

Actually, yes. The most high-efficiency central AC systems on the market use variable flow for exactly this reason. Below that are two-stage systems (low and high flow), then single-stage systems. Reliability and cost does play a part in all of this, though. [0]

AFAIK, the most efficient residential systems are ductless split systems, where you basically have a centralized compressor and distributed air handlers in individual rooms. This allows for variable flow and localized control, which reduces need.

[0] https://en.wikipedia.org/wiki/Seasonal_energy_efficiency_rat...

> But when either replacing equipment, or specifying new installations, a variety of SEERs are available. For most applications, the minimum or near-minimum SEER units are most cost effective, but the longer the cooling seasons, the higher the electricity costs, and the longer the purchasers will own the systems, the more that incrementally higher SEER units are justified.

Yes, exactly.

In most cases doesn't it take less energy to move head (aircon) and more to generate heat (resistive heating)?

That’s not how heat pumps work, heat pumps in air con can essily get 1:5 ratio the best radiative heating can do is 1:1.

Actually, heating with a heat pump is much more efficient than using a resistance.

That's mostly true, but only if the temperature difference isn't too high. During the coldest days of winter -- or when boiling water, I'd imagine -- the COP can be as low as 1, ie the same as resistive heating.

For the same amount of heat moved, resistive heating is much more energy intensive than the A/C compressor. Heat pumps are about the same, though.