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Comment by triceratops | Hacker News Reader

> Heating is considerably more energy efficient than having to run ACs all over the place.

That's actually incorrect. Heating takes considerably more energy.

When you're cooling you're looking at a temperature delta of at most 20 degrees C/45 degrees F between outside and inside. When you're heating, it can be easily be twice that. Or even more, in parts of Canada or the Great Lakes states.

Air conditioning is a heat pump and can achieve up to 300% efficiency. Whereas most houses in cold climates use furnaces, which have at most 98% efficiency.

Now that we have more efficient cold-climate heat pumps all of this may gradually change. But as of right now, what you're saying is wrong.

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Heating under usual winter conditions would consume more energy than cooling under practical conditions. Yes.

But that's very different than claiming than cooling is more efficient than heating. In A/Cs the ratio of energy (in the form of heat) removed to energy consumed is very low, around 10-15%.

In furnaces the ratio of energy added to energy consumed is around 90%, which was pretty good until the emergence of heat pumps. There almost all the energy consumed is added to the output in addition to all the energy removed from the other side. That's why the efficiency in heat pumps is beyond 200%.

In summary, no. Cooling is not more efficient than heating. But given the temperature differential when cooling the energy consumption is usually higher.

> In A/Cs the ratio of energy (in the form of heat) removed to energy consumed is very low, around 10-15%.

Why is that?

Obviously I'm referring to efficiency in practice. In theory, heating and cooling should be equally efficient for a perfect heat pump, right?

> Air conditioning is a heat pump and can achieve up to 300% efficiency. Whereas most houses in cold climates use furnaces, which have at most 98% efficiency.

No. You can't directly compare "efficiency" numbers across different energy sources. My understanding is that the efficiency of gas powered electricity generation is around 33%, meaning that powering a heat pump with a COP of just under 3 will be of similar end-to-end efficiency as burning the gas directly in a condensing boiler/furnace.

End-to-end efficiency calculations are tricky and depend on a lot of variables.

Sure, but that doesn't mean you can throw your hands up and make an argument comparing nameplate efficiencies of things with different energy sources.

I'm well aware the figure in my above comment could be off by a decent amount due to transmission efficiency, what specific plants are generating marginal power, etc. But it's at least framing the comparison right.

Even in your worst case scenario, heat pumps are about as efficient end-to-end as a natural gas furnace. And renewable power is only going to become more common over time so it could get better.

To a consumer, comparing nameplate efficiencies as you say makes the most sense. I can easily find out how much a joule of electricity and a joule of natural gas cost me. Then multiply those with efficiency to see which is cheaper.

I did not put forth a worst case scenario. I put forth an example of doing proper comparison.

Based on a figure I just saw from someone else describing their setup (1.75 COP at 5 deg F), and taking into account transmission losses etc, the worst case would seem to be significantly less efficient than burning natural gas directly in a condensing furnace.

This isn't meant as some sort of knock against heat pumps! In fact I'm tentatively planning to add a heat pump to my own home within the next several years. Focusing on a worst case is still not an overall comparison of performance, but rather just one data point in the evaluation of a given design.

Multiplying the cost of the energy source by the nameplate efficiency is indeed another sensible way of comparing across technologies. But that isn't what you did in your original comment, where you juxtaposed two different types of efficiency numbers. That was my only point in my original comment.

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