JamesPa

Yes, it's a mystery to me. And the Newark/HG tie up for the super expensive massive coil This is now on the Newark site. I've asked the question about yield relative to a same sized uvc. https://newarkcylinders.co.uk/hg-mini-store-coming-soon/ The massive coil cylinders are also interesting. It will increase efficiency by reducing the approach temperature, and reduce reheat time. Taken together this might allow you to move down a cylinder size for comparable yield. The dhw industry seems to be a bit 'seat of the pants' in terms of useful specs beyond volume and physical dimensions thus making it nigh on impossible to make any real comparisons.

As I conclude above the answer lies in the engineering detail not the fundamental physics. Unfortunately neither my spreadsheet models nor skills extend to computational fluid dynamics combined with thermal flows, that's more a Met office thing. We need to persuade Heat Geek or Newark to run the experiment.

Somewhere in the video the hp was operating at way less than max output. However only some hps are set up to modulate the water pump and, as you say, the turnover may be too large. Need to think about how the control loops work to understand this. I'm pretty certain one of the diagrams showed a diffuser at the flow inlet. Agree unless there is some efficiency advantage because the flow from the hp directly feeds the tank. All of the above is, in my view, an engineering unknown. And all of the above can be answered if we get the answer to the question about the comparison with a uvc of same size and operated at same ft (except some of the pump control questions which may be ho dependent). I wonder if they have thought about all if this or are just throwing it at the market following some basic experiments, to see what happens?

The hp in the video doesn't meet the demand to heat water from mains temp to dhw temp, we know that because the dhw eventually falls below temperature. However I think it does (and probably generally will) meet the demand required to restore the return from the tank to the tank temperature, just not at the (flow) rate sufficient to restore all the heat lost from the tank to the dhw. So what I think happens in a perfectly stratified tank is that the thermocline moves upwards at a rate slower than would be the case without reheat, eventually to the point where the thermocline is high enough in the tank that the dhw coil can't extract enough energy to heat the incoming mains to the desired temperature. I don't think they are particularly claiming efficiency gains and are I think heating the store to 50/55 (I can't remember which) which is not an uncommon flow temp for dhw, bearing in mind that there is typically a 5C offset between the flow temp to a uvc and the actual water temperature unlike the mini where the offset is zero. Obviously some run it cooler to maximise efficiency at the expense of capacity, this is a design trade off with any cylinder!. It all comes back imho to, how does the yield of dhw compare to a uvc of the same volume and at the same heat pump ft. Answering that question alone tells us the circumstances in which this is useful and any efficiency matters. Unfortunately it's not answered!

That's a very depressing thought! (But I'm not disagreeing!).

True, but so far we don't have any comparative performance figures relative to a uvc of the same size. What I really want to know is, if I need (eg) a 150l uvc to deliver x amount of dhw at y l/s, how big would this arrangement need to be? If the answer is 150l or less, then the only remaining argument for a uvc might be heat pump efficiency (which may or may not differ between the two). If the answer is within 10-20 % of 150l then I would say that in most cases, unless space was extremely tight, you would choose this over a uvc to avoid g3. If it's more than 20% larger to get the same yield, then the hg mini is only for situations where the g3 arrangements are much more difficult than accommodating a larger cylinder, and the solution for smallest volume (which seems to be what they are targeting) is actually a small uvc not the hg mini! Hopefully this detail will come.

If it's unvented then it still needs G3 which the hg one doesn't. However I do agree that the use of a phe instead of a coil is another variant that has legs. They are trying to solve a different problem to your situation, and the problem is that there are a vast number of situations and no ideal solution so we do need lots of variants. To continue with @JohnMo's thought track, there are lots of different models of car, none with fundamental differences, yet each with a market. I think they deserve some credit for sticking their heads above the parapets in a part of the industry that seems very reluctant to innovate (I would go so far as to say that it's stuck in a rut).

.. largely because the gutter press and certain politicians cynically exploit it for their own advantage and almost certainly don't care if what they are spouting is utter rubbish, so long as it advances their cause.

I think that the amount of energy required to satisfy the demand for luxury, which either has to be stored or generated on demand, means that there is no silver bullet. In large new builds a large uvc is an obvious choice, but in smaller retrofits much less so and so compromises must be made. There are quite a few variables to play with in the trade offs so it's almost certain that a variety of solutions are needed. Thus any innovation, such as this one is to be welcomed. It's just a bit disappointing that they have conflated small size with a new cylinder configuration, as it would help immensely to tease out the effects of these factors individually (particularly the latter). My qualitative argument above shows that the latter is dependent on the engineering not the fundamental physics, so we do need to understand how the engineering has panned out (IE what, if anything, is the yield difference relative to a uvc of the same volume) to make an informed decision in any particular application. As I say above, if the dhw yield penalty compared to a similarly sized uvc is fairly small, then the choice in a retrofit situation becomes (IMHO) almost a no brainer, simply to avoid the hassle and installation and ongoing expense of g3.

If Australia has excess solar (which is presumably the reason), surely this is good news. As @joe90 says you can always disconnect.

Agreed. The real question for me is, does it make UVCs a niche product? This depends on the yield penalty (if any) which we don't know

This was one of the factors which led me to question whether the real advance is size, or whether it's avoiding G3. If the yield penalty relative to a uvc is small, then why bother with UVCs at any scale? Cynically the other benefit is that the slight obfuscation might enable installers to justify departing from the MCS guidance, no bad thing as it might allow an installer to meet the customer requirements rather than the MCS assumptions about what the customer requires!

Definitely agree, but it needs to be done. The unreasonable expectations of the British public (which seems to differ quite markedly from some of our continental European neighbours) is a major challenge, and politicians aren't helping by telling people that they can have what they have been tricked by capitalism into believing they need. But imho the main question (to evaluate the actual innovation) is the comparison with a uvc of similar volume.

I think it relies on the system expansion vessel and PRV to deal with overpressure. Thats no different to a buffer tank with an inbuilt immersion for backup.

That's my understanding also.

On the other hand in a larger cylinder you can fit more dhw coil in the top (say) 10-20% reducing the percentage lost volume as a result of insufficiency of coil exposed to the top stratum. Agree that's how it is presented and very sensibly so imho. But my conclusion, based on the argument above (which I'm hoping you might critique in case it's wrong!) remains that a uvc of the same size would likely yield more dhw, so the main problem actually avoided is g3 not cylinder size. For the avoidance of doubt I still like it and applaud heat geek for doing something different to solve a real problem).

Regs don't need amending, they have vast flexibility and pretty much specify only that the system must be safe. It's the guidance that accompany the regs, which are followed slavishly by the industry, that is the problem. The guidance is neither obligatory nor does it guarantee compliance (this is stated in the guidance). Thus if the industry chose to develop something else it could

OK, I have given this some consideration. At first I thought it will be very easy to prove that a UVC performs significantly better (in terms of DHW yield) than the mini-cylinder of the same volume. However it turns out (I think) that its not as simple as this because of stratification. The UVC does yield more DHW than the mini-cylinder, but the difference is dependent on the degree of stratification and the effectiveness of the coil, particularly at the top of the cylinder. In the limit case of perfect stratification and a coil capable of infinite heat transfer, the performance is, I think, the same!. In other words it becomes an engineering challenge not a physical limit, and its perfectly plausible that a sufficiently well engineered mini cylinder performs nearly as well as a UVC of the same size, but without the G3 complication. The upshot of this is that the mini cylinder does not solve the size problem (a small UVC would do this just as well, in fact slightly better), but it does solve the G3 problem with potentially a small penalty in terms of DHW yield (or equivalently, cylinder size). So what Heat Geek have really shown in their experiments (if my argument below is correct) is 1. that the 200-400l cylinders we ‘need’ are not needed if you make some reasonable compromises 2. that a well engineered mini cylinder can perform sufficiently well to serve a useful purpose (albeit still not quite as well as a UVC of the same size), with the advantage that no discharge pipework/arrangement is needed. The crucial question is, how much is the penalty? If its 10-20%, which is very plausible for a well engineered design, then the trade off is going to be worthwhile for many and what Heat Geek have potentially done is knocked a major hole in the UVC market. If its quite a lot larger then the better compromise for most, if space is at a premium, will be a small UVC ---------------------- Rationale for the conclusion follows, however the implications (if the rationale is correct) appear above so you dont have to read on (but I would be grateful if you do since you are invited to critique in case I have made an error. We are going to consider the DHW yield of a UVC and a mini cylinder of the same size, both of which are heated simultaneously by a heat source. To do this we will work out how much useful energy can be extracted from the cylinder and the heat source. With a perfectly stratified UVC you can extract all of the stored water at the stored temperature and mix it with incoming cold mains water to produce DHW at the desired temperature. The amount of energy which is extracted from the store is VcdT, where dT is the temperature difference between the incoming mains water and the store temperature, and, V the volume of the cylinder and c the volumetric heat capacity of water. In addition, if you are heating the stored water simultaneously, you will be able to extract a further amount of energy equal to the energy transferred to the cylinder in the time taken to drain the cylinder, which, with an adequately sized heating coil, we can take to be PI where P is the power available from the heat pump and I is the time taken to empty the tank (which depends on the draw off rate and desired DHW temperature, neither of which we actually need to know. Actually its a bit more than this because a bit more energy is fed in during the time taken to extract this additional amount of energy. We don’t need to work this out exactly. With a mini cylinder the dynamic is slightly different. The incoming mains water is at the same temperature as the UVC case so will continue to be heated at least a bit until the initial charge of hot water is exhausted, but this is only useful if it is heated to at least the desired DHW temperature (this is the argument generally deployed o explain why heat stores are less effective than UVCs). However, if the mini-cylinder is also perfectly stratified, the bottom of the cylinder will eventually cool to the temperature of the incoming mains (just like the UVC) but (also just like the UVC) the top will remain at the original store temperature. Only when the amount of water left at the top has shrunk to the point where the area of the coli passing through this layer is too low to heat the water passing through it sufficiently, will the emerging DHW fall below the desired temperature. In the limit case, where the coil is very large, this mean that you can continue extracting useful heat until just before the initial charge in the mini cylinder is exhausted. Thus the energy you extract in this limit case is VcdT, the performance is the same (in the limit case). The extra due to the additional heat supplied during the extraction of the water is also the same in this limit case. Now obviously the limit case cant be built, but its quite plausible that its possible to design a coil (and shape the top of the cylinder) so that the heat transfer from water to coil is still sufficient when only 10-20% of the water in the cylinder (at the top) remains at the original store temperature. In this case the penalty is limited to this figure. What we really need to know is what this figure is. So, you are doubtless thinking, if this can be done with a mini cylinder, why not with a conventional thermal store. There are two crucial engineering changes relative to a traditional thermal store namely a) this stores heating water, so the store temperature is the same as thje flow temperature not reduced by the approach temperature of the primary heat exchanger and b) the DHW coil has a very large area, so the approach temperature is small and (presumably) only a fraction of its length is required to heat the incoming mains water to the DHW temperature, allowing much more of the stratified top of the cylinder to be used.

@JohnMois the expert on the regs (and more to the point the accompanying guidance which the industry has decided it must follow slavishly) but my recollection is that G3 applies to stored potable water, which is why buffer tanks don't need the vent arrangement.

In the cold light of day I'm now asking myself, does this arrangement perform better or worse (in terms of water draw off volume available) than a uvc of the same size, also with a large coil and the heat pump controls arranged to reheat more or less immediately. I will try to do the maths unless someone else has a ready answer. If it turns out to have the only a marginally poorer performance, then it's a solution to the G3 problem not the volume problem, still valuable but perhaps a little less valuable than it appears at first sight in a typical combi replacement scenario, where there is almost certain to be an easy route for the vent pipe.. If it's an equal or better performance, then of course it's a definite win (in fact at this point it becomes almost no brainer).

It's a cylinder containing water from the heating system, through which a (large area) coil runs that is fed with mains water and which feeds the hot water outlets. The cylinder is connected to the heating system via the usual dhw diverter valve arrangement. Call that what you will!

No stored volume of domestic water is below the need for G3. It's a buffer cylinder really with a DHW coil, not really a thermal store. Exactly as I listed in the image above. With the immersion heater option it definitely could boil. As @JohnMo says stored domestic dhw below G3, presumably it relies on the system prv and expansion vessel to deal with that (so the latter may need up sizing, as indeed it might with a buffer that has an immersion included as a 'backup'.) I agree with @JohnMothat the tank itself (but not how it is plumbed) is better described as a buffer with a dhw coil. I'm not sure I'd want to call it a buffer though, that has too many connotations in a heat pump context.

As a matter of interest, how does that work out during an extended recovery in cold weather?

The second video explaining it is now out and, as I am neither a father myself nor have a father who is still alive (its Fathers day!) I have the time to watch it. Its quite simply a small thermal store (filled with 'central heating' water but most likely at an elevated flow temperature as is normal for DHW) with a very large coil for the exchange of energy to the DHW, thus minimising the approach temperature. That's it. The only other 'clever' trick (if you consider it clever) is to arrange the settings on the heat pump to switch to DHW mode as soon as the store temperature drops by only (say) 3C, thus prolonging the 'life' of the store beyond that which you would calculate by assuming no heating whilst demand is present. On the video it was demonstrated with a 7kW Vaillant at OAT 7C. Nothing revolutionary so far as I can see except that it deploys pragmatism (ie it doesn't provide for all possible extremes of demands, just a reasonable demand), a feature that seems to be largely absent from the thought process in much of this area. Will the industry 'buy' it. Personally I fear not, because (I fear) there may be a preference to avoid altogether jobs where the customer may have to make a compromise (like waiting 15-20 mins between showers) and because (I fear) customers are now so used to getting everything they want that nothing less will do. That said, there are now lots of 'heat geeks' out there who will presumably be more easily convinced. Personally I'm very tempted, if only to avoid G3, but then I don't need (or even want) a 20 min shower at 20l/s and nor do I have over-indulged teenage children who are terribly concerned about climate change but haven't yet realised (or been told) that mitigating it may involve some lifestyle changes.

Plenty of 6-9kW electric showers around, similar to a more typical 8kW heat pump. I don't think hg are aiming this at large, highly insulated new builds where space for a large cylinder isn't a problem. Also will the cylinder run out in the time taken for a shower, their video suggests not (based on 10l/s delivery rate). Certainly if the 'requirement' is a 20 mins shower at 20l/s then nothing short of a vast storage vessel will do, but in reality 2-5 mins at 10l/s or less is easily sufficient to do the job. If we are to fix climate change then we will need to adapt and, dare I say it, make sacrifices. Also we need a wide variety of solutions to fit the vast variety of circumstances not the one rule fits all that MCS seems to like.