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Comment by Tor3 | Hacker News Reader

I think what the post you replied to meant is that when your home needs a 10kW-20kW heater to actually be able to heat your home, then spending tons of money on a heat pump which (for the largest models) can maybe pump out 7kW of heat (equivalent) under optimal conditions (when it's not that cold outside) then you have paid a lot of money and you're still freezing. So you may as well install something else, even a simple wood stove can provide 10kW or more, sometimes much more.

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If you need 20kW and you install a 7kW (equivalent) gas boiler you are in just as much trouble.

If you need 20kW and install a system that can output 20kW under the worst situation (resistive heating), you are probably averaging about 7kW of electric usage when the heating is on to generate 20kW (about 3:1 ratio on average)

Setting aside capital costs that's going to cost you 7kWh per hour of heating. An oil boiler will cost 20kWh per hour of heating.

If your oil costs 40c per litre/$1.50 per gallon and each litre delivers 10kWh, that's about 80c/hour to heat

If your electricity costs 10c per kWh, that's 70c/hour to heat, that's a win

If your electricity fosts 15c per kWh, that's $1/hour to heat, that's a loss

Absolutely - but the important thing is that you can actually get a 20kW gas boiler, but you can't get a consumer 20kW heat pump. You can buy the most expensive consumer heat pump you can find, and it won't do at all if you actually need 20kW. So you can as well save the money as you'll have to install a gas heater (or oil or wood heater) anyway.

> Absolutely - but the important thing is that you can actually get a 20kW gas boiler, but you can't get a consumer 20kW heat pump.

If even you could, you may not want to. Instead one external heat pump handle heads on the top floor, which is generally bedrooms, and not occupied during the day; a second external unit to handle heads on the main floor, which are generally not occupied overnight.

Each individual smaller unit runs less because the load is more focused in 'zones'.

The other issue is that a heat pump has to keep the output temperature relatively low to stay efficient - so just dropping a heat pump in to replace a wet heating system with a gas boiler will have two problems - the total power is less, and the amount of power the existing radiators can deliver to the room is too low. Effectively heating a house with lower temperature water needs big radiators or wet underfloor heating.

"The Daikin Altherma 3 H HT air source heat pump can provide water with temperatures up to 70°C – the same level as gas boilers – and can work when it’s as cold as -28°C outside."

https://www.theecoexperts.co.uk/heat-pumps/high-temperature-...

Given that there are a lot of existing houses out there, surely drop in replacements should be more prevalent.

That article was generally informative, but they forgot (as far as I could tell after a quick read) to include something very important: How much heat can that Daikin heat pump provide? 3kW? 6kW? 10kW? Can it provide more than a standard air-to-air heat pump which typically can provide less than 7kW under optimal (read: Not that cold outside) conditions? This is important to know before buying one (any type of heat pump)

"from £12,500" I guess it comes in different sizes

To be honest the prices I see out there are still generally 'luxury' anyway. If i am spending £20k on a boiler, then an extra £2k to have a secondary gas system that never/very rarely gets used wouldn't bother me at all.

It can make leaving water temps (LWT) of 70°C. It can work when it's as cold as -28°C outside.

What it can't do is both at the same time: make 70°C LWT when it's -28°C outside. It's designed for 65°C LWT (some models 60°C) and can only reach 70°C at a performance penalty (year-round) and can only maintain 70°C LWT down to -15°C and starts to lose max LWT, heating capacity, and even more efficiency below that. (Losing efficiency a few days out of the year is a minor concern. Not being able to meet the heat loss and heat transfer for the building for a few days is a much more serious issue for health and comfort.)

Heat pumps can't be more efficient than the theoretical Carnot heat engine running in reverse, whose efficiency is T_outside / delta_T. In this case it's (273-28)K/98K = 2.5.

I guess being 2x as efficient (cheap) as electric resistive heating isn't super-terrible, but it's not great either.

Compare this to a favorable groundwater heat pump configuration with good radiators and insulation where the 'outside' (groundwater) is maybe 10°C and the target temp 30°C (close to room temp): (273+10)K/20K = ~14.

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