SWMBO has decided an ASHP is a good idea, mainly because it's ideally fit n forget and I have enough on my plate with everything else.

She asks pointy questions like "why is this Chinese one a grand and that other one with the same rating is 5 grand"

At best I can explain that the performance of the jap one holds out a lot longer as air temps fall and that it has an inverter driven compressor so it shouldn't cycle as much, but I have no answer for the follow ups like "does it matter?"

I dunno.. Does it? With a big enou buffer, who cares about the inverter or lack thereof? And when one can buy a new Chinese ASHP every year and it be a long time before the outlay reaches that of a Japanese one, does it make sense to get a Japanese one?


I have he tluxury of living next door to a house that is broadly similar to mine, and I watch it's development. A Certain Supplier from every building mag going, has specked a 14kw ecodan for that property. Interestingly they tell me the final bits of paperwork (which I presume includes a decision about which ASHP to go for) will be done when the as-built configuration is given, which is done after the pump is installed. This seems arse about face to me, but who am o to judge?

So right now I'm looking at one of 2 options:

* Diy.a cheap non inverter one, mate it to a big heat pump cylinder and let it cycle if needs be, sod the RHI
* MCS install of a high end one, add umpteen thousands to the mortgage, use the RHI

There is also the option of buying a high end one for cheaper than RRP and not using the RHI, though financially there might be a limited circumstance where that's viable..


Prices wise, a DIY job would be around 1k plus the unit price I think
Ecodan is just shy of 7k commissioned (note, that doesn't mean installed) and rhi'd
Calor guesstimated 9k for a daikin altherma hybrid, RHI compliant
Samsung ones have been floating around for circa 4k as a second hand system, no RHI

I don't doubt my technical ability to do the DIY route, I just can't decide (with everything else on my plate from second fix, plasterboarding, flooring, kitchening, stairing, decorating and landscaping whether this is just one of those jobs where it's worth handing off to someone else. Essentially it looks like the expensive option, plus the extra mortgage, minus the RHI comes to about the same cost of buying a cheap pump and not using the RHI.. I just have to front the cash, which is a bind.. In essence my query is (I think):

Are inverter driven HPs worth the premium or is a somewhat agricultural Chinese pump that simply cycles on and off for a quarter of the price a better bet for a user who is prepared to have a large tank of hot water sitting there wi the aim of smoothing things out?

Have you built as air tight ad as well insulated as you can, fabric first approach will save you money.

Have you done a heat loss calculation? What sort if heating will you have?

The house next door does not sound like an example of best practice, so done follow his lead.

Are you aiming to do both the heating and hot water from the same ashp, if so the higher ashp output temperatures will lower efficiency. It may be better to keep the thermal store temperature lower to match hour heating temperature requirements and instal an in line water heater to boost dhw as you use it.

Do more research, there is quite a debate on thus subject going on in the other place. This may be worth a read http://www.ebuild.co.uk/topic/18157-towards-a-heating-checklist-please-comment/

That does sound a bit high for the level of insulation you've described. What type of windows are they? (apologies if you've posted before) You should be able to work a simplistic heat loss calc just for the rooflights if you think they're a problem.

You might be in the low-energy region where many models give dubious answers. It would be interesting to know what PHPP comes up with.

But your model presumably includes the sizes of the windows and what type of glazing and frame do they have?