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Comment by Ottolay | Hacker News Reader

The issue with heat pumps is that most of the ones currently being installed in South East of the US don't work well below +20F let alone 0F or -20F. Most of them are being installed with an electrical resistive backup heat, which is incredibly inefficient.

The problem is when a cold spell like Christmas 2022, with temperatures down towards 0F. All the heat pump users switch to resistive backup heat and it overloads the electric grid and we get rolling blackouts.

In my opinion, heat pumps are amazingly efficient at moderate cold temps, but they really need propane or wood heat backup for the really cold temperatures instead of resistive heaters.

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What is inefficient about resistive heat? Isn't electricity->heat basically 100% efficient?

Maybe you are saying that heat->electricity->heat is inefficient, since most electricity is produced from heat inefficiently.

I always get tripped up by this, since I live in an area where almost all electricity is hydro. In that case resistive heating seems fine.

That 100% efficient figure is correct - you get one Watt of heat for one Watt of electricity. It's just that heat pumps can deliver 3 or 4 Watts of heat for each Watt of electricity (usually quoted at around 300% or 400% efficiency!). Compared to that, resistance heaters aren't as efficient.

The explanation that made the most intuitive sense to me is that it takes less energy to move heat from one place to another (air at 273 Kelvin to air at 300 Kelvin, like a heat pump does) than it does to create heat from nothing (like a resistor does). That's why the heat pumps can get deliver more heat to you from the same amount of electricity.

That makes sense, but GP was calling resistive heating inefficient compared with propane or wood heat, which doesn't make sense to me.

The best natural gas power plants are 64% efficient. A modern furnace is around 95%. Sure electric restive heat is 100% efficient in your house, but the whole system us much worse.

Of course electric can come from many sources, if your is renewable at the time resistive is good. However you might also be using some old 1920s coal generator that is 10% efficient (these still exist, but are only used in the worst emergencies)

Natural gas (not sure about wood; in most areas it’s abundant) is about 1/3 of the cost of electricity in America, so it’s economically less efficient.

Resistive heat is 100% efficient, but heat pumps can operate at greater then 100% efficiency. That's because a heat pump doesn't actually generate heat, but just moves it around. Even when it's cold outside, there's still a lot of heat energy in the air, which can be moved inside to warm your home.

Due to the increased efficiency, heat pumps are better then electric resistive heat (when temperatures outside are within the heat pump's operating range, that is). This is regardless of the method of power generation.

Since heat pumps are moving heat around rather than actually producing it, they can be effectively better than 100% efficient so it's not so much that resistive heat is inefficient, but that it's less efficient than a heat pump.

Electricity offers resistive heating, and heat pumps. Heat pumps are much more efficient than resistive heating.

Otherwise, you have chemical fuel which burns, and a bit of electricity to pump it around (either by forced air or water pumps).

In terms of electrical input, resistive heating is the worst of the lot, even if it can be sourced in a carbon neutral way (unlike nat gas or fuel oil).

In the US, anyway, when people say electric resistance heat is inefficient they are comparing it to natural gas heat. It's the same story in water heaters; if you install a HPWH you are betting on not needing to resort to resistance heat because if you do that too much, you probably should've installed a cheaper gas unit instead. (I'm a happy HPWH owner weighing the timing of adding a heat pump for central heat.)

We’ve had heat pumps that work down to -5F for years. If you’re installing one that can only go to 20F in 2022(3?) something is very wrong.

Backup heat methods increase the complexity and cost.

I am just saying what the typical install is in the South East US. The heat pumps installed may produce some heat at 5F but they can't keep the temperature to the set value, so there are resistive elements (Aux heat) to make up the shortfall.

It's pretty common for people with heat pumps to have Aux Heat kick in during cold spells, which cause power grid overload issues.

I realize you can insulate a house well enough and have a good enough heat pump to avoid backup heat, but 5F or 0F days are rare enough that the codes do not enforce this.

When I researched this year on replacing my A/C / Propane Furnace system with a heat pump, I found that companies didn't seem to want to advertise what temperatures their heat pumps can operate effectively at. If I look at some marketing materials from Google it seems companies like Carrier and Trane are only willing to talk about their heat pumps working in low temperatures if it's regarding their top-of-the-line (very expensive) variable speed compressor units. No one talks about what temps the mid range units can handle, and I'm guessing it's because they don't work well below 20F.

My heat pump is advertised to operate as low as -13F. One thing to note is the efficiency of heat pumps is not optimal when they are operating close to their extremes.

Where I live, most people had electric baseboard heating before they installed their heat pump, so 99% of the time they are using less electricity and on those really cold days, they are using as much as they did before getting a heat pump, so we know our grid can handle that. The grid in the US is weak because Americans have traditionally depended more on fossil fuels or wood for heating.

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