@derek-m that appears to be how it is rigged and over the last 4 days we are pleased with the results. I have rigged 21 degrees with a fallback of 18 and set the system running from 4am to 10 pm so this morning at 8 the house was 20 degrees and i am hitting about £11-12 per day in electrical use which is below my usual electrical + LPG + wood daily rate so happy - so far!

Just to update after a month with some quite cold weather. We now have a house that is sitting at 19-21 degrees all day with the lower night temp set at 18. Our COP is 3.1 for the whole of march but on the days when the temp was above 8 it was over 4 my costs for the actual heating of the house varies between £1.44 and £7 per day. My LPG was £9 on the old fuel price plus the cost of wood for the wood burner which has not been on for the last 4/5 weeks. Water had to be turned down to 45 as it was far too hot (except for the weekly Legionella run) and the heating costs above allow for hot water including a large bath every night.

our total installation was a shade over £25k but this included 2 pumps but mainly the difficulty of re piping the whole property which due to the oddity of the build is rather difficult. I had months of doubt after ordering the system even though i had done my homework, the results far exceed my expectations to the extend that it has been heard whispered that the house is a bit hot - something not heard here for 38 years

@sapper117 

sounds good.  Cheaper, warmer and most importantly a happy partner.  Our wood and coal burners are now also nice to haves rather than the essentials they were.

Did you get your RHI application in on time? 

@kev-m hopefully yes but as we took the scottish govt 7.5k cash back our RHI is reduced by this. I did all my calcs simply by removing LPG and wood purchase, and to top that we are just waiting for clearance to fit an 8KWh plus storage unit in the field to the side of the house (agricultural land so we need planning but i would like to see them turn it down in the current climate!) if our rate goes up to 28p p KWh then it will repay itself over 5 years then really make us a good return 

@Sapper117 - this may be a bit forward but would really like to get some detailed info from you on your system and installer, particularly as you are roughly in my area of the world (I am in Edinburgh with an 1880 sandstone house). Can you share info on your installer? I'd also love to see the units in place to get an idea of sizing etc. but realize that may be asking a bit much. A contractor here in Edinburgh indicated that 2 x7kW Vaillant units with a 250L HW tank would be what I need, so your setup seems like it could be a copy and paste of what I could end up with.

The firm are Renewable Clean Energy they have fitted over 50 systems in the area and cover Fife. There are many companies sprinting up like the old solar panel days but unless they are trained and really know their stuff it is you who pays!  i strongly suggest you give then a ring. We have a very similar system to that you mentioned perhaps a slightly smaller tank with buffer in attic above tank. 2 pumps as in the photo quite large but quiet - our bedroom is the window to the right of the right hand pump and poses no problem even when it comes on at night which it does to keep the main house at 18 degrees ready for the boost to 21 at about 6am

i have just checked the amount of electricity we used for the full march and it is showing that we used 1137kwh so at our price bans march with the cold weather cost us £199 so i am very happy with that we have also found that after the first week/10 days as the house warmed up our costs seemed to drop a little. The expensive bit was ripping out all the micro bore we had installed 38 years ago which at that stage was easily installed under flooring etc we had a much bigger job and decided in the end to run some pipes surface - next job is to box them in but not critical! The large rads and wide flow pipes have ensured the main aim is achieved and we have never been this warm and saving money 

if i can be of any help please let me know we are north east fife and quite exposed and yet it all works (so far!)

@sapper117 - thanks, really appreciated!