@boblochinver I went through the same thought process.  However, I am slightly different in that I have expensive and inflexible NSH heating with coal, wood and daytime electricity to help out.  I should (I hope) save quite a bit on the running costs and will have a better heating system than I have now.  Having said that without RHI it would have been a more difficult decision.  As it is, I hope to recoup most of the costs over the 7 year RHI term when I include savings on electricity and coal.   

In terms of the installation costs, you might have to factor in some new radiators.  The people doing ours said they were really reluctant to retrofit ASHPs to some existing systems because the rads were too small for the lower flow temps.  And the RHI won't be around forever or it might become less attractive as with PV.  It's an incredibly good deal at the moment.

For me it's a bit like getting an EV.  It's not going to save me money compared with what I've got because of the purchase cost but I like the idea of having one. And if I were buying a new car anyway then it would start to look more attractive.  

Hi Bob,

Here is something that I have been looking at as an alternative to Air to Water ASHP and the RHI/Green Home Grant route. Two or three of these sited around the house could utilise your solar output and reduce your oil consumption. You would not get any RHI, but who cares at this price.

You also get the added bonus of Air Conditioning on the odd day or two in the Summer when you may need it.

APPLIANCESDIRECT.CO.UK "https://www.appliancesdirect.co.uk/p/eiq-12wminv/electriq-eiq12wminv-wall-split-air-conditioner?refsource=apadwords&mkwid=sTq41ApBF_dc&pcrid=316375285866&product=eIQ-12WMINV&pgrid=60291385853&ptaid=pla-446815102020&channel=googlesearch&gclid=EAIaIQobChMIoeDOj5Gk7wIVqIFQBh1mjQoUEAYYAiABEgJlffD_BwE"

electriQ Easy-Fit 12000 BTU A++ WiFi Smart Wall Mounted Air Conditioner with Heat Pump - 4m Pipe Kit Included eIQ-12WMINV | Appliances Direct

Buy electriQ Easy-Fit 12000 BTU A++ WiFi Smart Wall Mounted Air Conditioner with Heat Pump - 4m Pipe Kit Included eIQ-12WMINV from Appliances Direct - the UK

First things first.

You don't need underfloor heating to install a Heat Pump. You may not even have to replace 'wet' radiators. Many are oversized in the first place.

You are correct in working to reduce your heat load in the first place with all reasonable measures like insulation and, maybe, better windows.

Radiators are fairly cheap to buy (a 1200x 600mm is £52.00 including valves from Screwfix). I have increased or replaced some of myradiators over the years in key areas.

As mentioned by Bob, the addition of Solar PV or Thermal would be a bonus but not many people can afford the extra expense.

The basic point is get a heat load calc done by a good heating engineer to give you guidance on the size Heat pummp required. The HP itself is not that expensive but retro fitting UFH can be.

@derek-m

Air to Air heat pumps are viable and many people have them. From LG you can get a Multi system that consists of one outdoor unit plus up to five indoor units. There are bigger systems available but with increased costrs.

The choices of fan coils are wide and varied and its quite quick to install. You also get the benefit of air conditioning in the Summer.

This is useful if you have got a relatively new boiler system. It is also quicker to heat up the space.

@bobbt9866, are the heat emitter guides/reports generated by installers usually quite accurate and a good gauge of whether radiators need to be upsized to keep rooms with ASHPs?

Steve seems quite happy with his Panasonic air-to-air ASHP: 

RENEWABLEHEATINGHUB.CO.UK "https://renewableheatinghub.co.uk/panasonic-wall-split-heat-pump"

Panasonic Wall Split 7.1kW Heat Pump & AC | Renewable Heating Hub

In February 2020 we installed a Tuin log cabin in our garden to act as an office, gym and hobby area. By the time we had protected it with three coats of stain inside and out, installed lighting, power and internet, the warmer weather was upon us and no thought...

@editor

A good guide was  issued by one of our very good heating engineers recently on Twitter. Turn your boiler water temperature down to heat pump levels and see how each room performs. Its not a hardship in current conditions but will give you a guide.

There does seem to be an annoying trend that rads have to be changed on every install, when this is not the case.

If in doubt, get one or two heating engineers to do an assessment and take their advice. Not all rads may need replacing. As I said before, they are not expensive anyway.

@editor

It seems a large unit for the size of the building but, i am guessing, that it is single thickness wood with no insulation on all walls, floor and ceiling. Panasonic produce very good split systems and this one will be inverter drive so will have a wide range of performance.

We're pleased to announce that @bobbt9866 has agreed to become a moderator and will be sharing his experience and insights as a heating engineer and expert.

@bobbt9866 has over 40 years’ experience in various sectors including renewable energy, mechanical services, maintenance service and heat pump product technology. @bobbt9866 has also designed and supplied heat pumps to leading high street brands since 1980 and has been involved in the evolution of domestic heat pumps since the mid 90s.