I'm not sure how a hybrid heat pump works, is it just a full-sized heat pump and a full-sized gas boiler in one package... and you switch between the two. Heat pump OR gas.

Or does the heat pump and gas boiler work in series... that's more what I'm interested in. Heat pump AND gas. (More accurately, I'm interested whether there's any mileage in a gas AND heat pump arrangement.)

Grant do their Vortex oil boiler as a hybrid - basically an outdoor vortex combustion boiler plonked on the same base as one of their (well, Chofu's) 17kW ASHPs. Uses whichever fuel is more efficient. So that basically means oil at lower outdoor temperatures.

https://www.grantuk.com/products/hybrids/grant-vortexair-range/

It's not very easy on the eye, designed by the MD as a cut'n'shut product? Ideal for larger properties which may need to provide a lot of heat energy quickly or owners who want the reassurance of oil to fall back on in case the ASHP goes down.

I'm not sure how a hybrid heat pump works, is it just a full-sized heat pump and a full-sized gas boiler in one package... and you switch between the two. Heat pump OR gas.

Or does the heat pump and gas boiler work in series... that's more what I'm interested in. Heat pump AND gas. (More accurately, I'm interested whether there's any mileage in a gas AND heat pump arrangement.)

At the moment we have a gas and heat pump arrangement, but it is an Air to Air heat pump, that we run when there is enough solar PV. We ran the heat pump today, and it shutdown the gas boiler for 6 hours.

I'm not sure how a hybrid heat pump works, is it just a full-sized heat pump and a full-sized gas boiler in one package... and you switch between the two. Heat pump OR gas.

Or does the heat pump and gas boiler work in series... that's more what I'm interested in. Heat pump AND gas. (More accurately, I'm interested whether there's any mileage in a gas AND heat pump arrangement.)

That's interesting, but OFTEC is the oil fired technical association, and one can't help but feel this sounds like self-preservation? If there's adequate 240V power supply to run an ASHP, there's probably power in hand to supplement heat output electrically during defrost or very low CoP periods. Efficiency would be poor: 100% for electric and maybe 90-05% for oil.

The real issue here is the cost disparity between electric (increasingly renewably sourced) and fossil fuels per kWh.

To combat climate change, we need to 'stop burning stuff' and producing CO2. Along with coal and bottled gas, oil is one the most polluting and carbon intensive home heating fuels.

The planning constraint on ASHP permitted development is 0.6m3 unit size if I recall correctly. That means the evaporator coils on most units has a very narrow fin pitch. Far narrower than you would find on commercial refrigeration and cold store plant which is designed to allow ice to accumulate on the fins for much longer before defrosting. Domestic ASHPs ice up in no time when the outside humidity is high and the refrigerant evaporating temperature falls a few degrees below freezing. If ASHPs could be brought to market that defrost only once every 2-3 hours rather than every 45-60minutes, the doubly whammy of low CoP and parasitic defrost energy requirement could be mitigated. Such ASHPs would need to be physically larger however.

Did they state how this 'boiler' is powered?

It's least efficient at lower ambient temperatures when there is less enthalpy in the outside air to capture. The ASHPs' Achilles Heel.

That takes us back to my original question. On a really cold night, why don't we just take a small flame to heat up the ASHP evaporator just a little bit to make it efficient again.

I obviously don't understand the thermodynamics involved! And I'll probably sound really stupid to someone who does. But imagine that, on a really cold night, I park one of these in front of the ASHP outdoor unit. Portable Grey LPG Space Heater 15,000W - Screwfix

Worst case scenario: I improve the ASHP's performance back to 'normal'... but I pay exactly as much in gas as I win in electricity saving on my ASHP. Net gain = exactly zero. (Or worse when I take into account transaction costs.)

Best case scenario: it actually takes less gas than I expect to get the ASHP's performance back to normal and then I'm quids in.

I suspect that the real world is like in 'worst case scenario' because a heat pump doesn't generate energy... it just moves it from one place to another place.

But as I say, I don't really understand the thermodynamics. Is there anyone here on the forum who can explain?

Rest assured, I'm not trying to turn lead into gold or invent a perpetual motion machine! Just curious.

As you stated, an ASHP does not magically take a 'dollop' (technical term) of energy and multiply it by 3 or 4.

When operating at a COP of 3, a heat pump uses 1kW of electrical energy to extract 2kW of energy from the outside air, to supply a total of 3kW. As Allyfish correctly stated, the ASHP's Achilles Heel is that there is less heat energy in the outside air, at precisely the weather conditions when the heat pump is required to supply more heat energy. Less energy from the outside air means that the heat pump has to take more energy from the electrical supply.

Once your central heating system is filled with water from the mains supply, it then operates as a sealed system, so there would be no requirement to pre-heat the mains water.

Of course! Stupid me! Mains water doesn't go into the heat pump, the heating return goes into the heat pump. What I should have said is 'divert the heating return to run through the solar thermal system first, so that it can get preheated before going into the heat pump'.

Your idea is to use the thermal store on the source side and what would be needed is some kind of collector inside your thermal store. I can visualise this quite easily with a GSHP (I've got a GSHP myself): the brine loop runs under the ground, it collects heat and returns to the heat pump, but, before the heat pump, we run the brine loop through a collector in the thermal store. That's easy. With an ASHP, I just can't see how to convert the heat in the water in the thermal store to usable heat for the air source collector. Unless you maybe run a hot water heat exchanger over the outdoor unit fan or something like that?

Your suggestion of using a thermal store to pre-heat the return water to the heat pump should indeed work, but on cold days the return water from the heat emitters could already be as high as 40C, or even 45C, which would limit the amount of heat energy that could be extracted.

By pre-heating the air, rather than the water, it should be possible to extract heat energy from the heat store down to virtually the outside air temperature, which could be -5C or even -10C. The water in the heat store would of course require an adequate mixture of antifreeze, but this could be the type used in vehicles, rather than the more expensive kind used in heat pumps.

There is a detailed write up somewhere buried in the forum, that explains the design.

Water to air heat exchangers of various sizes are readily available, but for a trial an old car radiator or two would probably suffice.

It's least efficient at lower ambient temperatures when there is less enthalpy in the outside air to capture. The ASHPs' Achilles Heel.

That takes us back to my original question. On a really cold night, why don't we just take a small flame to heat up the ASHP evaporator just a little bit to make it efficient again.

I obviously don't understand the thermodynamics involved! And I'll probably sound really stupid to someone who does. But imagine that, on a really cold night, I park one of these in front of the ASHP outdoor unit. Portable Grey LPG Space Heater 15,000W - Screwfix

Worst case scenario: I improve the ASHP's performance back to 'normal'... but I pay exactly as much in gas as I win in electricity saving on my ASHP. Net gain = exactly zero. (Or worse when I take into account transaction costs.)

Best case scenario: it actually takes less gas than I expect to get the ASHP's performance back to normal and then I'm quids in.

I suspect that the real world is like in 'worst case scenario' because a heat pump doesn't generate energy... it just moves it from one place to another place.

But as I say, I don't really understand the thermodynamics. Is there anyone here on the forum who can explain?

Rest assured, I'm not trying to turn lead into gold or invent a perpetual motion machine! Just curious.