Michael_S

To be fair, last week with the easterly flow DPs were very low across the board despite the solar warmth. I have just bought a bunch of computer case fans and and going to see how they work with our double rads and flow temp controlled to be above dew point; we will probably run the dehumidifiers in the summer when we have excess PV so minimal cost which should also help with this issue - at the expense of some additonal heat input from the dehumidifiers.

Just wondering how heat pump quotes are calculated. For example my quote of 6200+7500 for a zero disrupt install (no cylinder, no rads) with a heat pump costing 3.5k, a 10m cable run and a 3m pipe run just seems crazy unless I am missing something?

I would like an effective zero disruption install. I think I could use a 9kw Grant Heat Pump costing 3.3k inc VAT run the cabling and pipework myself and even position the unit (total cost about 5k?) so it would just need someone to connect the electricals at the meter end, sign off on everything, inform the DNO, confirm it meets the BUS requirements and whatever else in order to get the BUS grant. Even without this would seem cheaper than the 6k after BUS that I have been quoted. Thoughts?

Space is fine, existing cylinder is properly supported with a steel, but was installed during an extension and the loft hatch is too small for a 600 diameter cylinder to pass. I never understand how the quotes are so much - this one is 6200 after the 7.5k has been taken off for basically install unit on side of house, run about 3m of pipe and tee into existing heating/hot water circuit with a diverter valve needed.

Yeh - strange on the cylinder, with 5 beds I thought MCS would insist on bigger but the optional cylinder upgrades are only 175l or 210l ?? There is no specific mention of whether they would be using a plate heat exchanger rather than the existing coil. Our loft hatch won't fit a standard cylinder so keeping the current one has advantages.....

Yeah, currently we do immersion set to 70c heated overnight at 7p per unit which gives us 5 showers, at say 45C we would need to reheat during the morning shower period but should be doable at say 7kw reheating. [Say cold water is 10c, 70c x 180l gives us 360l at 40c whereas 180l at 45c only gives 210l so I will then need another 5.25kwh - say 45 mins - to heat the remaining 150l of 40C water needed each morning] Either way we also then need to reheat for evening showers. Zero disrupt - I have already changed enough rads that I know we stay warm with a flow temp of 45 (it went a bit higher when we were -5 and below outside but not over 47C) Plan is also to not change the DHW tank and accept the hit to COP.

Indirect 'gas' cylinder Viessmann 180l, about 15 years old

AL4 postcode district - I know the design temp is -2.7 for here but I also know what weather we get. I also currently have a heat meter so know that the heat loss calc is pretty much bang on the money with a 45 flow temp. The quote is using the existing hot water cylinder.

Thanks for the link to the Czech tables, have looked at them before - at 45c for the heating and -2 the output looks sufficient. What worries me is the hot water in addition, I know we use lots and with no tank upgrade we are talking a high flow temp to get the tank to a high enough temperature (happy to aim for 45-50c and use an immersion to do legionella cycle) but that still means a flow of 55-65, possibly towards the higher end due to having to limit tank heating time due to the space heating demand. At -2 / 55 the COP also falls off a cliff, again suggesting that a non-negligible part of the day will be needed to heat the water and thus wont be heating the house. Also these numbers don't include any allowance for defrost cycles.... I can see it might almost work if there was a slight overnight setback during which the hot water tank was heated and then perhaps in the middle of the day when it is warmest there could be another hot water window but this is -2.7. Most winters we have days that average this level or slightly below and we often have -8 coldest nights.

So we had the £250 Heat Geek survey. They have come back with a proposal for a 7kw Vaillant heat pump (apparently they only do Vaillant under this offer) Their heat loss calc says that at the reference temp (-2.7 here) the heat loss is 7.83kw. I know this is about accurate and our rads support this at a flow temp of about 45C WE heat our hot water by immersion and use of order 24kwh per day. There is no proposal to change the tank so lets assume that at the -2.7C day the cop on hot water is only 2. That means another 0.5kw of heat energy needed on average taking it to 8.23kw. And that is before there is any thought about recovering an overnight setback or other unexpected heat loss (power cut, windows open for painting or whatever). Too me the 7kw sounds under powered, are there any charts for its output at different air and flow temps to suggest that it would be powerful enough? What would people think would be the correct size heat pump for this heat requirement - I think we need a unit that provides 9 or 10kw at -2.7 air / 45 flow. Thanks

Do we know how much of an impact a larger delta T has on efficiency - is it just that to have the same average T the flow temp needs to be higher (eg av flow temp of 35C could be achieved with 40 flow, 30 return or 37 flow, 33 return) or do heat pumps actively dislike a large delta T? Edit: Google suggests a 1C decrease in flow T could result in a 2% increase in efficiency so in my example above the 3C lower low rate would save 6% of energy use

Back to the OP. WE have rads up and down, had to use the balancing valves to heavily restrict the flow through the downstairs rads to force enough heat upstairs. Is there a way for you to restrict the UFH flow to increase the upstairs circulation? We followed advice on here to stop using the TRVs on the downstairs rads to try and achieve the same effect as it tended to just make the HP cycle more and the upstairs never got as warm as we wanted, now the upstairs is slightly cooler (20 vs 22) due to rad sizing and flow.

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.