Hi November Romeo, 

Good luck with all your quote comparisons. Just a quick thought about water tanks - have you considered a Sunamp heat battery as an alternative to a hot water cylinder? They are compatible with ASHPs and take up much less space. They are like a box full of that hand warmer stuff - the heat energy is converted to chemical energy, then back to heat energy to give you instant hot water. Unlike lithium batteries, they have a much longer life span and can be charged and discharged many thousands of times. www.sunamp.com 

We haven't got one but I wish we had known about them before we got our ASHP installed with a big cylinder and buffer cylinder that take up quite a lot of space. We have both of those in an outhouse which is a single skin brick building. We use it as our outdoor airing cupboard because despite good pipe insulation, the various pipes and cylinders still leak heat into the room. 

@pumpo-sorcerer Sunamp Batteries are not compatible with all heat pumps, they require a flow temperature of above 60 Deg C which not all heat pumps can reach and to produce this high temperature is very expensive. For a heat pump to be efficient, you must keep the flow temperatures  low, we design systems to never go above 40 Deg C with radiators and 30 Deg C with under floor. Hot water heating, if the cylinder is correctly designed, should have a flow temperature of no more than 55 Deg C. The cost of domestic hot water with a Sunamp battery is nearly double that of a conventional, well-designed cylinder.

We do not install them for this very reason (and they cost nearly 2 times that of a standard cylinder), they do however work well with solar PV.

@novemberromeo

A cylinder can be installed outside in a well insulated shed, however, you must remember that if the cylinder is not within the heated envelope of the building, any heat loss is waisted and costs money to produce/ replace.

Without a proper heat loss calculation and system sizing, it is impossible to judge the size of the heat pump required. I would ask all installers if you can speak to some of their previous customers and actually go and see the installations, ask about running costs, quality, performance of the installation and after sales service before making a decision on which installer to choose. We always encourage future customers to speak to previous customers.

I hope this clears things up a bit and helps you to make a decision. If the system is designed and installed properly, you will have many years of trouble free operation, if it is not, it will be a nightmare. Do your homework and you will be OK.

@novemberRomeo, a few points (I'm not a qualified professional heating engineer btw)

Your quotes are in the same ballpark as the ones I had.  The quote we went with was around £16k but was for everything including radiators and pipework. 

Your RHI payment is nothing to do with your actual heat loss or energy use.  It's calculated from your EPC-defined energy use (which is capped) and a nominal COP for your ASHP that your installer will tell you.  The COP used for mine was 3.7 (I know I will not achieve this in reality). You can find the calculations in the Ofgem documentation.

You should be able to work out how many kWh your oil system is delivering per year.  It's a fair assumption that your ASHP will have to deliver a similar number.  You can get some idea of the size of ASHP required from that, although it will be an estimate. The estimates I had prior to getting the heat loss survey used the numbers from my EPC and they got the ASHP sizing estimates broadly right.   

Good luck and keep us updated.

@heacol Interesting...I didn't know this!

@kev-m Cloce but there are a few things that need to be clarified.

We find that the actual energy used once a heat pump is installed, it is usually higher than previously used with oil,  this is because a heat pump is more effcient, people tend to heat their houses better.

A well designed system on radiatore, should achieve a SCPO (anual COP) of 3.7 easilly.

Be very cairfull with using the EPC to scize the heat pump, it is a very inaccuarate doccumant and bears little resemblance to real life. (I am an energy assesor and produce EPC's)

A proper heat loss calculation is critical for the correct scizing of the heat pump and a rom by room heat loss calculation is critical for scizing the radiators, the installer is obliged to supply you with them to comply with MCS.

@heacol, can't disagree with any of that.  I never thought about an ASHP delivering more energy than the old boiler; I always though the relatively high cost of electricity would make people less wasteful (wearing a jumper, keeping doors closed) rather than just cranking up the boiler. 

Thanks everyone.  More food for thought.  I'll look to get an up to date EPC done in the meantime, so at least I will know for sure what the RHI payment will be from each supplier once I have the COP of each system.

@novemberromeo That is not a bad plan, when you are choosing the installer, take note of the predicted flow temperature at duty point (-1 to -5 depending on the area in the country), the lower the flow temperature, the lower the running costs will be, the higher the grant will be but it may be more expensive. The extra expense is well worth the investment in the long run. In addition to this, ask them what the weater compensation curve will be set to. it should start at assound 20-25 deg C at 15 Deg C ambient  and  45 Dec C radiators , 35 Deg C inder floor at duty point. If they are running a flat curve, your installer does not understand heat pump opperation and your heating bills will be very high.

DIY Alternative Technology has been a long term interest for me since the early 1970s, what with 3 day week electricity blackouts and especially since the oil crisis back then. We built a small aero-generator from bits'n'pieces to charge car batteries, and a thermo-circulating hot water system from old black painted CH radiators - all amazingly inefficient. This publication was key back then https://undercurrents1972.wordpress.com/contents/

Later it was a 70s-80s renovated super- insulated Norfolk cottage on a smallholding, with occasional long distance cross-country trips for inspiration at CAT in Wales.

I have made several of my own solar HW panels and installed the whole systems, in 3 different countries, which worked fine, especially when combined with woodstove and backboiler; probably too old to do it myself again, but we are just about to move into a bungalow with low pitch roof (50 sq m of south facing surface) which is SO tempting. It is a 50s build which needs much insulation, DG, and new heating so have ventured onto this forum to get up to speed with current approaches, particularly PV. Do we make decisions just for our last few years remaining or the future of the planet? A reasonable compromise probably, I suspect.

Hi everyone. My partner and I have just moved to a house near Bristol and the plan is to get solar on the roof, a Mixergy cylinder, a home battery and possibly an ASHP. We had solar and Mixergy in our last house so I'm sort of happy with that side (although a good, usable, single app to control everything would be nice) but the battery and ASHP are new to us. So I'll be asking questions and posting progress... Looking forward to finding out more as the project progresses 

@alastair, welcome to the forums and look forward to your questions.