an ASHP cannot create energy - all it can do is extract it from the air supplied using power. When someone says an ASHP is 300% efficient, it isn't really. Its just using 1kwh of energy to extract 3kwh of heat from the air. So I cant really see what benefit your system would really have beyond extracting the diesel heat from the air and moving into the central heating. Yes it would work, however I can't see it being magically more efficient. If you wanted to do that maybe you could heat the water in the central heating system via heat exchanger which would have less environmental losses - but again the same thing would happen; You would be moving heat around.

Posted by: @batalto

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an ASHP cannot create energy - all it can do is extract it from the air supplied using power. When someone says an ASHP is 300% efficient, it isn't really. Its just using 1kwh of energy to extract 3kwh of heat from the air. So I cant really see what benefit your system would really have beyond extracting the diesel heat from the air and moving into the central heating. Yes it would work, however I can't see it being magically more efficient. If you wanted to do that maybe you could heat the water in the central heating system via heat exchanger which would have less environmental losses - but again the same thing would happen; You would be moving heat around.

If one considers how a heat pump functions.

Say a heat pump is consuming 1kW of electrical energy and producing 3kW of thermal energy, then it would be deemed to be 300% efficient. Analysing how the electrical energy is being used, there could be up to 200W being consumed by the water pump, fan, electronics and heat losses, so only 800W may be being converted into thermal energy by the compressor and refrigerant system. To output 3kW of thermal energy, the refrigerant gas must therefore be extracting approximately 2.2kW of thermal energy from the outside air.

If the outside air temperature falls, such that the heat pump can now only extract 1.7kW of thermal energy, then to still output 3kW, the compressor must now supply 1.3kW, giving a total electrical energy consumption of 1.5kW. The efficiency is now only 200%.

To bring the heat pump back up to an efficiency of 300%, it would be necessary to add 0.5kW of thermal energy to the air flowing through it. I doubt that the heat transfer will be 100% efficient, and the diesel burner will not be 100% efficient, so it may have to consume the equivalent of 1kW of energy.

Therefore there cannot be an efficiency improvement from the energy consumption point of view, whether it is more cost effective would depend on the relative costs.

The only true way to improve overall efficiency, would be to capture and store thermal energy from solar thermal or even solar PV panels, and then use this stored energy to pre-heat the air supply to a heat pump during colder weather conditions.

Both replies are on the same way I was thinking, I probably didn't explain it in correct terms.   Just wondered if anyone had explored this idea.

Lets say the optimum outside temp for an ashp to operate at its 'most efficient' was 44°F, and it uses 1kW used to output 3kW you mention.   

But when the outside temp is <34°F, the hp has to work harder to extract the 3kW... so it's using more than 1kW to still extract 3kW.   (i.e. using more electricity)

The idea of the Diesel heater was, lets say its winter and -2c outside (28°F) and to put a type of big thermal-jacket over the ashp, with the DHs heat warming the air surrounding the hp to ~44°F, the hp is now operating with 'optimum' temperature air and using 1kW instead of 1.3kW.

I'm not sure if using 1ish ltr of diesel/kerosene (~1gbp) per day during the cold months is cheaper than the difference of an ashp using 1.3kW instead of 1kW.

And on the other point mentioned about using the DH to heat the water in the central heating system, yes the exhaust pipe itself gets red hot. so it could probably be extended to loop into a water tank somehow to heat it but still extract the exhaust gases outside.

Solar PV are just too expensive at the moment for the return in investment.

@johnsm1 your idea it's not far from wishful thinking BUT:

The energy you want to use in order to help the heat pump(any type) needs to come for free or be residual/waste/lost.

Let's say you take the exhaust gas/air from the diesel heater and you blow that through the evaporator of the heat pump, that is a gain without extra cost. If you just burn anything to heat the air in order to help the heat pump, you did not gain anything.

A different design of the same concept would be an air source hp where the refrigerant goes first through the evaporator to take as much from the air then to have a second evaporator/water heat exchanger that take extra heat from water. Put it differently, a water heat pump that first takes some heat from the air and then from the water/brine.

Much of the struggle with ASHPs in winter is the defrost method. ASHP Evaporator coils have very narrow fin pitch, 1 to 1.5mm. That gives a large surface area for heat transfer in a small compact coil, but in winter it means they can ice up solid in no time and need continually defrosting. More than once an hour is common in damp, cold humid conditions.

Permitted development conditions state that an ASHP has to be no more than 0.6m3 volume to comply. So they are made with very small evaporator coils. Commercial refrigerator evaporators have a much wider fin pitch, 5mm is not uncommon, with far fewer defrost cycles. They may defrost once or twice a day, not every hour.

When ASHPs defrost they take heat energy from the heating circuit to defrost, which removes heat from the very location you want to heat - the house. Most ASHPs have a 4-way reversing valve, because most are heating and cooling circuit designs. (In the UK, under MCS, configured for heating only). They reverse cycle defrost. It's madness. Hot gas defrost using superheated compressor vapour, or stored heating medium (charged from, say, condenser sub-cooling) would avoid robbing heat from the heating circuit. The energy requirement to defrost is constant, but sourcing that energy from compressor input power (direct for hot gas defrost, indirect for sub-cooled enthalpy storage) rather than the water side (home heating circuit) would have a far less detrimental impact on space heating inside the home.

What we mostly have on the UK market today is a legacy of mostly Far-Eastern A2W ASHP designs adapted from A2A heat pump designs, using 4-way reversing valves, with evaporator coils not designed for our cold and damp winter weather. ASHP winter performance could be much improved by reducing the frequency of defrost and defrosting by a different process. CoPs will always be lower in winter, that's just physics. But losing 10-15 minutes of heating function each hour to defrost (5 mins to defrost and 10 mins to recover LWT) can be designed out quite easily if manufacturers had the mindset.