How do you judge ASHPs?

[markocosic]

They don't have load compensation. Fairly useless for radiator setups. Less of a biggie for domestic hot water.

 

They also don't have sensible control strategies for heating hot water cylinders (they just run balls out against a hysteresis stat) or for coming up from setback (again runs balls out until target flow temp achieved which isn't useful).

 

You can mitigate by bolting on a 3rd party control such as Homely but this would mean being beholden to a startup company and internet connectivity and subscription fees. 

 

 

The second line was supposed so say "all FGas units" but got spelling autocorrected without me noticing.

 

FGas is the high global warming potential legacy rubbish that lazy manufacturers use to delay investment in environmentally friendly refrigerants. Samsung are launching their R290 unit in a bit of a catch up rush late this year. Give it another year or so to iron out the crinkles and it'll be ready to use. I wouldn't touch it until then myself but I'm keen to avoid being guinea pig with cars etc too for the same reason.

 

You might not care. R32 works. It's just crappy of them to be putting more of it on the market in this day and age.

[JamesPa]

I guess it might help OP to provide a list of key specs (and gotchas) to consider. Here is my starter for 10 (simplest, not necessarily most important, first)

 

Output, specifically

• Max output at low ambient temperature and your design flow temp – some ‘rated’ outputs are overstated at low ambient
• Min output at high ambient temps – this matters if you are trying to design with a small or no buffer (but note that system volume for defrost also affects whether a buffer is needed or not)

Max flow temp – only matters if you are designing for higher flow temperatures (eg >50), which you shouldn’t be doing.

 

Physical size and placement constraints. All units require space around them and R290 units have some more specific constraints. Have you got somewhere it can go? Also will it meet the permitted development volume constraint if PD is a consideration

 

Appearance – can you tolerate it?

 

Weight – only matters if it is not ground mounted

 

Sound POWER (nb NOT sound pressure which is quoted on an inconsistent basis). Can you meet the noise requirements (eg for permitted development) given the specified sound power? MCS provide a spreadsheet making this easy to calculate.

 

Controls – Weather compensation is a must, certainly for flow temps above 35C.  Less important at low flow temps but still highly desirable.  Load compensation is desirable unless you have only UFH and/or your house responds very slowly to temperature changes.  Night time set back also desirable/essential depending on your control/heating strategy and house time constant.

 

Ideally weather compensation should allow a non-linear/multi point curve to be programmed, very a few do this. However its only worth a % or two in efficiency depending on your specific scenario.  Load compensation to some extent negates the need to have non-linear weather comp.

 

As mentioned by @markocosic and if you can find them out (which, mostly, you cant), things like the intelligence in the control strategy when its heating up (as opposed to running in steady state) make a few % difference to efficiency when heating DHW or recovering from setback.

 

Third party controls, eg homely, can mitigate deficiencies but, as others have said, it means being beholden to a third party (or writing some code of your own).

 

Claimed SCOP at your design flow temp - (but possibly treat with a pinch of salt?)

 

Refrigerant - R410a (still around) has a GWP of 2088. With a typical 3kg refrigerant charge thats 6 tonnes of CO2 equivalent if the refrigerant is released(eg on disposal). R32 (very common) has a GWP of 675 (so about 2tonnes CO2 equivalent if 3kg is released) and R290 (the latest available, not universally rolled out) a GWP of 3, ie negligible. By way of comparison heating a typical house with gas emits around 3 tonnes CO2 equivalent per year. I have no idea if refrigerant leak is an actual problem – perhaps others do!

 

Availability and servicing availability – as discussed extensively

 

Quiescent power consumption - some older models (particularly, but not exclusively, Mitsubishi) consume 200W or so when doing nothing, to keep the compressor warm.  This problem seems to have been fixed on new models, but there may still be some rogues out there.  Expect approx 25W or so. 

 

Is it MCS listed if this matters

 

Price

 

 

None will score top on all of these, but this list will eliminate most for any specific application.  Of course that wont stop salespeople trying to convince you that their wholly unsuitable unit is just what you need! 

Edited April 2, 2023 by JamesPa
Repeated word removed

[SteamyTea]

@JamesPa Useful list, thanks.

[Iceverge]

There's a live list of real world performing heat pumps on a website somewhere. Someone posted it a few weeks ago but I can't remember where..

 

Will try to dig it out unless someone else remembers. 

[DamonHD]

4 hours ago, Iceverge said:

There's a live list of real world performing heat pumps on a website somewhere. Someone posted it a few weeks ago but I can't remember where..

 

Will try to dig it out unless someone else remembers. 

https://heatpumpmonitor.org/

[Iceverge]

That's the one. I think there was one mentioning international installs too but that too has escaped my mind. 

[markocosic]

https://www.waermepumpen-verbrauchsdatenbank.de/index.php?button=verbrauch