gravelld Posted October 12, 2021 Share Posted October 12, 2021 After a recent disaster with our oil tank, I'm checking the feasibility of getting an ASHP installed. A local installer with a good reputation have appointed someone to do the heat loss calcs. These came back and are as follows: Energy required to heat property 26,570 kWh Energy required for hot water 4,588 kWh Suggested ASHP Vaillant aroTHERM Plus 10kW VWL 105/6 A230V S2 Size of Hot Water Cylinder 250 ltr Our current energy use for space heating and DHW combined is less than 18,000kWh - I know this because our oil tank is 1800L. I guess the discrepancy is because we don't heat the whole house to 21C or whatever the MCS survey requires. Also, air tightness wasn't considered in the assessment, which I've worked hard to get down and has been measured at 5.8m3/hm2. And neither is our 3G ("double glazed"). I have suggested oversizing to the 12kW model as it only costs a few hundred more and it should give us some breathing space (and better efficiency?). AIUI the efficiency is related to the flow temperatures. How do I ensure the specifiers have not underspecced and we can still achieve a high efficiency? The specs for this range of heat pumps are here: https://www.vaillant.co.uk/for-installers/products/arotherm-plus-71424.html#downloads I expected more like a 300L HWC to be specified. We have three bathrooms with showers, four residents, but also occasional visitors. If anyone could take their valuable time to run their eyes over that and flag up anything worrying or missing I'd be grateful. Link to comment Share on other sites More sharing options...
ProDave Posted October 12, 2021 Share Posted October 12, 2021 You need to know the heat input on the coldest day of the year. Assume 18,000kWh is for the whole year, and you use heating for half the year, that is an average of 98kWh per day in the heating season. then assume the coldest day is twice that, that so 196kWh. If the heating is on for 12 hours per day that comes out at 16kWh so I would not want to be going lower than the 18kWh heat pump. More important is how you deliver the heat to rooms. If normal radiators they will probably all need replacing to work at a lower temperature. Link to comment Share on other sites More sharing options...
gravelld Posted October 12, 2021 Author Share Posted October 12, 2021 19 minutes ago, ProDave said: If the heating is on for 12 hours per day that comes out at 16kWh so I would not want to be going lower than the 18kWh heat pump. I thought it would be on for more than that - that's the recommendation for a heat pump, no? By these calculations, the 12kW (you meant kW right?) pump would be ok if on for 16 hours... but that's the coldest day so I'm less worried (plus we have a 12kW wood burner to top up - although I don't want to depend on that). Is there a reason you said 12 hours per day? Not sure about the rads - the quote says "Radiator upgrades included if required". Is there a way of deriving a desired m2 of radiator given the heat pump size or heat demand? Link to comment Share on other sites More sharing options...
Nickfromwales Posted October 12, 2021 Share Posted October 12, 2021 7 minutes ago, gravelld said: "Radiator upgrades included if required" Eh? Does that mean a rebate if they're not?! Link to comment Share on other sites More sharing options...
gravelld Posted October 12, 2021 Author Share Posted October 12, 2021 4 minutes ago, Nickfromwales said: Eh? Does that mean a rebate if they're not?! I'm guessing this is the RHI rearing its cost externalising head again. i.e. the installation is overcharged to a non-trivial amount, and the tax payer picks up the bill. I guess? Link to comment Share on other sites More sharing options...
gravelld Posted October 12, 2021 Author Share Posted October 12, 2021 By the way, my own PHPP model comes out at 10kW maximum heating load down to -2C. It also has a lower overall heat demand, I guess because the proper U values of the windows and the actual air permeability is taken into account. Link to comment Share on other sites More sharing options...
ValleyBoy1958 Posted October 13, 2021 Share Posted October 13, 2021 Gravelld - can I be cheeky please and ask what cost you were quoted? Link to comment Share on other sites More sharing options...
SteamyTea Posted October 14, 2021 Share Posted October 14, 2021 Starting with your DHW, if you have a ΔT of 20K i.e. heat from 30°C to 50°C, that will be 5.8 kWh, call it 6 kWh. If you use that amount every day, with no reheating, then that will be ~2.2 MWh/year, you have around double that figure, which seems reasonable. Your 10 kW at -2°C is about right for the MCS minimum temperature (supply 99% of the time). See chart. You can work out your heat loads from the minimum temperatures and how long they are likely to last. Link to comment Share on other sites More sharing options...
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