JamesPa

Thanks for this explanation, since its very plausible I'm happy to believe it. However the leak reduces at reduced flow temps (ie compression ratio), which is where you are operating (assuming WC) when you need to reduce output. So at least the physics works to assist the engineering for once. I grant this is ugly and arguably deceptive, but if its only done to extend the modulation range which is otherwise achievable with a reasonably state of the art compressor, is it so bad? If the condition in the prior sentence is true then a) you don't have to go that far and the pump would still perform as well as any other b) (assuming you have got your HP sizing about right - unlikely with the MCS over-specification approach I grant) then you are only doing this when ambient is high, flow temp is low, and thus COP is pretty high anyway and in the periods when a minority of the total annual energy consumption occurs. In this case does it really matter in the grand scheme of things if, as a result, more heat pumps are sold to replace gas boilers*. I'm not condoning deception, but its perfectly legitimate (I would argue essential in some cases) to provide people with (declared) sub optimal options which might just make the package viable whilst not degrading overall performance too much. Obviously if others use a more state of the art compressor that natively achieves a better modulation ratio that that is to be preferred, but you seem to suggest above that this is not the case. Engineering is about making sensible compromises within the parameters of available technology. Sometimes a few percent loss of efficiency (or whatever metric you are dealing with) is worthwhile in the overall scheme of things and necessary to find an acceptable solution. * to clarify Im not advocating 'dodgy tricks' to sell more heat pumps for any given manufacturer. However I do regard the overriding objective as being to sell as many reasonably performant heat pumps in total as possible to displace as many gas boilers as possible in the shortest possible timeframe. Even a lousy CoP is better for the planet than a gas boiler, and every one sold reduces the political pressure to continue artificially inflating the price of electricity relative to gas, which is a major disincentive to making the switch.

Not necessarily. My house, like many, has two stories. So 6kW to each on 22mm is just fine for my 7.5kW(measured) or 11kW (calculated using MCS assumptions) demand. 6kW to the tank is also fine, throughout the summer and much of spring and autumn my dhw is heated from solar PV using a 3kW immersion. It's only a few years old and plenty large enough thanks. I don't have a problem with reheat times even if I turn my boiler flow temp down to 55 (or even 50). No need at all to make a ruddy mess unless some idealist insists on running 28mm to my tank, upgrading the CW flow to the tank from 15mm to 22mm, or can't find a way to make do with the existing coil. Let's consider one fact we know. With current practice we are installing way too few heat pumps even with a whacking govt subsidy to hit anything like what is necessary to mitigate climate change.. So we need to change practice unless the technology is going to change radically and bail us out. Of course the industry is probably quite happy to continue in high price low volume mode for as long as it can. The planet isn't.

It sounds plausible, but why is this (my knowledge of compressors is small) It wouldn't. Hence the talk about point of use heaters having a 15l local store. Yes. I have used these in community centres where we have replaced gas with ashp. They work a treat, but of course people aren't taking baths or showers.

Building on this, 11kW HP (larger houses) will do instant hot water at the same speed as 9.5kW electric shower. 8kW HP (medium houses) plus a 3kW willis heater ditto. Is this plus saw 15-30l stored at say 50C (ideally 15 because then you don't need the venting arrangements) sufficient? Biggest issue is probably all the water in the distribution pipes!

I cant see this is do-able. 20l/min at a deltaT of 28C (which is optimistic in winter assuming you want tap water at 48C) is 39kW. Even 10l/min is 19kW. But there might be some middle-way combination. Point of use hot water heaters often store 15l of hot water. That's enough for most requirements and, combined with a 3kW heater enough to satisfy everything domestic other than a bath or a shower. The recommended shower temperature is actually only 38C, perhaps there is some more 'finely tuned' engineering to be done here.

Looks good if you have a 4kW pump. I think the ideal design objective is to transfer the full output of the heat pump at the desired final DHW temp with the delta T on the hot side equal to 5 or 7 or whatever you are running the HP at. A lesser (the minimum?) design objective it to transfer at least the minimum output of the heat pump. The advantage of the former is (obviously) faster reheat time at the expense of a larger PHE and higher pressures in the PHE circuit. Solarcoil doesn't look to have a particularly large area to me so I cant see it does the job, and anyway, for me, I want to retain my immersion heater for backup and for solar PV.

Can you explain this please. I cant work out what the challenge is (and I feel I should be able to!)

I grant it's horses for courses. In my case the room insulation had been increased since the rads were installed so most of the upgrades were 11 to 21 or 21 to 22 with little change in dimension. I found I could easily do 2 per day in a fairly relaxed 5 hour day, a plumber would presumably be twice as quick. I have a couple of difficult ones yet to be tackled where I need 33 or possibly fans. But overall much less painful than anticipated. I think what this whole thread shows is that a) we all (pretty much) agree that there is a problem and b) we need a range of options to fit various retrofit scenarios and customer preferences, not a one size fits all approach.

Not sure I understand this, can you explain how it works and/or provide a reference. Also what is the modulation ratio of Samsung hps , I can't find any specs (not that Samsung are alone in this respect, only Mitsubishi seem to provide really comprehensive specs)

Why 28mm primaries, one of the MCS brigade who surveyed said that too. Is it because hps are run at lower delta t and if so is there any reason this couldn't change for the DHW circuit if we are running it at an elevated flow temp? Or by primaries are you talking about the heating not dhw circuit (no problem in my case as the ch feed splits just after the diverter valve upstairs/downstairs each 22mm)

You may very well be right, as I said I didn't enjoy fluid dynamics at uni; I think this may need an experimental determination! I think I'm currently leaning, if its feasible (which it may or may not be), towards simply running a HP at higher temp for DHW only. This is purely on grounds of simplicity. If only I could get a handle on the modulation ratios and control strategies, which few manufacturers seem to publish, despite the importance. If it isn't feasible then PHE and a pump looks like a good bet, or maybe a combo. Whatever the outcome I'm now 100% convinced that the prevailing 'throw away your DHW system and start again' mantra, whilst undoubtedly theoretically 'the best', is hopelessly (and unnecessarily) idealistic in the real world. It comes down to this - from a consumer point of view, fitting a HP is functionally equivalent to replacing the boiler, little or nothing more. Most simply won't make this choice it its significantly more disruption and significantly more cost. I regard myself as pretty 'green' and determined to go HP within 12 months, but am still reluctant to follow the received 'wisdom' of tearing everything out and starting again if there is a realistic alternative. We definitely need a broader range of options. I suggest that anyone who doubts this spends an evening in their local pub (unless its in Brighton of course) speaking to the customers and trying to convince them both to pay £10-17K to replace their boiler (instead of 3-5K), and accept a tax increase to fund the government grant in perpetuity. Good luck with that!

I can't now remember where I got the heat transfer coefficient from, but the very simple model effectively assumes that the whole contents of the tank warms at a uniform rate. That's roughly equivalent to stirring it, your suggestion. Convection might produce stratification improving the performance, or might be insufficient to take the hot water away, degrading it. Not sure which, I never liked fluid dynamics! You research on PHEs seems to have identified some reasonable options. Given that the mixergy 'kit' (comprising pump, phe and hp interface box) is about 400, I would expect the components are perhaps 200. Pumps seem to be about 100 so 50-100 for the PHE seems in the right ballpark.

True, but the cost and disruption isn't! I'm also not following the below, can you clarify? But I doubt many manufacturers want to install 2 kg of gas at 20p MPa next to a structural wall of a building. Especially if it unnecessary with existing technology

Interesting, I too was wondering about storing at a lower temp. Max allowed temp to taps in new build is 48.

Nice theory I grant you but ... back in the real world... almost all new houses are being built with gas boilers and (doubtless) a cylinder (if they have one at all) with a coil insufficiently large for LT operation replacing an existing perfectly functional DHW system is a severe disincentive to fitting heat pumps, particularly if you are told 'there is no choice' fitting a buffer tank in a small house is a severe disincentive, if you can find anywhere at all to put it 'It' hasn't 'passed' at all. We have 10s of millions of houses out there with cylinders that have a 0.8 sw m coil and which urgently need to be retrofitted with heat pumps at a price and level of disruption the residents will tolerate. Currently they are faced with a bill, after the government grant, of 10-15K. To the consumer all they are getting is a new boiler worth max 5K. Why are they going to bother (we know the answer to this already, they don't) You aren't thinking about the retrofit from the consumers point of view and, until we as a nation (and in particular the installation industry) do, we wont fix climate change.

Not sure what point you are trying to make/question you are trying to ask? All I am doing here is to explore the boundaries of the engineering, to find out whether it is really necessary to swap out the DHW system for most (so far as I can tell) retrofits (and in particular mine), and whether it is really necessary to have a buffer tank. Taken together these are, so far as I can see, major impediments to mass adoption because of cost, space, and disruption. Modulation ratio is the key determinant of the answer to both, with max flow temp being a second important factor (but much less so if the modulation ratio is high). I am aware of the basic physics, but the engineering of particular products, whilst constrained by the physics, doesn't go to the boundaries, so we need the specs.

Building further on this, high temperature heat pumps (eg the latest Samsung) can go up to 70C. So in principle, if they modulate low enough, they can heat the DHW with a standard 0.8 sq m coil with no need at all to change the DHW system (provided you have some sort of tank). The smart thing would be for the pumps to adjust the flow temperature according to the measured DHW temperature in the vicinity of the coil, ramping it up as the DHW heats up. They would then be more efficient still than having the immersion kick in when the DHW temp rises above a certain value, because the CoP even at the higher flow temp will still be >1. As shown above, providing the bulk of the DHW heating is at a flow temp of, say, 55C, the annual penalty by using higher flow temps to heat the water through the last few degrees, even with todays high electricity prices, is modest and does not make a convincing case for swapping out a functioning DHW system. Samsung don't seem to publish the modulation data for their HT range. Nor do we know whether they will adjust their flow temperature down in the required circumstances. But if the modulation ratio is decent and they do at least some adaptive FT control then these pumps (and likewise other HT pumps with a decent modulation ratio) are capable of working with existing DHW tanks. That's two 'ifs' unfortunately, but it may well point the way forward even if the current control systems/modulation depth don't play ball

Not got there yet. @JohnMo seems to have identified a useful starting point, but Im not yet sure I understand the calculation fully enough to be confident, nor how the results can be transferred to other manufacturers. Only working on this intermittently, my wife is getting fed up with the amount of time I've invested in ASHPs.

Clearly HPs are not a drop in replacement and as you say the physics negates the possibility that they will be. But, at the same time, an industry which loads what, to the consumer, is 'just a new boiler', with a shedload of difficult replacements/additional requirements, is not going to get the mass take up that it must. I think the case for radiator upgrade is clear. I attempted to design to avoid this initially, but have now done it and discovered its an easy job, whether DiY or outsourced, and really quite cheap. And I got shiny new radiators into the bargain, which certainly look better than out old ones. This is, if correctly put, a saleable proposition. The case for DHW system replacement and a buffer tank is much less clear from a consumer standpoint, and much less clear from an engineering standpoint also. Fix modulation depth and a buffer becomes unnecessary, other than to the extent is required to service the defrost cycle. DHW system replacement is a big deal in terms of cost and disruption and, whilst there are benefits (but also downsides) if its replacing an exiting vented system, there are no benefits, only downsides, if its replacing an existing unvented system solely because the coil isn't big enough. In the latter case its barely a saleable proposition, so better solutions are needed. Modulation, possibly combined with higher, albeit less efficient, flow temps for DHW only, has the potential to fix this problem also, but perhaps we need some staging points hence my thoughts on this thread.

Thanks for this, its going to take a bit of reading through. It looks like this unit is designed to be attached to a thermal store and transfer heat to incoming cold water, but of course the principle is still the same. I've had another crazy(?) thought. The problem that I'm trying to solve (or convince myself that there really is no solution for) is the idea that in every HP retrofit (and mine in particular) you need to swap out the otherwise perfectly satisfactory DHW tank. This adds, let us say, 3K to the cost, generally involves a lot of disruption, and has little apparent value as far as the consumer is concerned. My gut feeling is that, until this problem (and the obsession with buffer tanks) is solved, HP retrofits will remain a minority market. I (and thus likely others) need at least to have some options, rather than having the disruption and cost foisted on them by the 'cant think out of the box' brigade. So what if we just accept the existing small coil (typically 0.8sq m) in whatever DHW tank the homeowner has. For now we will ignore the large number of homes with combis, thats the next level of problem. Many heat pumps will in fact heat to 60C and some HT ones (much derided on this forum. for understandable reasons) to 70C. I grant that using these high temperatures for space heating is bad economics, but what about using the higher temp just for DHW? There are two factors to consider, loss of efficiency and minimum output power of the heat pump before it starts short cycling. For the rest of the argument I'm assuming that the target temperature for the stored water is 50C as that seems common, with a weekly anti-legionella 55C boost. For example the Mitsubishi Ecodan 11.2kW has a COP (ambient 7C) of 2.8 at 55C o/p and 2.3 at 60C o/p, and will modulate down to about 4kW. With a 0.8sq m coil you can get the DHW up to about 45C before the heat transfer in a 0.8 sq m coil drops below this minimum. So I could operate the HP at 60C to heat the water to 45C, then and use the immersion for the final 5C? Is this so silly? The average household uses about 120l DHW per day according to various sources (some say 80l, Im using the higher figure). In the 'conventional' arrangement (which, in a retrofit situation, generally involves replacing the DHW cylinder), and running the HP at 55 (COP 2.8), assuming an input mains water temp of 10C, this will require 2kWh per day input power. Alternatively if we don't replace the cylinder (or any other part of the DHW system), run the HP at 60C (COP 2.3) until the DHW reaches 45, and then use the immersion (COP1.0) to heat it up to 50, the input power goes up to 2.8kWh. Annualised, at current very high electricity prices, that's a difference of only £115. I'm not at all sure that makes a compelling case for all the disruption and cost involved in swapping out the DHW cylinder. If course its still desirable to do so, but we must not allow perfection get in the way of very good. The fundamental requirement is to get off of fossil fuels onto renewables, in practice switch from gas to electricity. We need to make this as easy as possible for the mass of people! Just another out of the box train of thought.

Does anyone know how to read the heat transfer specs of PHEs. Some seem to be specified in kW (under what conditions?), many arent really specified other than by number of plates and physical size. I'm trying to work out how one might make a selection if I decide to go down this route.

Does anyone know if Octopus are actually intending to turn the RED design into a product. So far as I can see nothing has changed on the RED website since they bought it. I suspect RED was quite small and Octopus may have bought them for the expertise and maybe floorspace. A few people who actually understand the engineering and design of ASHPs, and can therefore go beyond reading the specs, might be really valuable to Octopus in negotiating with manufacturers and as consultants on the design of their installs. For example, this expertise would allow Octopus to bypass all of the backside-covering in the manufacturers documentation, and thus install solutions with fewer unnecessary components and install tasks. Just speculation, I only know whats been said in public, which isn't a lot!

I assume this is after deducting BUS. Octopus are 'productizing' hp retrofits and, from what I have heard, are doing a reasonable job. Hopefully they will destroy the business model of the frenzy feeders in the market, which appear to be the majority. If they will hold the price irrespective of what they find in the survey (or they give you the option to get your money back if they can't hold the price) it's almost certainly a good deal. If someone offered me that (subject to the above) I would take it. Unfortunately my house doesn't fit their current profile.

Not sure I followed this, is it a comment specific to your system or generic. Does it relate to a vented cylinder or a UVC with insufficiently large coil (the latter being the motivation for adding the PHE)?

I understood that for a UVC an annual inspection by a g3 qualified engineer was mandatory, but I haven't actually read this anywhere other than secondary sources, so it may well be plumbers drumming up business not an actual requirement. True. It's a trade off I accept, but houses are. Ideally I'd completely rebuild my 1930s house with the latest building technology throughout, but it's disruptive and expensive. So I generally prefer to upgrade when components reach their end of life or are being displaced for some other reason. Still not decided. Maybe outside the south east, but in the south east the MCS bods seem to want 15K absolute min, and frequently 20K plus, to do a 11kW hp retrofit including a UVC, excluding radiator upgrades. Ok you get 5k off in govt grant, but it's daylight robbery.