Jwenty

Another cleaver work around. I'll be sure to have a look at the immersion location. Thanks

thanks ReedRichards another interesting point. I think my case the PV would meet max ASHP load but will check. Thanks also Gmarshall, i will take a look. but for not it does seem the timing ASHP DHW demand and using immersion for the excess is likely the simplest. I guess it will be some experimentations with the actual loads and generation i can achieve. Thanks again. Joe

Thanks all this makes sense and I see this issue with the ASHP min load. As none of its installed yet, i will see what the actual loads and generation look like and start to have a play around. Thanks again. Joe

Hi We are looking to install an ASHP (vaillent arotherm plus 7kw.) in our mid terraced house (significantly renovated). We are also looking to install a small (3kw) PV system. I wont have the cash for PV + batteries and as they don't add any CO2 reduction overall I'm not that interested. But we will have the hot water cylinder as a good place to use excess solar. But it seems most systems just dump the PV into the immersion which is OK, but with a heat pump with with COPs of 2+ for DHW is seems it would be better to use the solar to drive the HP? Are there systems that do this? or is it just better to set a timer on the DHW on the HP to coincide with the solar peak? Thoughts welcome. Many thanks Joe

thanks for this too, it makes me have more confidence in my measured usage vs the calculated usages. As the house is both heavily renovated and mid terrace it seems the MSC value of 120w/m2 is OTT (heat geek sugesting 110 for an uninsulated single glazed detached house ). my measured heat loss of 56 kwh/m2 seems to fit well on their scale.

Thanks for this tougher response. this what i had understood. good point about the variable heat output at varying out side temperatures. the ASHPs i have been looking at look to have good performance and maintain their rated power to the very low temps. I think all I can do now is re calculate the heat loss and compare the three figures and work from that.

thanks for the heads up, i will check this but none of MCS surveys have mentioned it yet....

hi Thanks for this it looks like i need more calculations to compare too. interesting point about assuming the neighbours stop heating. that would make a massive difference in our tall skinny mid terrace. in that case we might need a huge HP! we do not have an up-to-date EPC (it no longer has to be 2 years old it must not recommend loft of cavity insulation. ) but interesting his calculations are even higher than the old epc from before the improvement to the loft , extension and insulated basement slab. although i get the impression the EPC calcs are quite basic?

thanks i will do this

I divided the heat use of the house by the total annual degree days, giving the heat needed for every degree difference between in and out then working of the coldest average day being -2 i multiplied by the temp difference of 22. so this should give the peak load rather than the average as i understand it. Even if this is incorrect the overall all annual usages are a factor of 2 different. agreed that all engineers want to lean towards oversize.

Thanks steamy, I'm having a go at the calculations myself, but I'm finding it slow going, its such a mix of materials and finishes. but I'll get there... i will explore this plumber option, but this means i need to be even more confident of the sizing if i take the decision solely on my shoulders. the cupboard where the boiler is, is also the best spot for the hot water cylinder and is not big enough for both. as its a narrow mid terrace there really are few alternative locations on external walls for a flue outlet. and really we are looking to leave gas behind. thanks

we do not have heat recovered ventilation. we have extractors for the the bathrooms, and trickle vents. the closed off fire places have louvered vents so can be opened or closed manually. From memory the fire is small enough to not have required a dedicated air brick.

Hi Olf, space does not allow keeping the combi boiler and we are looking to use the UK 5k grant when the gov get round to launching it. so we do need to use MCS fitter. we have several people quoting so I'm not ties to this one and i think i could make them tweak the calcs to be closer to my readings to get the job done (e.g. increase the airtightness they have used. ) we do have back up of a small wood burner so there is back up allowing us to risk under sizing slightly. Although in Bristol there is the risk the wood burner will be banned at some point.

Hi, perhaps someone with more experience than me can help. My gas usage for the last year is around half that suggest by a recent MCS ASHP survey. I really don't want to pay for and live with a double height HP unit if its not needed, but don't want to undersize either. I live in 160m2, 4 storey mid terrace house in Bristol. C 1870 Extended to the rear and loft so half of the exposed walls are essentially new build levels of insulation and air tightness. Floors are fairly well insulated. Mixture of rads and wet UFH. Fully double glazed, half the exposed walls are 300 and 400mm uninsulated solid brick/ stone. as mentioned we are looking to install an ASHP. my measure gas usage does not match the ASHP installers calcs. My figures for heat loss at at -2c are 4- 5kw and theirs come out at ~9kw. (i calculated this from measuring gas usage over the year, used degree days to establish a heat loss per degree difference in/out. then see what it gives at -2c ) Annual usage is equally off. Our gas usage was 9000kwh for the year and the calcs suggest 19000kwh. (no calculation by me on these numbers) For refence that gives a measured value of 56kwh/m2 vs calculated 119kwh/m2 Finally I ran our combi at 40 degree flow for the past winter (coolest average day was 2c ) and maintained 20degree internal temp fine. but the calculation suggest the radiators would only keep interior temp of 20c with flow of 55c and outside temp fo 3 degrees. which would be far from ideal. Do calculator tend to over estimate? I have worked hard to reduce drafts, perhaps the clacs are based on an assumption for an old drafty house ? any advice on if my measured figures seem plausible? The renovation has been done well but not to enerPHit. opinions on whether oversizing or under sizing is the better option welcome. Our primary aim of ASHP is CO2e reduction not financial. Thanks in advance. Joe