JamesPa

No, that's a separate issue. Necessary I grant, but not the problem I'm trying to discuss/solve, namely that the transition to ashp isn't happening (and as things currently stand isn't going to happen) because it's too difficult, too expensive, and too disruptive in the way we currently do it.

Ive added lettering above so I can refer a) Yes (at least according to the modelling), provided weather compensation is turned on and correctly adjusted b) Gas boilers are massively oversized because its easy to do so. The average house needs about 6-8kW. c) Gas boilers don't have the same characteristics as ASHPs and don't benefit massively from running slow and low (although they probably do benefit a bit). But so what, we need to make gas boilers history and give people a practical and affordable path to adopting ASHP else we are toast (literally!). Most gas boilers are set up inefficiently so we have only to do as well cost-wise as an inefficiently set gas boiler and the transition becomes a no brainer! Of course, that's a given. But the result is a more comfortable house so, once people get over it, its not a difficult transition.

I'm being deliberately provocative here to stimulate out of the box thinking, please bear with the explanation and numbers, but then feel free to critique. In summary I am proposing that, for retrofits (at least in the South of England, where many of the UK houses are), we should abandon the insistence on achieving low flow temperatures in favour of accelerating roll out, which is necessary to mitigate climate change. So here goes: I think we all know that ASHPs are the way to go for low carbon heating, and that low carbon heating is an essential part of combating climate change that the way to make ASHPs work efficiently is to minimise the flow temperature that the current model, whereby ASHPs are subsidised by government yet still cost up to 3 times as much as a gas boiler to install, is not working, in the sense that it is not gaining the traction hoped, let alone the traction required to meet climate change goals that large ongoing government subsidies are likely to be unsustainable politically, however strong the environmental arguments that most ASHP installations 'require' replacement of at least some radiators, almost all 'require' the replacement of the DHW tank and other sundries, and that this contributes massively to the cost and disruption that, because we 'need' different flow conditions ‘specialist’ design is required that local plumbers, who are surely the mainstay of the retrofit market for gas boilers (which is the retrofit market), are not engaged in the process because its too complex, onerous and expensive and because of point 6. Now if we could fix #5 and #6 then #7 is fixed more or less automatically, #3 no longer need applies and we can achieve actually #1, which is the real goal So what is getting in the way - answer: a) regulation and b) #2. Regulation is ultimately fixable, but #2 is a function of the physics. Several manufacturers now do high temperature heat pumps which run at 65 or even 70C. These are much dismissed on this forum because they are less efficient, and the received wisdom (which, until recently, I, physicist by training, bought into) is to design for the lowest possible flow temperature, which is what triggers most of the other challenges. From a physics point of view designing for low flow temperature is undoubtedly the right answer, but engineering, particularly system engineering, is about trade offs between practicality, performance, cost and other factors. So what if we designed for a flow temperature of say 65 (and a delta T of 5)? The average radiator temperature would be about the same as the common 70/50 combination, so we almost certainly don’t need to change out radiators. At a flow temp of 65 you can easily heat the DHW, with a standard coil, to 50( provided that the HP has a decent modulation ratio) so we don’t need to change out the DHW tank. In many, perhaps most, domestic environments the primaries split near the boiler into 2x22mm (upstairs and downstairs), which is sufficient for 6kW, 7-9kW at a push, on each leg, sufficient for most households, so we don’t need to swap out the primaries. And we don’t need to upgrade the ‘standard’ 22mm feeds to the DHW tank because they are already good for 6kW at a delta T of 5C, which is sufficient and twice what an immersion heater delivers. We will have to swap out the existing diverter valve (a few 10s of £) and we will need to connect up the controls differently (maybe half a day for the electrician), but that’s about it. No need for MCS, extensive replacement of functioning hardware, extensive lifting of floorboards (or solid flooring), replacement DHW tanks etc; a regular plumber and his electrician friend can just do it in a day or two. But, you say, the efficiency will be terrible! Well sure it won’t be as good as it could be, but there is almost always a trade off between capital investment and long term cost and that trade off is not ‘one size fits all’. I would suggest that, while a SCOP of 4 or more is desirable, it is sufficient, in the real world, if the SCOP is such that the running cost are about the same as whatever the current system is. In the case of gas, the predominant heating in the UK, this means an SCOP of about 3 currently (because electricity is 3 times the price of gas), a figure likely anyway to reduce as the Government (hopefully) reduces the wholly artificial weighting in favour of gas that exists at present. Some weeks ago I posted a model of weather compensation (which I attach to this post) and this can be used to estimate the effect of increasing flow temperature. I have taken, as an example, the LG figures for their 12kW model. This shows, for conditions in the South of England (where a large proportion of the UKs houses are situated) an modelled SCOP of 4.4 (with weather compensation) and a ‘design’ flow temperature of 45, frequently the target for retrofits where radiators as are present. If we increase the flow temperature to 65, the SCOP reduces to about 3.5. That’s a big reduction, but arguably a sacrifice worth making in return for the advantages in terms of up-front cost and disruption. Most importantly, even allowing for poorer-than-model performance, its good enough by the definition above. It is dependent on a reasonable (but by no means perfect) setting up WC, but if installers weren't spending so much time on replacing perfectly functional equipment, they could perhaps do this and pop back a couple of times in the ensuing heating season to adjust. Of course radiator and DHW upgrades can, and should, still be offered, but as an option not as a requirement, which is how they are currently positioned. This decouples the part which is transition to low carbon heating, and the part which is system improvement and, vitally, gives consumers choice. The upshot is, I think, that we could for a good proportion of households, eliminate much of the ‘compulsory overhead’ associated with retrofit of a heat pump. Yes it means compromise, but the rewards in terms of climate emissions are enormous. The MCS brigade probably wont like this, but that’s just tough. The ASHP market cannot possibly continue as a niche sector if it is to achieve our climate goals, and the constraints imposed by MCS confine it to just that place. This has to change! Discuss! (but in doing so please bear in mind that the objective is to find practical solutions to achieving mass retrofit with existing, or near-existing, technology, not to achieve perfection). WC Simulation.xls

Neither of those seem to serve my area (Herts/Essex borders). I have tried every MCS registered organisation within 15 miles, and many beyond, but admittedly none that do not claim to work either in my area or nationwide.

If I ate the amount off fish and chips I could get for the 25K I was quoted for 2x12kW installed plus 6 rad swap outs, I would likely die of a coronary! But I guess that this is mostly installation costs not the units themselves.

I have, sadly, been at this for over 18 months now. Given the big discrepancy between calculated load (11kW) and measured (7.5kW), I may yet delay until next year to get another winter's worth of data. Unfortunately my smart meter stopped communicating during the November cold spell so, whilst I was diligent in checking the readings frequently, I don't have any for the middle of the night. Also I don't seem to get anything like a linear relationship between degree-days and consumption (I don't have real time temperature monitoring so am reliant on local degree-day reports). This obviously makes me suspect readings. The plus point is that, over this time, I think I have developed a much better understanding, will be better prepared, and have worked out what I don't need to do (or at worst can leave out initially and retrofit only if there is a problem) notwithstanding the industry insistence.. I have also narrowed the choice down to three units, the 11.2kW Mitsubishi, the 12kW Samsung HT or the 9kW LG. The latter, whilst well priced and seemingly well featured, a risk however. There little margin on capacity and so depends on my measurements being accurate and in addition there is a very relevant inconsistency in the spec (which I have asked LG about and am awaiting a response).

I also have recent experience with Good Energy. Based on a form-fill and telephone discussion they estimated about £5-8K after grant (I cant remember the exact figure) for a 12kW Midea, likewise the best offer I have had. I did pijnt out to them (in writing) that it wouldn't meet the permitted development noise spec, but they said that they might be able to find a work-around. They then (rather generously I felt) offered to do a full survey free of charge, which I accepted. Following the survey (which took 2.5 hours, appeared to be thorough, and was done by a very nice lady) I received a quote for £K25 (before the BUS) covering the installation of 2 12kW Midea pumps (total 24kW) plus six rad replacements, designed to run at a LWT of 55C. Their calculation of the demand was a little over 16kW. In fairness another organisation, who I had paid earlier to do a survey, came out with about the same figure. That other organisation (from whom I requested details of the calculations) ignored the upgrades to insulation from the original solid walls/better wall construction for extensions, and also sundry other improvements. They also counted losses from one room to another but not gains, double counting about 1kW. I can only presume Good Energy did something similar, but don't have the detail of their calculations. I made a very specific point of telling both surveyors about the upgrades. If I use the MCS assumptions, take into account the upgrades, and of course subtract out the double counting, the calculation yields about 11kW. During the long cold spell at -2 in November (I live in the South East so this is the typical design temp) I actually measured a consistent 7.5kW. If I were to sign up to the Good Energy offer within 30 days, and pay a 25% deposit, I get a discount of £1750. My wife is worried, as she fears that the survey was for purposes other than heat pump installation (for the avoidance of doubt I am not making any allegations). I'm really not sure what to make of this, why do a full survey for free and then make a silly proposal, unless they need cash quickly and hope to get some deposits in by playing a numbers game.

A Surrey/York flange might be useful here.

If you want to get really creative (and take some risks it might not work) fit destratification and, then when you swap out your boiler for an ashp, add in a plate heat exchanger. Then you might not need to change out your DHW cylinder because the PHE will augment the coil area to make it large enough for ashp. That's how the Mixergy 'heat pump conversion kit' appears to work. This idea was discussed over in the ashp forum, but is not tested, so far as I am aware, other than by Mixergy. However if destratification works then, with the right PHE and correctly adjusted pump, it's difficult (at least for me) to see how the idea could fail. Some will, however, argue that vented hot water systems are an abomination and should be swapped out at the earliest possible opportunity. I think (someone please correct me if I'm wrong) a destratification pump is just a pump designed for domestic water, and is commonly used for recirculation loops eg in hotels. They are more expensive because they use better materials and meet the WRAS standards.

For the avoidance of doubt my reference to 'the industry' was meant as a reference to installers (in the UK) not manufacturers (who by and large seem to be doing a good job). Like you I fear that lack of training, basic numeracy and basic scientific understanding drags the UK down. We were all told by our politicians, not so long ago, that we don't need experts, and this plays straight into a culture which values celebrity more than engineering. In fairness to our current pm, he has highlighted that aversion to maths is a drag on the economy, I look forward to him implementing a coherent strategy to fix this (but won't be holding my breath).

Fair enough. To some extent it's 'of our own creation'. If the majority of customers expect a heating system just to work and magically be the correct temperature whatever happens, then the industry will deliver just that. There is, of course, a price to be paid for this luxury, either in efficiency or system cost or both. Who knew until recently that turning down the flow temp on your gas boiler was _necessary_ to make it achieve the designed-in and much touted efficiency? It's just sad that we don't get to make an informed choice, instead the industry does what it thinks is right (and for the majority of customers probably is right) rather than presenting customers with options. I suspect those in the industry will say, doubtless with some justification, that giving customers choice with trade-offs to evaluate is too complex for most customers. We are victims of our wealth (and perhaps our laziness?)

+1 on the thread title. The latest 'mcs compliant' quote I have received is recommending 2 midea 12kW units for a price of £25,086 less the 5k government grant. Their own estimate of house consumption is just short of 17kW, my estimate using mcs assumptions but the correct u values for walls etc is 11kW, and the actual measured consumption during an extended period when temperatures both day and night were -2C (pretty much the recommend design temp here in the south east), is 7.5kW. Oh and they propose to run the system at a flow temp of 55C. I can get a 'discount' of 1750 on the price if I accept within 30 days, entailing a 5K deposit. Need I say more?

@IanRs argument (which, ultimately, was fairly convincing) depended on stratification, which in turn depends on (all of) a well designed tank, the thermostat controlling the hp being located correctly, the pumps being more or less well adjusted, and a good height to diameter ratio. Yours doesn't have the last of these, your measurements confirm 100% mixing in the tank and, as you correctly say, this will be a 10%+ efficiency hit solely because of the mixing and hence elevated flow temperature. Losses from the tank will add to this and if WC is not correctly adjusted (which seems unlikely given that your installer clearly has not prioritised efficiency), will add further. If the buffer tank is essential for the warranty (which I doubt), plumb it as 2 or 3 port but definitely not 4. If it's plumbed as 2 port, you will need to lose a pump. Unless there is a good reason that you have not stated, this is a poor installation. However that's nothing special to heat pumps, since condensing gas boilers were introduced (and widely touted to be a good thing) most installations seem to have been poor, ie with a flow and return temp adjusted to exceed the maximum figure at which condensation occurs this sacrificing 10% or so in efficiency. Installers, it seems, frequently prioritise avoiding support calls over efficiency, and there is no reason on earth why new technology will change that behaviour.

Only 7.3kW at -3 though, not enough for my application.

Until recently I had dismissed LG because I was looking for 11-12kW single fan (based on calculated heat loss) with sound power of 60dB(A) or less (to meet pd sound levels). This combo of specs basically comes down to mitsubishi or samsung ht, from the commonly available brands, so far as I have been able to determine. But this winter I measured max 7.5kW even during the sustained cold snap in November. So the LG 9kW unit (which seems to be available cheaply and seems to be well specified) is now a distinct contender, albeit that the appearance isn't going to win design awards! However nowhere can I find specs for modulation capability, min flow rate (given that ch14 low flow rate seems to be a notorious problem), and whether it does timed setback of flow temp. Can anyone who has one advise enlighten (obviously I'd also appreciate any other comments on this machine and have read most of the posts here which obviously discuss it).

I put 4kW in 12 years ago and am now trying to upgrade which is proving difficult. Wish I'd put more in from the start. Electric vehicles, ashp and better export prices are here or on their way and these only increase the attraction. Furthermore overdimensioning means you still get meaningful output in the shoulder season. I'm not sure how the maths works cost wise, but it's a long term investment and electricity is the way to go.

A2A is often sized, in a commercial context, for cooling which is more challenging than heating. That may be part of the explanation. Having said that it does sound a bit excessive. If they modulate down efficiently then efficiency might not suffer, no idea how A2A performs in this respect.

Completely agree on all counts. There are some cases where replacing a cylinder is easy, but many, like mine, where the knock on effects are huge. At least we need an option to consider, even if its sub-optimal. That's how I started this thread (with an idea to solve just this problem). We ended up with two, either of which could probably be made to work. Will the industry do anything, doubt it but hope so.

A final(?) comment/question on this. The HP industry, including the lobbying side, needs to get its skates on big time. Almost every day I see a headline somewhere telling me (erroneously) that heat pumps are going to destroy Britain (that was in the Telegraph recently) and hydrogen is the solution. The hydrogen-ready brigade are coming for you, and will surely find a way to continue to sell gas boilers on the promise that one day they can be powered by 'clean hydrogen' from the same pipes. They wont care about retaining a vented cylinder. however crappy it might be. They wont want to rip up flooring unnecessarily to fit 22mm cw feeds to the DHW in a house where the mains pressure is 9 bar (!), and they wont need to swap radiators out. Nor will they need planning consent, or MCS, or government subsidy at point of sale, or a complete redesign of a heating system that may not be perfect, but which the customer has got used to. But they will be (doubtless already are) lobbying government in the background to allow gas boilers to be fitted so long as they can, at some future date, be run off of 'green' hydrogen. Then, a few years down the line, it will transpire that hydrogen is at least 30% more expensive than electricity and, because gas has been retained, there will be pressure not to make the switch 'because people cant afford it'. Furthermore, in the intervening years (because more gas installations have been retained than are necessary), the political pressure to maintain the artificial relationship between gas and electricity prices, which today disadvantages heat pumps, will remain, and proposals to fix this will be watered down. You can see this all coming. The Telegraph recently ran another article 'Heat pumps won't work in old homes, warns Bosch' where Bosch was represented as a heat pump manufacturer, thus making it more plausible. My cynical suspicion is that Bosch are, in reality, scared about loss of boiler sales so need to find a way to perpetuate the unconscionable. Currently perfection is, to my perception anyway, getting in the way of the good, with a very real risk that the result is a 'victory' for the disaster that is climate change. This is not intended as a criticism, just a comment on how things may need to change very fast if the battle is not to be lost. I am a believer (because I have a background in science so trust facts about climate change not opinions), but haven't yet put in a heat pump, and am really frustrated by the industry for always presenting problems not solutions. I will now almost certainly end up doing it myself, most wouldn't bother. I understand the need to walk before running, but there is a runner right behind you ready to take your lunch and our children's futures. How can we collectively help?

This is the current headline article on the renewableheatinghub. Shocking indeed. https://renewableheatinghub.co.uk/shocking-truth-kw-and-kwh-are-not-the-same-thing

That's actually a particularly interesting one. I am pretty certain that my comfort temperature is higher at +4 than it is at -2. I assume this is humidity related. I wonder if this is common. The late November cold snap was -2 day and night here just north of London, for 4 days in a row. Great weather for experimental work on central heating. I had pretty much concluded that but, short of making measurements (which some on here do) don't know what to substitute. My own calcs already split fabric and ventilation which is part of the reason why I am pretty confident that is a large part of the difference.

Fair enough. For some reason I had always had 300W in mind, but on the basis of 2000Kcal/day (which is high) thats 140W, so I will go with 100W. So only 600W accounted for with baseload leccy consumption and two adults. Still significant though.

Incorrect assumptions about insulation accounts for 3kW. Basically they ignored me when I told them which walls had been upgraded or were of a different construction because they were a later extension, and assumed they were all solid brick, which is far from the case. Also pessimistic assumptions about windows which have also been upgraded. Their calculation actually came out at 15kW (and they based their quote on this) bu,t when I asked for the detail, it transpired that they had had also counted room losses in the house total, but not room gains, thus effectively double counting 1kW. I paid £300 for this particular survey. I'm struggling to explain the difference between the 10.5kW I calculate with the correct insulation values, and 7.5kW measured. Part of it is certainly temperature, we tend to heat to 20 max. I suspect however that the bulk is air changes, I don't think the house is particularly well ventilated. Solar gain, people and appliances are a further contribution, our baseload leccy consumption is 400W, and I guess most of that eventually appears as heat. Two people there most of the time is another 600W, so that's a kilowatt accounted for! Perhaps, given this experience, you can understand why I am suspicious when told that something seemingly sensible and entirely within the bounds of physics is impossible! Helpful/good points. Doubt I will convince any MCS installers though (at least based on my experience to date)! Its going to end up as a DiY job I fear and still be cheaper than if I get the grant. Not the outcome I'm hoping for, just the one I'm now expecting.

Thanks for the comments, most helpful. As its my house I can decide (or at least should be able to decide) the level of risk I am prepared to accept. Im minded to try it and retrofit if it doesn't work, the 20% VAT is a factor, but the retrofit can be done by any old (or young) plumber so will probably be cheaper than if I were getting an MCS plumber to do the job anyway. My biggest design concern, to be honest, is whether to go with the consumption actually measured during a 5 day consistently cold spell (7.5kW), and thus fit say an 8.5kW heat pump, the calculated consumption using MCS assumptions but known insulation levels (10.5-11kW) and thus fit a 11-12kW heat pump, or the consumption calculated by an MCS surveyor (14kW), and thus fit a 14-16kW heat pump. My gut tells me the middle of these. That's very interesting and clearly potentially useful. Not sure what the problem is with running three pipes through a wall, a lot easier than the work associated with fitting a replacement cylinder, at least in my house! Fair enough, but if this is the case then perhaps the installers should be up front about it, as in 'at the moment we are only doing certain types of property and if yours doesn't fit then we will consider it, but may have to put in a solution that requires more disruption/cost than may be the case in a couple of years time (as opposed to what most have said to me namely 'its my way or the highway'. Being upfront is exactly what Octopus are doing, to their credit.

I agree, Hopefully I wouldn't. Sadly I have the impression (but no solid evidence) that oversizing the heat pump may be all to common. MCS contributes directly to thus by insisting that the hp must be sized to meet 100% of the heat demand, leaving no room for downside error. Sadly I also have the impression that 'avoiding support calls' is the dominant system design criterion, both for the products and more so still for installers. The thing is this. I am fortunate enough to be able to afford to scrap a working tank unnecessarily. Many aren't, and many more won't (I may, but not if I can avoid it). Yet they are told by the installation industry that 'there is no alternative'. So they find their own alternative, and fit a replacement gas boiler instead, which will continue to exacerbate climate change for the next 15 years or more. Thus the whole heat pump installation industry, by being rigid, idealistic, uninventive and having the cheek to presume they know what the customer wants, is directly contributing to climate change. Shame on them! Thus I come back to needing a greater diversity of solutions and a more flexible approach.