JamesPa

From my reading on here, listening to Heat Geek videos, personal experience with prospective installers, and comments and evidence from another forum Im becoming increasingly convinced that poor system sizing (generally oversizing) is one of the key factors to root out over-engineering leading to poor performance, significant excess cost and significant unnecessary disruption. The fact is that, in a typical retrofit, there are real unknowns due to: invisible fabric upgrades (eg wall insulation) the unknown performance of double glazing, which can have a U value anywhere between 2.8 and 1.4 later extensions of a different construction to the original building the unknown ventilation rates It is thus perhaps inevitable that theoretical calculations err towards oversizing, in my case and others of which I have heard, by a factor of two. However the destructive effects of this are quite devastating, in terms of cost, disruption and the negative effects on performance. Its all very fine blaming installers/surveyors (as I have done in spades because they have ignored what I have told them about the fabric), but, realistically, when they cant verify the key parameters of the key calculation, what else are they going to do if they are anxious to avoid 'im too cold' service calls? The more I consider this problem the more convinced I become that this cannot continue and at the same time lead to satisfactory outcomes. Heat pumps with a high modulation depth help, but don't fix the consequent overengineering of the system. Range-rated heat pumps would similarly help, but again don't fix the consequent overengineering of the system. Of course there is an alternative, namely experimental measurements of whole house heat loss. With a smart meter, half hourly meter readings are available and can readily be correlated with average OAT/degree days to get the required load (and with a bit more manipulation the 'thermal mass', which could help with setting up control parameters). If no smart meter is present then annual consumption plus some information about heating patterns could, at worst, be used as a 'sense check' on the calculated figures. In another thread there are references to companies that make special measurements to determine experimentally the heat loss which are a further alternative Of course some room by room calculations will be needed to size radiators, but getting a radiator too big is nothing like as serious a problem as getting the system too big. Discuss! Has anyone got a better idea of how we might practically deal with the real fabric uncertainties in a typical retrofit? If not then how else do you suggest we square off the problem that we simply do not, in many cases, know, with any degree of accuracy, the key parameters to do the theoretical calculation that is the basis of almost all much heat pump system design.

Thanks for this. Is cascade mvhr the fresh-r system with fans between rooms. I was looking at that and as it happens it might suit my layout. Does anyone else do something similar as fas as you know?

@kron77Remember that this is arguably a desirable feature of dhw tanks not related to the heat source! Stratification means you get some usable dhw more quickly than you otherwise would. Occurs with immersion heaters and gas sources just the same, depending on tank layout. Of course there are also some downsides, but to overcome it requires a pump so it's rarely done because of the positives.

Im looking at fitting two or three dMVHR units to my extended 1930s house, which suffers in some areas from poor ventilation. I've ruled out whole-house MVHR (too much disruption), extract only/passive intake (don't want to make the heat loss worse) and PIV (targets the wrong areas if fed from loft, which seems to be the norm to re-use otherwise wasted heat). There seem to be three principal types/operating modes from DMVHR 1. Units which operate in pairs, in theory forcing air through the house, each unit alternating between extract and intake 2. Units which alone, alternating between intake and extract 3. Units which operate alone, with simultaneous intake and extract Logically (1) seems potentially the best, (2) the worst and (3) somewhere in between. What is the general view, and are there any other considerations or things I should be aware of?

Myson ivector2. Reading the spec again it's unclear whether this offers above dew point cooling or not. I can't see a condensate drain so I think it probably does. Also I then its stelrad that offer a variation of a normal radiator with the fins cut along the middle so they can insert computer fans in a line. I have no idea of any of these work, they are all expensive.

A couple of the fancoil companies advertise 'light cooling' mode, basically keep the flow temp above the dew point and lightly blow air over the radiator. It's entirely plausible that this is sufficient to make a difference in the UK climate and could be done by adding accessory fans to ordinary radiators.

So far as I know they are all reversible because that's how they do defrost. Vaillant require a 'key' to operate in cooling mode, costing £200, it's a 5,p resistor in a 20p plastic case, others may have an installer setting disabling cooling mode. But cooling is not just about the hp, the emitter and distribution system needs to be suitable too. I don't think the government wants to or should encourage the routine use of Aircon hence the BUS conditions.

Correction, this does still appear in MIS3005-d as part of the pre-contract requirements, where it forbids system sizing based on actual consumption. This prohibition is not repeated in the post contract requirements, so it is open to interpretation whether actual consumption can be considered, even if only as a sanity check, or not. The interpretation that is safe for the installer is 'cannot', the interpretation that, at least arguably, is safer for the customer is 'can'. I leave it as an exercise for the reader to guess/discover which interpretation will be used.

Focussing on this bit rather than the experiment with the immersion, I would suspect (as others suggest) stratification in the tank and measurement errors. The hot water will drift upwards from the coil and form a layer at the top and so the sensor lower down might not perceive the rise at all until the whole tank is heated. As others say meters may be accurate to 1C/1kWh so with such a small change you might reasonably measure something completely different from the fact. Also, depending on the flow temp and the size of your coil, the heat transfer might be poor (is it a 'Heat Pump cylinder'?) possibly causing excessive cycling. I would repeat at a lower starting temp for a longer time and also perhaps post some info on coil size and flow temp so a bit of modelling can be done (I do have a crude model of DHW heating with a HP albeit that it assumes the tank is stirred). Of course you may already have ruled all of this out and I might be saying something which you find obvious (in which case I apologise, no offence intended).

Assuming you need MCS you will likely be limited by what the installers will agree to, see (many) other discussions on this forum. The MCS rules come close to requiring a replacement (they don't actually go that far, but they come fairly close) and many MCS installers, particularly in the South East, want to fit pre-plumbed cylinders provided by the HP system manufacturer, because its dead simple. DHW and Heating do not run concurrently, its either/or. Opinions will differ on whether replacing DHW is necessary/a good idea/silly. In your case I imagine that you get solar heating of your DHW for half the year or more and losing this seems like a bad deal, but as I say you might not have a choice and of course if you are concerned about reliability then submitting to general view that a new unvented cylinder with a big coil (3sq m) is the way to go that may be enough to persuade you. Others may well comment in a more opinionated fashion, personally I would recommend reading a few related threads and forming your own ideas of the trade-offs (feel free to test them here but be warned some have very strong opinions). I would also recommend not being too committed to any particular solution if you need MCS, because you will quite likely not get the option! Apologies if this wasn't the definitive answer you were hoping for.

I think you have spotted the main ones (20% uplift for intermittent I think unless the design is for the 99.6% OAT case), beyond this and reheat time the scope is limited. MCS 3005-d expressly mandates minimum design room temperatures. The draft of MCS-d also expressly forbade using actual measured demand to size the system (why - its the only known fact!), I cant find that in the released version so that avenue might be worth exploring even if as a sanity check only. It also says that they must design dhw based on the greater of for bedrooms +1 or #pccupants. Possibly it might help to say that everything will be at the same temperature, at least then they wont double count room to room losses, but of course it might push up rad sizes. Its basically a manual of bad system engineering designed to protect the installer from being called because the house is cold. I would strongly advise putting in writing in advance any 'fabric upgrades' (relative to the original house build) and also any extensions which have been built to a higher standard. if possible I would ask them to confirm that they have/will include these before paying. This was the major source of error in my surveys, they assumed the whole house was uninsulated solid brick which is far from the case, as I told the surveyor when they visited. I would also ask for a copy of the detailed design, you have paid for it so you should get it, to check that they have done what they say they have done. However they may well be determined to do it their way and have the luxury of choosing their customers. Good luck and please tell us what happens.

Yes indeed you heretic! If I had known when I was getting quotes what I know now I would have, before asking for quotes 1. Read the MCS rules more thoroughly to look for loopholes 2. Designed the system I want 3. Worked out what lies I need to tell those quoting so that they can use the MCS tools/rules to create the system I want ie reverse engineered it from what I want to what I need to tell them so that they can deliver within their rules. Perhaps you could ask the Vaillant guy why their units are so heavy. I used to work for a telecoms equipment manufacturer, they put chunks of metal in handsets 'to increase the perception of quality', with the result that you got arm-ache on long calls. Just saying...

I'm wondering if you answered my question with "65C or 35C"? Presumably 65C for DHW so you don't need to change the cylinder (and since it's probably only 10-15% of your total demand, who cares if the cop is only 2). Is this your thought process?

I'm intrigued, because you seem to be following a similar thought pattern to me, as to what flow temp you are designing for and whether for dhw only or both dhw and ch.

Interesting that they are charging you, they did mine for free.

Are you sure you need 28mm copper as much as you are being told? Installers appear (from what people say here and my own experience) to like to recommend 28mm copper when its not actually necessary mostly, I suspect, so that they can whack in the heat pump suppliers pre-plumbed cylinder using a rookie plumber. Obviously it depends on your layout, demand, and current primaries

I hadn't, but having looked it up no more than a largish ASHP which are frequently wall mounted (or for that matter a small is Vaillant one, which seem to be particularly heavy). Better to put it on the floor I grant though. Its still an interesting concept.

That's not a bad idea given all the talk on here about the virtues of A2A. I'd rather try to find an f gas engineer than an MCS bod to be honest, the commercial HVAC industry has plenty of the former and they aren't high on government grants. Repackage it to the same form factor as a combi and you have a drop in replacement as far as the dhw is concerned with no need for G3 which can be installed by any commercial Aircon installer, of which there are plenty. I'd be tempted at the right price.

Thinking about it, a TS with a water to water HP drawing its input heat from the TS might be a good way to go. Heat the TS with the main house ASHP to say 40, pass the incoming mains water through the TS to heat it to say 35, then use a W2W HP mounted above the TS to heat from 35-48. Needs 20kW to get 20l/min (do we really need 20l/min I ask) which is a lot, but starting with a source of low grade heat from the TS at 35 (and water not air so much more compact), perhaps not unachievable. The TS could also serve as a buffer tank for the CH if it needs one. Mixergy have probably patented this by now, they seem to be innovating at the boundary between heat pumps and water heating.

Funny you should say that as I have the same feeling. I'm not particularly keen on having 200l of hot water at 3bar in the centre my house either. But i also must concede that fresh mains pressure hot water is better than stale low pressure water Before ashps came the scene thermal stores seemed the way to go, at least if you have a loft. I'm not yet quite convinced that we won't end up back there eventually. Maybe a thermal store and a small secondary ashp to heat, real time, from the ts temp to the desired dhw temp? Or some clever stratification like Mixergy are playing with.

Surely that's wrong. The cop of the dhw will suffer yes, but if you are suckìng heat from your house that you need (as you do in winter) it's the combined cop that matters. Effectively you now have a two stage heat pump between OAT and water temperature. Is that more efficient than a single stage heat pump? Maybe but I doubt it. And if your first stage has a cop of <1 because you have used an ir heater, not all of the energy from which will end up in the HP, I would think definitely not. On the first (drawing heat from the house) I'd check the figures and work out the combined cop. On the second (using a separate pre-heater) I wouldn't bother. Obviously in summer, when you might be thankful for cooling the house, its a different matter.

I've got a 12V pump that looks a bit like that on my narrowboat https://www.midlandchandlers.co.uk/products/johnson-compact-water-circulating-pump-12v-18-5lpm-20mm-10-24502-03, rated for 24*7 use at 100C. Its pumping the CH water not potable water, and I don't honestly know if it meets WRAS rules for potable water (its OK for fish tanks apparently). Not cheap, £180, but perhaps an example of a small, reliable, 12V pump. Check out Midland Chandlers and you will find a range of small robust 12/24V water pumps for boats.

Confused by this statement. The digram in fig 4 above shows the pump (I assume the open circle is a pump) in the heated DHW circuit essentially taking it out from and returning it to the bottom of the tank and thus needing to be WRAS compliant/bronze. What am I missing?

So is the inference that you get better cop if you circulate the water than if you don't, in which case adding the Mixergy 'heat pump conversion kit' or equivalent to an existing cylinder, rather than upgrading it to a hp specific one with a large coil, is actually, potentially, rather a good idea!

This product it's a heat pump cylinder in itself, but instead of having the condenser coil inbuilt around the exterior of the tank, they try something different by having the HP module separated on top, by piping the water through a condenser heat exchanger pulling from the bottom so that the heated water it's fed through the top. The tank it's specifically designed to a degree to avoid mix. What literature have you seen? Apologies, my comment was based on the standard Mixergy cylinder with the heat pump conversion kit. I hadn't twigged that they had brought out a cylinder with an integrated HP.