But you also need to factor in replacement costs. Youe ASHP will probably start to wear and cause issues before a GSHP that is inside a building. That is my view anyway.

Only read the abstract so far, so may be talking rubbish. But is the lack of difference between GS and AS down to our climate?

Posted By: tonyis it right to take the 75% percentile?

Still not read it. But if it referring to the top CoP performing 25%, then no.
A normal mean would be the useful gauge as you are taking your chances on it being supplied the right size and fitted correctly. If there is a mode, and it is over 50% of the test, then it is meaningful, though there is some scope if one model was accounting for the mode. You would know how that model performs.

Trouble is we probably know more about these than the people that fitted them. Or more to the point, know more about the pitfalls.

Is the SPF measured for each KW of heat used or for each day and then averaged?

Air source can give a very good SPF on the days you don’t need us much heat….

Thanks Ed,

I would also question if it was a cold or warm winter.

(I get the same issue when choosing who to buy gas from, should I pay a higher standing charge, so I get a lower per KW rate that will benefit me if the winter is colder than last year?)

Results of the Energy Saving Trusts phase 2 field trial, where they intervened to improve the performance of a selection of heat pumps to see how good they could get, concur:

SPFs Mean
ASHP 2.45 (vs. 2.34 above)
GSHP 2.82 (vs. 2.74)

Their figures suggest high performing GSHPs achieve about 3.0-3.2, so again consistent with 75th percentile stuff above. High performing ASHPs manage 2.8-3.0, again consistent.

Overall, that's a wealth of data to suggest:

- ASHPs and GSHPs, even high performing ones, provide somewhat lower SPFs than (certainly I) would have expected, particularly the GSHPs (depressing given how much they cost).
- the difference between GSHPs and ASHPs performance seems surprisingly low.

BUT, on the positive side, DECC's quite large sample results (page 8) suggest that:

- for ASHPs, it is perfectly possible to achieve SPFs of 3.0-3.5, presumeably if you have a very well installed, high spec unit. 34 units achieved this or better.
- for GSHPs, it is perfectly possible to achieve SPFs of 3.0-4.0. 35 units achieved this, again if similarly high spec/well installed.

Interestingly, not a single GSHP exceeded 4, while three ASHPs did (but I'm inclined to treat those as outliers).

My key takeaway is that on the basis of these data, GSHPs are absolutely not worth the significant (Ă‚ÂŁ5k+?) price premium over ASHPs, as the performance premium is marginal to zero. Better installing an ASHP, and spending your extra cash on more insulation.

Thanks for the Danish info Chris. I'll dig into it. As suggested by the Swedish stuff, and based on common sense, I still believe that AAHPs will routinely manage SPFs of 4.0+ for a fraction of the cost of the water based ones. DECC's data give me more confidence of that (if 35 air to water units can achieve 3.0+, with their extra complexities and losses, then simpler AAHPs should do significantly better).

Posted By: GarethCMy key takeaway is that on the basis of these data, GSHPs are absolutely not worth the significant (Ă‚ÂŁ5k+?) price premium over ASHPs, as the performance premium is marginal to zero. Better installing an ASHP, and spending your extra cash on more insulation.

Have they been around long enough for unit lifetimes to be assessed? Aside from the loop (& even that's underground), most bits of a GSHP set up can live in a benign environment. At least part of the ASHP set up must inevitably be in a reasonably exposed position. Maybe that's a factor to consider, especially if you live near the sea?

There is the noise of ASHP along with finding somewhere to site it.

Also I expect that the building with ground source may have been more demanding on their locations or heat need.

My current view is that if you have a digger on side, the space, and the garden is being destroyed by building work anyway that it is worth putting the loop in for a ground source pump, otherwise take the easy option if possible and use an air sourced pump.

Of cause given mains gas, it is hard to justify using anything else and in a few years time when CHP becomes affordable, it will make the case for mains gas so much more.

Agreed in our current home, if we did not have mains gas, then air to air would be good at this time of year.

However in a new build, with much better insulation, so little heat would be needed when it is 10C outside.