Maybe they're referring to cases where the heat pump can't supply heat faster than what the building is losing through the uninsulated walls. In such cases, you must first move out of the holey tent before installing a heat pump.

You just need a bigger pump!

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.

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 make 70°C LWT when it's -28°C outside. It can only do that down to -15°C and starts to lose both max LWT and heating capacity below that.

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.

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'.

To understand the difference in a short and simplyfied way:

1. Your examples are heating up the inside air by using energy (burning fuel). Doing so will always be less than 100% efficient, some heating technologies are as little as 10-20% efficient (energy per kWh)

2. Heatpumps are instead using energy to do heat transfer. Moving heat from the outside to the inside.

The latter is way more efficient, with easily 300-400% efficiency. But obviously the colder it gets outside, the less heat is in the air to extract and the efficiency goes down.

Heat pumps take much longer to heat the system, to get it to the desired temperature. Combine that with a property that is not insulated to very high standard, and you get a heating system that is incapable of keeping you warm.

Of course, you could throw more money at it. But it won't be cheap, and you won't see a return on your investment any time soon.

Speed / response is a perfectly valid difference. Had not considered it.

Leaky houses are already throwing money at the heating problem, and perhaps with a slower response time you would 'idle' the heating circuit at a passive 25C against 18C room temperature, and throttling up from there. Throwing money at it works!

It's not different but the amount of heat that each system can be generated is different. And when it's cold (i.e. when you need the extra heat to be compensate for your draft home) your heat pump is going to struggle most.

> I dont understand how heat output from a heat pump vs heat output from a gas / oil / wood burner / resistive heater are at all different.

A gas/oil/wood burner are not 100% efficient in creating heat, and release carbon into the atmosphere.

A resistive heat is at most 100% efficient: all the electrons go to making the coil glow, like old school light bulbs. So 1 kW of electricity is 1 kW of heat (which has some BTU equivalent for old fashioned folks).

A heat pump does not create heat, but moves it from one place to another with refrigerant and pumps. So 1 kW of electrical usage can move 3 kW of heat.

* https://en.wikipedia.org/wiki/Coefficient_of_performance

* https://energyeducation.ca/encyclopedia/Coefficient_of_perfo...

The $/capacity may be quite different. The capacity of burner systems is generally cheap, so you oversize them and if you need more heat, you just burn more fuel. The capacity of a heat pump is relatively expensive, so you size it up to something reasonable, and in an unusually cold day you may hit the limits of how much joules you can get out of it.

Also, the $/fuel is different - if one system gets three times more joules from the same fuel, it doesn't mean it's more efficient as the other system may be using four times cheaper fuel; so a 300%-efficient heat pump is more efficient than a resistive heater but may be less efficient than a furnace burning cheap fuel.