Michael_S

It is all a bit Heath Robinson, we got second hand heat pump to heat the kids pool then the gas boiler broke so we teed it into the rad circuit as a temp solution until we get an official install. This was delayed as we were waiting for the regs to change so we can put it on the side of the house less than 1m from the boundary. That is now allowed so we should get round to getting it done but with a big battery an average scop of 3 is not unaffordable.

I had been expecting posts here and MSE re units freezing up so it is very reassuring that there have not been any. Our install is DIY using a second hand unit teed into a set of radiator tails so we have a bit of a flow issue (about 0.7m3/h). My 'design temp' was 45c rad temp at -3 OAT. IN reality due to the rad chosen for tee in we have 3 rads not working and flow temp of 45 at -3 with return at 36 so I guess average rad temp of 41ish but are still holding temp at 21-22C [By my calcs that is about 7.5kw (45-36)*0.7/0.84 at -3 when the heat pump is using about 3kw so cop of 2.33. When we got to -6 flow temp went up to 48, return 38 so 8.3kw at 3.8kw input 2.2 cop]

I guess an experiment might be to shut off all the rads and see if that helps with the downstairs. Also check there isn't a bypass valve somewhere that is allowing the suspected 'short circuit'.

Nope, should be a constant burn, sounds like your emitters are too small or there is a circulation issue such as TRVs restricting the flow.

Sorry for not being clear, the minus 7 was an example arbitrary personal decision on what outdoor temp you want to be able to cover, I can see this was not clear. Obviously there are the MCS 'rules' based on a set room temp, outdoor temp (by geography) and number of bedrooms which I guess become a minimum if you want the grant but you could obviously decide you want to exceed these to support more hot water or higher room temps or a lower minimum outside temp.

Very impressive heat loss. In theory that outside air temp is actually slightly lower than the heat pump design standard. For heat pump sizing you need to find out if a quoted x kw at -7 (or whatever you decide your actual min point should be) includes defrost cycles. You obviously also need to factor in hot water demand, we do hot water via immersion so know that the 5 of us use more than 25kwh per day so at least 1kw per hour to add on to the space heating demand.

Your boiler is probably about 95% efficient including the hot water cycle so need to downrate the gas consumption by about 5%| Do you know if your cylinder has a suitable coil for ASHP? How far is the installation location from your main consumer unit? Heat pump warehouse has suitably sized Viessman for about 5.5k inc vat - not sure if it is not the ex vat price that the installer will pay, plus any discount they have negotiated. https://www.theheatpumpwarehouse.co.uk/product-tag/viessmann/

OP said they already had 30kwh battery. There is a cycle life and therefore a cost per cycle and of course cycle efficiency to update my first order approximation.....

Suppose the heat pump gets a cop of 4 (not impossible given the low flow temps you are already operating at) 14.3k gas usage at 5.5p per kwh costs £786pa 14.3kwh gas input at 95% efficacy gas boiler divided by a cop of 4 at 7p per unit off peak rate using battery timeshift costs £238pa In theory your heat pump install should be cheap, only need a small unit (6kw?) and only change to plumbing might be to replace the hot water tank with a large coil one and hopefully your battery would be big enough to timeshift your total demand even on the coldest days? Risk is that the new govt plan to prioritise batteries may lead to a reduction in the potential savings from timeshifting due to increased arbitrage.

WE have an older series that has a paddle flow switch. My initial thought might be that you have TRVs that were closing down the flow but with a 4 pipe buffer this should not be the issue. I assume there isn't a mixer valve in the circuit somewhere that is for some reason set to a low value?!

The only supplier I have found with good info on modulation is Viessmann where they show a shaded 'output area' for min and max output at each OAT temperature, possibly even different charts for different flow temps.

WE are also on the 7 hour eon tariff (6.7p) and with V2H and 2 EVs we use up to about 80kwh from batts for our heat pump and general usage in the day rate period. Likely to try for utility warehouse E7 tariff when our eon ends in May, not sure about export at that point.

Wow £1500 compensation for a financial loss probably running in to the 100s of thousands Perhaps if proper levels of compensation (ie to cover the actual loss) were paid the maths would change for these projects but that is only right.

Interesting to see so much rewriting of history- the exact bits depending on which side of the political divide you sit. I think we need a better way of compensating for nationally important infrastructure. If I was going to get a pylon at the end of my garden that would reduce my house price by 200k but was not close enough to get compensation then I would be pretty peeved at being the one who had to take the hit for the greater good. Yes expensive, but why should the national interest mean unlucky individuals?!

I guess reinforcing local grid is an issue for the DNO not the energy supplier so the economics becomes more complicated. I also guess in theory the more short term storage is available the more prices will be arbitraged across the day and thus the less financial gain will be possible from possessing a battery - so the big differences between day and night rates will become a thing of the past and the return on battery systems will fall?