Does anyone know of a cheap ASHP ("air to water") with an output in the region of 3 kW or so? I've looked but can't find one.

The irritating thing is that you can buy 2.5 kW to 3.5 kW heat output mini-split ASHPs with SCOPs of 5 or better (the new-ish R32 units from Mitsubishi and Daikin) for about £700 to £1000, but they're all sold with a single indoor unit (the wall-mounted indoor unit houses a refrigerant-to-air heat exchanger with a fan, plus some controls electronics, but no more than that).

If you want air to water heat pumps, you're quickly into the £3500+ zone for larger units with lower SCOPs (which use the less efficient, but higher GWP R410a refrigerant).

If you are feeling adventurous it's possible to convert these sub £1000 units into "air-to-water" heat pumps. To do this, you substitute the whole indoor unit with a refrigerant-to-water flat plate heat exchanger (available for around £100), with some adaptation to or replacement of the indoor unit's electronics to keep the outdoor unit happy. In the UK this would need an "F-Gas" certification, and I suspect any warranty claims might be tricky...

Another approach I've found people using is to buy a second-hand de-gassed older generation mini-split (R22 or R410a based) for circa £100 to £200, carry out the above air-to-water modification and then charge it with propane (R290) - which has a very low global warming potential, so doesn't need F-Gas certification.

Since propane is flammable, you'd want to keep it outdoors by putting the flat-plate exchanger outdoors (much like the Mitsubishi Ecodan and other commercial heat pumps do anyway).

To add to the list of creative heat pump hacks that google has turned up, I also found a few examples of people converting small ASHPs to be hybrid air-source and ground-source heat pumps, by adding a small ground loop to bump up the COP. In heating mode the evaporated refrigerant passes through the refrigerant to air heat exchanger as usual, and then goes through a second refrigerant to water exchanger (the water then passing through the ground loop).

This has the advantage of needing a fraction of the ground works of a conventional GSHP (to the extent that for a small unit, you can make do with throwing in some plastic pipe when you're trenching for drains and telecoms etc. anyway), and that you can automatically turn off the ground part when the air temperature is higher than the ground temperature (e.g. spring/summer).

Perhaps it's better to wait for the 5 kW Mitsubishi Ecodan to come out in a higher-efficiency R32 version, and then wait a couple more years for them to come onto the second hand market - after some new-build houses fit Ecodans only to have the occupants later change them out for bigger gas boilers because they can't cope with the actual heating load...

I've seen lots of seemingly cheap ones on ebay and amazon in Germany e.g. search for Michl 4kw in ebay.de, less than a grand COP 4.2

related question, aside from price, what are the (presumably negative) consequences of over-sized heat pumps? I wanted to explore the idea of 2 or 3 smaller (and cheap) heat pumps rather than a large one and/or separate heat pumps for DHW (water) and heating.

Posted By: SilkyI've seen lots of seemingly cheap ones on ebay and amazon in Germany e.g. search for Michl 4kw in ebay.de, less than a grand COP 4.2

Interesting - thanks, looks like a reasonable alternative to the Mitsubishi EcoDan

related question, aside from price, what are the (presumably negative) consequences of over-sized heat pumps? I wanted to explore the idea of 2 or 3 smaller (and cheap) heat pumps rather than a large one and/or separate heat pumps for DHW (water) and heating.

I think they're similar to gas boilers - they operate more efficiency when they're modulated down (AKA "turn down ratio"). I think (BICBW) that some are able to modulate down to about 20% of max output (so similar to a lot of gas boilers in that respect too), but also that some don't modulate at all, or only slightly . It would be worth checking with the manufacturer for the model in question, since that's not often published in brochures I think.

However, once you want less than the minimum modulated output, they will start cycling, which is bad for efficiency and mechanical wear+tear on the heat pump, so best avoided.

Also worth considering is the total refrigerant charge, and the global warming potential of the type of refrigerant it uses (equiv CO2 quantity of warming if it all ends up the atmosphere)... The smallest 5kW R0410a Ecodan has a GWP of 3.5 tonnes of CO2. The R-32 replacement version which is out this year I believe is likely to have a GWP of about 0.8 tonnes. If you can recover some of the refrigerant at the end of life, you do better than that, but I doubt that in practise much actually gets recovered.

The in-use emissions from the gas boiler are about 1 tonne a year currently, and this would fall to about 0.35 tonnes if it were replaced by a heat pump. So an R-32 ASHP would "pay-back" in under 1.5 years, but an R-410a one would take about 5.5 years.

If you have a relatively high thermal mass building, or high thermal mass emitters, (or thermal store etc.), then a smart control system should be able to minimise cycling and run them at a high-ish efficiency.

Posted By: TimSmall
However, once you want less than the minimum modulated output, they will start cycling, which is bad for efficiency and mechanical wear+tear on the heat pump, so best avoided.

That depends on how much buffering you have with a buffer tank, or a high thermal mass UFH system along with how your control system works.