sharpener

Well yes, I have no idea what happens with the average domestic install, I suppose it is done with the boiler "service" or not at all. Your case is a bit unusual in that you have a recent vented installation,@Beelbeebub has many for reasons above. In a commercial setting I am not surprised a G3 ticket is an insurance requirement (though the course is not that expensive). If there is a suspicion of tenants fiddling with the system then it might need someone with a G3 ticket who knows what he is looking for to spot that. I am not a professional landlord but in the past when I rented a flat out nothing surprised me, best was a tenant who wanted me to pay for an electrician callout, turned out he admitted to knocking the standard lamp over and the bulb blowing also tripped the RCD, muppet! How did they know to get their (unknown) UVC inspected then? Or was it bc you put them all straight <g>? Yes, the concept of the tundish is pretty rubbish, at full flow from the relief valve it splashes out of the cheapo plastic ones and goes everywhere. Since they are usually tucked away somewhere the idea that the overflow will quickly come to someone's attention is risible. My D2 pipework was installed by G3 qualified person and is fully compliant regarding size vs length, but still will not quite take the full flow when I test it without backing up to the tundish and overflowing, and that is after I had to relocate the bladder trap myself - before which it was much worse.

From https://www.vaillant.co.uk/specifiers/products/cylinders/: coil SA is 145l slimline 1.0 m^2 200l 1.4 250l 2.4 300l 2.8 So you are correct, they are a long way from meeting their own recommendation of 1.75 m^2 for the 7kW unit, and a factor of >2 out on requiring 3.0 for the 12kW.

You've mentioned this before, I am not sure it is mandatory to pay a third party to do it, what is the basis for yr thinking it is? For those paying for an annual boiler "service" it is a trivial amount of additional work and I think routinely included.* Just looked it up, my UVC mfr (Ariston) says the following should be done annually by a "competent person". Hard to argue a degree in physics is not sufficient(!): Clean the filter screen in the PRV, test the pressure vessel pressurisation and the temp and pressure relief valves, check that the drain pipes (D1 + D2) run clear. Check the pressure reduction valve works OK by measuring the pressure at a draw-off point (I doubt the professionals ever do that, if the spring in it fails [which has happened to me] flow/pressure will be catastrophically low anyway). I test the valves more like 4 times a year as by repute they have a tendency to not re-seat properly if not used in a long time. *Not that they do any work! This is another racket, Domestic and General which underwrite a big proportion of maintenance contracts admit that their policies only cover an annual inspection and they do not carry out the service procedures recommended by the boiler mfrs.

FYI I had the following from Vaillant tech support in respect of the Arotherm Plus (R290) HPs: "The rule of thumb for sizing a coil surface area for a heat pump is 'for every 4kw of heat you need 1m2 of surface area'. This would indicate a 12kw unit would need a 3m2 surface area coil within the cylinder." which to my mind demonstrates a woeful lack of understanding of the product that they promote as being capable of 75C flow temp. I am continuing to press them for a better answer.

I understand why even with weather comp S-plan is less than optimal because you then feed the CH at the HW flow temp. But Y-plan? FWIW the HP would replace a 28 y/o oil boiler. We currently have its stat set to the min which is 65C. Weather comp not possible because this would result in acid condensate all the time. It has an economiser in the flue and this produces a small amount on startup, the condensate tray has been replaced twice as it is. No, since the HP will increase the flow temp until it can get rid of the heat, the reheat time will be no worse than before. 6kW min o/p from an HP will still be twice as fast as the immersion heater. In any case it is not a consideration for us, the 210 l tank only needs heating once in 24h, atm entirely by PV. No, the objections I have to replacing an OSO s/s tank in good working order are none of the above: (i) massive disruption in order to dismantle and rebuild the airing cupboard, and lift carpets and flooring to run new pipework or site new tank in utility room meaning there is no space for the thermal store I also want (which will fix the turn-down problem anyway as it will be a parallel sink for the heat) (ii) cost thereof vs the small marginal savings given electricity is E7 night rate or free PV (iii) waste of resources in scrapping off a tank prematurely (iv) it is an unthinking brute force approach which totally lacks elegance in its design.

Just so. I have said to Vaillant that if they can relax the automatic requirement for a new cylinder it would be a competitive advantage for them as they will sell fewer cylinders but many more heat pumps.

Well done for persisting @JamesPa, another small step in the right direction.

That's the situation I inherited. It struck me as very unusual but I can only think that (with an S plan setup) it is so the coil cct (which is very short) does not starve the radiator cct when they are on at the same time. Otherwise I agree with all your calcs, I have put similar to Vaillant - as they make a point of pushing their 75C flow in their advertising, wot is point if you cannot make use of it to avoid having to change your cylinder? Will report here in due course. PS would not fit a Stuart Turner booster again. Despite what they say some products are rebranded Far Eastern imports. Have had a shaft seal fail, no spares stocked, zero support after 18 months. Full price replacement has recently developed an unexplained crack in the plastic impeller shroud, with drastic drop in o/p pressure, have now built one good pump out of the bits of two. Endless hassle with it, will buy DAB next time.

Thanks, presumably you didn't want to publish the tables here. Even if the turndown is better than I expected the big takeaway for me is that they will not achieve 65C flow below an OAT of 5C. As you may have read on this thread, because of the small coil I will need a high flow temp to drive the heat through the small surface area. Hence my interest in R290 HPs which will provide up to 75C flow, the CoP is not good but it would be running on E7 whlile producing the DHW.

Precisely my point, which means if the 16kW has 2:1 modulation (which I don't know but would be reasonable) and turns down to 8kW then so will the 12kW which is quite poor and would not cope with the small coil in my cylinder without cycling.

Other things being equal then it is important that you try and avoid getting a model which is software restricted, because you will automatically get a worse turndown ratio. The York range seems to achieve quite a high output from their middle frame size, 16kW from a package 1040 x 865 x 410mm is pretty good - a "thin" design and well inside the 0.6 cu m. Unfortunately I am looking for 12kW which is the lowest of the three ratings with 1.8 kg of R32, so the turndown ratio would almost certainly be poor (?min o/p of 16kw/2 so maybe only 1.5:1). In these circs better to choose a different mfr with the break point at a different rating. From the engineering point of view I am not sure about the modular concept, it's nice in theory but there are a lot of things (pcbs, reversing valves, pumps) where the costs don't increase much if at all with rating). And installation would be a faff unless like Pylontech batteries you just physically stack them one on top of another and all the interconnections are made automatically. Which I can see being expensive on the hydraulic side.

The first unit may be PD*, but after that they all need express Planning Permission. *principal requirements are MCS install and not more than 0.6 cu m, which is likely to apply to small units. If the mfr varies the refrigerant charge I would expect the min would vary along with the max. If they just limit the max by a software truncation then it wouldn't, seems like York are in the second category. So are Vaillant, the 10 and 12 kW uinits both have 1.3 kg R290 and AFAICS the output map for the 10 is simply a subset of the 12's with the top end chopped off.

Wow, thanks for the full model. It seems not to be quite linear in delta T but by a bit of extrapolation it looks to transfer about 7.18 kW at W = 55 and falls to 6kW at W = ~61C, would that be about right? It seems plausible at least and so a bit of margin for scaling (not that I will get any with the rainwater, on the outside of the coil at least). Separately I calculate 34 litres/min required to shift 12kW at delta T of 5 deg, which in 28 mm pipe is 1.1 m/s so all good. I will tackle them with this but am not overly hopeful, I fear they will not countenance running at 75 any more than they will run the rads at >55 or the UFH at >45.

The thought experiment was about adding the TS and DHW systems myself after the MCS install but that wouldn't work. Certainly I could reprogram it how I want once installed. Yes but only I think within the timing window you programme for the DHW. Also they typically have a programmable max duration and lockout times to prevent it hogging the heat supply. Anyway a 210 litre cylinder takes about 10kW to heat from cold so with a 300 TS as well the total will be about 25kWh. Worst case a 12kW HP would take 2 hrs, leaving 5 hrs for space heating. So it would probably be split it into two sessions either by choice or by operation of the default settings. I envisage the HW/TS getting up to temp by 0600 so the UFH can come out of setback before we go downstairs. In the depths of winter there will also be the AGA to warm the kitchen.

With a 2:1 disparity yes I think you should. Either one of them is going to cost you much too much or the other one will not heat your house. Remind us of its age, construction and floor area and we might be able to suggest something. For reference I have had several estimates now and am reasonably confident a 12-15kW HP will be enough for a 200 m2 barn conversion in the SW.

I have had no luck on this score with Vaillant tech support or their recommended installers despite the following claim in their publicity material: I am assuming that if you can run it up to 75C the heat transfer to water at 55 will have no difficulty in getting rid of the minimum o/p, maybe you can check for me if you have got the formulae handy for a 0.8 m2 coil @JamesPa. According to this chart the 12kW unit will defo turn down to under 6kW and that is for the stated conditions "Topný faktor (COP) a topný výkon při A/W 65-57" (Heating factor (COP) and heating output at A/W 65-57) They don't publish figures for 75C but they can hardly be any greater. Running off E7 as I propose to do even a CoP of unity would be fine and it achieves this OK (at 65C) but I get met with "the CoP will be awful" to which I reply "E7 is so cheap in comparison (1/3) that I do not care in the least". Though heating up from cold it will not be as low as that for most of the journey anyway: If it turns out to be less than 1.0 in practice I can always use the immersion. Thinking about it maybe I should tell them that is the plan, no that won't work, MCS says it has to be the HP even if less efficient. HTH

I have my immersion which is fed from the solar diverter set to 55C, and it has priority over the separate thermostat for the oil boiler which is set to 50C. They were originally 5K higher but the rainwater dissolved copper from the pipework which ended up as a green deposit in the shower tray. Never had a problem with any organisms, perhaps they are poisoned by all the Cu? Shipbuilders would put a copper coin under the foot of the mast as it prevents fungal attack.

Maddening. A friend of mine had a newbuild where they had wired the room stats to all the UFH actuators completely at random so the entire house went into wild oscillations. Sounds like the same kind of thing.

Ah, slightly unusual situation. Well done, Cool Energy seem to have a good rep on here, are you pleased with it? (You allude to problems) Have sent my three penn'orth to both, will see what happens.

How did this come about? Did you have the calcs and the design from another supplier? Did you fit the cylinder and buffer yourself beforehand? Were Nationwide happy about finishing off a part-complete project? Or have I got it all wrong?

If you can let me have details of your contacts there @JamesPa I will send them the following: <Government's Air source heat pump (ASHP) grant system only benefits the closed shop industry. It costs more to the end client and taxpayer, and is a hindrance to the UK reducing emissions. I have been trying to install a heat pump in our barn conversion in place of the existing oil boiler. I have contacted numerous installation companies as well as two utilities that offer heat pumps, Good Energy (who have quoted) and Octopus (who say our installation is too complicated for them). 1st problem: the house is large and has solid stone walls and so needs a large (12kW) heat pump. These are bigger than the 0.6 cu m limit under Permitted Develpment (PD) so it needs formal Planning Permission. This costs £270 and will put a minimum delay of 10 weeks into the project. Some planning authorities also use this to impose impractical noise limitations far more stringent than required under PD. 2nd problem: the existing hot water tank has a heating coil that is "too small". All the installers insist I will need to replace the current tank which will mean dismantling the airing cupboard to get it out and ripping up the landing floor to fit new pipework, at an extra cost of £2k or more. I am a Chartered Engineer and have proposed three different technical workarounds: an extra pump, a separate heat exchanger or an extra plain tank. None of the installers will listen. 3rd problem: the government's Boiler Upgrade Scheme offers a grant of £5000 to fit a heat pump, but this is only available if you use an installer registered with the Microgeneration Certification Scheme (MCS). In addition, the fitting of heat pumps is only ever exempt from Planning Permission under the Permitted Development rules if you use an MCS installer. This requirement has nothing to do with Planning or the environment but was written in by MCS themselves. So the MCS installers are a cartel which has little incentive to compete on price and will not entertain ideas that would reduce the installation cost (see 2. above). In practice the £5000 grant is spent on the additional paperwork and the over-specified systems that risk averse MCS installers like to fit, and does not benefit the consumer. 4th problem: ordinary plumbers that are used to fitting gas boilers are kept out of the marketplace. The additional costs of registering with the MCS scheme and completing the resulting paperwork (anecdotally over £1000 per job) dissuades them from entering the heat pump business, so perpetuating it as a closed shop. 5th problem: householders cannot easily do the work themselves. If they fit even the smallest heat pumps it requires Planning Permission. The major heat pump manufacturers only offer extended warranty terms if they are fitted by an approved installer (in practice, an MCS registered firm). So by keeping out the ordinary plumbers, and houselholders who are competent to fit their own systems, it is a nice business for MCS companies. But there are far too few of them to meet the target of xxx installations per year so the requirement to use them needs to be abolished if we are to get anywhere close to meeting those targets.>

Output. You could connect the 3.4 to the output and that would keep going and the 3.1 to the input and it wouldn't. If you had the 8kVA unit then both could be kept going. Or sell the 3.1 and use an MPPT like my setup. Depends rather on the value you place on riding through blackouts, and whether you need more than 5kVA (4 kW continuous) to power your loads. If you want to integrate yr generator then the Quattro range have a Grid input plus a Generator input, they can be configured in all sorts of ways e.g. autostart the generator according to state of charge etc. But more expensive. 5-9kWh of battery strikes me as small, I put in 7.1 to begin with but upped it to 10.65 as it wasn't enough to keep everything going until the E7 cheap period. Also I needed more capacity to use all the current available from the second PV array. 2 hrs endurance in a blackout is a good rule of thumb for min size. Various solutions to this on the Victron forum.

Victron is the go-to brand, it will do all the above using its ESS software with fully automatic changeover, no external switches or relays required. Can connect an equal amount of PV to the o/p and they will keep going in a blackout. What size are the two you have got? I have had the MultiPlus II GX 5kVA for 18 months now, DIY install, is large enough to supply the whole house, works a treat with 10kWh of Pylontechs. I have 3.68kW of AC PV and another 3.24 of DC connected via an MPPT converter. Good support and excellent user forum here.

Yes, it struck me that way, there was some earlier discussion (and a link to the Solarcoil website) in this thread but I don't think we found an answer.

Made worse by the only real measure of success being the number of grants doled out not the reduction in carbon emissions. Govt departments really really want to spend the cash once it has been allocated by the Treasury, because having them claw back unclaimed money is an admission of failure. Quality of outcomes ranks a very distant second. But given the current handwringing about the poor uptake under the BUS I do have faint hopes they might make it easier to get at the money before the next election.