sharpener

I think this is defo over-specced. (5 bed + 1) * 45 is 270 litres so a 300 l tank meets the rules and prob sufficient. Heat Geek say allow 20 - 40 W/m^2 for a post-2006 new build so worst case that would be 11.2 kW. (For comparison, two recent full heat loss surveys for my 200 m^2 uninsulated stone barn have come in at ~12kW) NiBe are notoriously expensive and mandate the use of their pre-plumbed tanks and other quirkiness.

AFAIK it has to be "visible" somewhere, Ariston MIs say ideally at both locations but if "practically difficult" one is sufficient so just at the tundish will do, mine is in the airing cupboard. D2 goes into a waterless trap on the soil stack. Was signed off by the plumbers who did it. Have moved the trap lower down so there is more head but on test at full flow I can still hear it backing up to the tundish, D2 complies with the rules, have stopped worrying.

I recall being told in the past that MCS specify 45C max flow for new build and 50 or 55 for retrofit situations. But I can't find chapter and verse for that and it may of course have been superseded, or was just a myth from an earlier installer trying to defend his design decision. Not entirely, only that Paragraph 5.10 of Part L1 2021 quoted here does not seem to be relevant. Other parts may apply, there are also a CIBSE design guide and BS/EN that may be applicable. Would like some arrows in the quiver as I am expecting some pushback if I ask for rads to be spec'd for a flow of 52.5 (which the HP will just do against the predicted heat loss). Much less and we are into rearranging the furniture and massively thick K33s. Yes some of it looks conservative beginning with the min OAT of -1.41 which seems unlikely as the HP is at sea level and 25m from a tidal estuary on the S coast. Someone here must know the facts, maybe the self-builders!?

Have thought of fan assistance of various types. But OH will not have a fan of any kind in her study, which is the most difficult of the rads to re-size. I might try and identify a removable fan accessory which could be fitted later if necessary to placate the installer. But the main thrust is still to find out what is actually allowable under MCS and other rules.

I have now got two quotes for a heat pump install. One is for a flow temp of 45C and the other is for 50C (with mean water temps of 2.5C less). The heat loss calculations have various other differences which make a direct comparison difficult, however the total losses are 12.3 and 11.5kW respectively. The exisiting radiator sizes for my boiler CH are mostly too small already. So to get reasonable sized replacement radiators when I move to a heat pump I may need to have a flow temp higher than 45C. (The upper bound of 52.5C is set by the 12.2 kW max output of the HP at the -1.41C design OAT, any higher and it will not meet the total heat loss. In warmer conditions the WC will of course result in lower flow temps.) MCS's publications MIS3005 and its accompanying Best Practice Guide say only AFAICS that flow temps >55C are regarded as "high temperature heat pump" installations, and need to have a standard HP quote alongside for comparison. There is also MCS 021 - Heat Emitter Guide but that sheds no further light other than explaining the star rating applied to MWT. So what are the actual rules about this? The Building Regs Part L have recently been changed to specify 45C for new builds with HPs, and 55C for existing buildings for any complete heating system renewals inc pipework (which is not what I am doing). As a further twist the feed to the rads will mostly come from a thermal store heated at off-peak rates. So the absolute efficiency is perhaps not as much of a driver as might normally be the case.

Yes, on your figures 24kWh is 24 x 860 = 20,650 litre-degrees. But in my analysis I was assuming 55C max (as that will in parallel charge the HW tank to 50), and 30C min (as the lowest temp at which the rads will give any useful heat to the bedrooms). So exactly half i.e. 12 kWh thermal with a 400 l tank. Hence Cosy looks good bc you get two bites at the cherry every 24h and the 1300 - 1600 is when there will be relatively warm air outside as well. Am now a bit more hopeful I can get it to work as I have at last discovered an MCS installer who understands the concept (and has given me a reasonable quote). Rather than others grudgingly saying they will follow the Vaillant schematics at my expense if I really really want. But as upthread the main reason is it will avoid needing to replace the HW cyl, the TS is actually a bit cheaper so the marginal cost is just the cost of 1 pump, 2 extra valves and some pipework.

You cannot design an HP installation on the hoof like this and expect it then to work well or indeed at all. Stop and think. Plan the work then work the plan.

IIRC @markocosic informed us that Samsung use hot gas bypass to achieve their (not very good) turndown figures and this nadgers the performance/efficiency. I wonder if this is true of the current model range?

As an illustration of their thoroughness I have just had 3 more MCS installers do a (first) site visit. None of them said anything at all about noise despite the suggested location being visible from a small window on the adjoining property and by inspection about 5m away. A previous installer and my own measurements confirm that as the Vaillant 12kW is a very quiet machine for its size we should be OK, but nevertheless it might require a partial or even a full screen to get the last 5 or 10 dB. Fortunately we have express PP. The planning decision appears to have been based on an (erroneously generous) EHO report as follows, the quoted 36dB relates to the 5kW unit, the 12kW is 6 dB worse. We have considered the application and the proposed make and location of the heat pump. The pump is a low noise pump with sound pressure level at 3m being around 36-38dB. The nearest neighbouring residence is further away than this and there would be no direct line of sight with windows to a habitable room. Therefore we do not anticipate that there would be any unreasonable noise impact from this unit.

Or IIRC in old money 0.0002 dynes/cm^2 (I think that's where the figure 2 comes from) being a round-number approximation to the threshold of human hearing.

Presumably they would make a site visit and if it appears from a quick look that the MCS requirements regarding distance, screening etc are met then not go down that route. I was alluding to yr own particular concern that the EH people can and do apply much lower noise thresholds than MCS.

AFAIUI a Certificate of Lawful Development is a way of regularising something that has already been built (e.g. our loft extension which was not constructed as designed). The process is anyway much the same as applying for full PP in advance and IMO this latter is the way to go. It is what we have done for our 12kW HP which falls outside the deemed PP on account of its size. I was careful to specify it would comply with the noise aspects (only) of the MCS standard. This was to give scope for a non-MCS install. The LA granted permission with a condition that it is installed according to the plans, but with no mention of noise or any other conditions which I thought was surprisingly lax. The fly in the ointment is that the high end HP mfrs will only offer extended warranties via their own approved installers who are almost invariably MCS registered. Hence I have recently had site visits from 3 more in an attempt to get some competition on price. If there is a complaint about noise then as others would point out it becomes an Environmental Health matter and would be dealt with under quite separate legislation. The fact that the MCS standard and hence planning law was complied with (whether by an MCS installer or not) might help but would not necessarily be a complete defence. It seems this wording was put in to allow for the possibility of other standards bodies writing equivalent standards but AFAIK none exist though @JamesPa has in the past referenced some entity which is working on an "equivalent" scheme. IMO it would have been better if e.g. BSI or BSRIA were responsible for the standards rather than a body with a very clear vested interest.

Yes I now remember I did some sums and would have needed 12in ducts to bring enough air in for complete heating/cooling. Might be useful as a topup or for specific rooms I suppose. OH likes fresh air and would have all the windows open given a chance. OTOH I would wake up with a headache if the CO2 level was too high. Michael Podesta has written about using CO2 levels to assess air changes/hr on his blog protonsforbreakfast but I was more confused after reading it than I was to start with. And am normally quite numerate.

We have a Villavent 400 MVHR system with inlets in the bedrooms and extract points in bathrooms and kitchen. The noise on medium speed is perceptible but not a nuisance. It is on a timer for setback at night, when it is almost inaudible. When frying we put it on boost which is noticeable in the kitchen (but not as bad as a fan hood or the control panel cooling fan in the cooker(!)). You can get fancoil heating units to put in the ducting, I don't know if they work for cooling, you would need to do sth about condensation. To keep the ductwork clean we have added bonded acetate fibre filter discs to all the extract points which we change every 6 months, they collect a lot of cooking spatter and towel fluff, I am surprised they are not a standard fitment.

I think the S2 models are what are shipped to the UK market. The installation manual on the Midsummer website is specifically for those variants. It has come up in the past on the Arotherm plus FB page.

Do you need to have it done to maintain the warranty? £200 - £300 is not unusual. Otherwise DIY as above, also clean any Magnaclean or Y-strainer filters etc. Inspect for leaks. Check glycol concentration if used and system pressurisation. Check condensate drain is free. Crack open the pressure relief valve.

Sounds expensive, what part of the country? You can price it as a complete kit on the Midsummer Wholesale website. IIRC cooling was not allowed under the BUS scheme, I can't find a reference so best to check on the OFGEM web site. The Arotherm plus needs a plug-in coding resistor to do cooling, over £200 UK retail but you can import them from the EU for less, I have read it is the same resistor as fitted to gas boilers for some purpose (presumably not cooling).

As someone else in the SW peninsula I would have thought you too were exposed to >1000 mm rainfall/year<g>. Yes, when there is sun OH likes to use the rotary airer for "freshness" but I don't see much point, now we have kitted out the second roof we have 6.9 kW of PV which gives us a surplus most times of year.

Which means that you can't get the 70C plus flow temp that R290 brings. Midsummer Wholesale has this wonderful typo: *For Vaillant aroTHERM Split systems we will require evidence of your F-gas certifiction*

I think this is a retrograde step and shows how little the politicians know about the science. Another of HMG's perverse incentives, they delay the surcharge on gas boilers but then tip the balance back towards HPs with this. Not sure I would want cavity wall insulation (if I had wall cavities). But loft insulation is a total no brainer, the ROI is better than almost anything else you can do and it might reduce the size of HP you need as well so in that case essentially free.

https://www.vaillant.ua/downloads/proekti/0020222099-10-compressed-2406863.pdf?fbclid=IwZXh0bgNhZW0CMTAAAR2cgInGE7BtvCgpdxYDszZUgKRAQE8jm3AIb_4C0KwHKwd_pHVLc5itfYA_aem_AaxrDlhR_6iT7VEGsQhhKVj5KjwLaeJTS-GNzLG_Gy5tBQRLd1zxg8__lVP42DpLQzO2V26rFwsW0FhdKIEFWMSo

No. Most Vaillant installers I have spoken to do not know they exist. Nor did the V regional tech mgr for the SW. I have not found any explanation for why they are not available in either English or German (which I can read well enough). There is also a Planning your Installation doc of which the EN and DE versions are very different, it's all rather a mess, on the FB page there have been various discussions about this.

You can download the full set of tables from the Arotherm FB page here starting at p34. So I can't see any way the 5kW model would satisfy this even with massive rads and a flow temp of 30.

Heatpunk gives 7763W as the output at 53C flow and -2.3 OAT. So by choosing 53 not 45 as the flow temp it would still more than match the heat loss, and you might well need fewer radiator upgrades and maybe saving more in capital than you would lose in running costs. You will probably not find an MCS installer willing to do this though. When I lived in Hitchin there was snow on the ground for some weeks in I think the winter of 1968 - 69, but it was most unusual, I don't suppose you will see -2.3 very often nowadays.

That's probably why they "highly rate" them! Also as an umbrella provider it de-risks the relationship with the actual installer.