JamesPa

So if there is no immersion in the cylinder, doesn't that satisfy the G3 requirements on its own without all the vent faff?

I guess the question is does it really, or is it the easy solution given that (I guess) most gas installations will also have an immersion. Also do gas boiler manufacturers tell you that their product can't get above 100, I don't recall seeing this in the specs. Only a small proportion of the plumbers I have encountered think out of the box, so I wouldn't expect much innovation. Re your 'kind of foreign' comment, I would say most definitely. Also 'new' and of course 'new' things need more regulation don't they, heat pumps being the obvious example in this context.

I know this has been raised before and I also posted a similar question on the Hot Water Tank thread (so apologies if you have read it there) but... I realised the other day that the R290 Vaillants don't rely on an immersion heater for their legionella cycle, they do it natively. Presumably others will follow suit as they introduce R290, possibly as an option, possibly as the only option (as it is with the Vaillants). These being the case, is it to re-think how G3 is satisfied with a UVC connected to a heat pump? The requirement (as opposed to guidance) is (3) A hot water system that has a hot water storage vessel shall incorporate precautions to: (a) prevent the temperature of the water stored in the vessel at any time exceeding 100˚C; and (b) ensure that any discharge from safety devices is safely conveyed to where it is visible but will not cause a danger to persons in or about the building.' If we omit the Immersion altogether requirement, 3a satisfied automatically, so far as I can see without vent valve, tundish, vent pipe etc, and thus 3b does not apply Now if we do want a backup, whats to stop us putting that in the heating flow pipes to the tank, rather than in the tank itself. Answer - nothing, its called a backup heater and many HP manufacturers offer one. Now this could boil the tank, because its in a pressurised system, but we can incorporate backup devices into the heating pipework, rather than the tank pipework to prevent that happening. For example the usual immersion cut out valve plus a valve which diverts the heated flow away from the tank if it gets above say 95 - a bit like a car thermostat. The point of all of this is to avoid having to run the vent pipes from the tank, which is often in a space inaccessible or not conveniently accessible to the outside. Furthermore, as all of the safety features are contained in the heating system itself, the HP manufacturers can incorporate them and tank manufacturers can do what they are good at, making tanks. Have I missed anything (apart from the difficulty with getting the heating industry to think outside the box (or, in this case, cylinder)?

Well the D1 D2 stuff isn't actually in the requirements, it's in the guidelines. But my key idea here is to remove all of that from the cylinder so you can put it somewhere more convenient or do something more convenient. The rules of physics are older than the hills, but still people keep inventing things.

Could you put it in the loft (if you have one) but within the thermal envelope (ie no loft insulation below, but 'loft' insulation all around) Thats my current plan which will be less lossy than the vast hole I currently have in the loft insulation to keep my cw storage tank above freezing.

Perhaps I should have been a bit clearer where my thoughts were taking me. Im trying to think 'out of the tank' both metaphorically and literally. The principal practical pain with G3 is routing the vent pipe. So my thought process was - can this be avoided. The requirement is: (3) A hot water system that has a hot water storage vessel shall incorporate precautions to: (a) prevent the temperature of the water stored in the vessel at any time exceeding 100˚C; and b) ensure that any discharge from safety devices is safely conveyed to where it is visible but will not cause a danger to persons in or about the building.' So if we have indirect heating from the HP, we know that cant boil the water. If we also have indirect heating from the backup immersion, then I grant that it can. So it has a safety cut out as usual, plus some other mechanism, not involving the tank, to ensure it cant boil the water in the tank. A simple overtemperature tank bypass (similar to a car thermostat) would do the job. Result, tank cant be boiled and all the safety features can be located somewhere convenient, not tied to where the the cylinder is. How would that not satisfy the requirement?

It looks worse than the inside of my airing cupboard which yesterday I was easily convinced should be stripped out in its entirety. The airing cupboard is the result of successive mods, not a one off design.

My understanding is that it isn't much of a problem for A2A. The modulation depth of A2W is much less and this is where the problem lies AFAIK.

Makes sense. Also with low temp bio contamination of the circulation system must be a risk and sealed eliminates that. Less important though if the ashp does the legionella cycle natively as opposed to just firing up the immersion, which I now learn Vaillants do.

All of that agreed. They have the potential to be a very simple solution for difficult living areas and, because the fan speed is easily controlled, come very close to auto balancing for room temp, which is whats really needed. Of course A2A is really just another fan convector so it does beg the question... I will need one, maybe two and will probably end up buying the Mitsubishi ones made in Italy. This will be an interesting part of the journey. Prices need to drop though to become mainstream.

All that piping just for dhw? As you say it seems kinda mad, surely a tank with integrated hp is better for this situation.

Alledgedly the smiths ones are cr*p. You pay about 500+ for supposedly better ones. I'm guessing the components are sub 50 in total.

Put the numbers into the excel I posted (I'd do it for you but ukpn are changing out their bits of my elec installation, I only requested a fuse upgrade but they took one look and said 'we will just do the lot, it's that old').

They do, does anyone design to use them I wonder. Interesting if as @JohnMosays, Grant advocate undersizing.

Yeah, after assuming unreasonable air change values, ignoring fabric upgrades then rounding up to the next size. I don't think sizing for 99.6 could be described as undersizing.

Thats the way to do it, anticipate the inevitable, make as many changes as possible at a convenient point in time. Alternatively stick your head in the sand and expect MUCH bigger bills, migration like you have never even imagined before, wild fires, flooded cities, societal breakdown and dictatorship. The political effects of climate change are, in many ways, more scary than the physical ones. Oh and re rad size, what about fan convectors? OK you might not want them in a bedroom but in the living areas whats the real issue? Are your fridge, TV or for that matter your kids silent?

Well done! Can you share the sizes/temperatures, it would be good to calibrate my model to check if I've made a mistake in the logic.

The guy who is doing my ashp is fitting a PHE and pump to a off the shelf UVC. It's more or less his standard install, although he has also retrofitted same to an existing VC. When he does it I will find out what control he uses, probably your simple (second) suggestion above.

Bit silly of Warmflow - they might have had a ready market! Mitsubishi also do a plate loaded cylinder I think, not sure if it suffers from the same stupidity.

If its leaking heat that fast surely some improvement is possible with insulation in at least most cases and would pay dividends in cost terms. In the limit you can get heat pumps with almost any capacity! That said I wouldn't necessarily advocate installing one now in this circumstance, but at the same time its a myth that heat pumps cant heat larger/leakier buildings, they can and do. However I grant that the timing/sequencing might be wrong at present. Fossil fuels have to go, that's just how it is. Heat pumps appear to be the only realistic answer for domestic heating in almost all cases. The longer we leave it, the more difficult the fixes become - had we been serious about climate change 30 years ago (when scientists told us more or less exactly what was very likely to happen, which turned out to be right) we wouldn't have such a tough time now!

I'm beginning to wonder if there is a good argument for undersizing a (A2W) heat pump, particularly one with poor modulation range, or if not for deliberately undersizing, at least for erring towards undersizing, rather than the 'received wisdom' of erring towards oversizing then adding 20% for (insert your reason here). The BRE model https://tools.bregroup.com/heatpumpefficiency/index.jsp and data seems to suggest their might be in at least some cases, although until I understand it I remain sceptical about some of the figures it throws up. Without more robust data on the degradation due to both short and normal cycling, it's difficult to be certain. However it's entirely plausible that having a supplemental electric heater and suffering a cop of 1 for 20% of the load 10% of the time (when the cop would otherwise be only about 2-2.5 anyway) is a good trade off vs compromising the performance for much of the heating season.

I'm resurrecting this thread to clarify a possibly interesting scenario. The ashp I'm about to have installed does it's legionella cycle natively (ie using the ashp not an immersion). Clearly it can't itself boil the water in the tank. The cylinder currently proposed has an immersion solely for 'backup'. The reheat mechanism is an external PHE not a coil in the cylinder If either (a) the immersion were removed altogether or (b) replaced by a Willis heater or similar in the pipes feeding the PHE (and suitable safety devices incorporated so the willis can't send steam through the pipes), does the cylinder itself need the full G3 works (and in particular the visible, continuous fall vent/tundish)? As it happens it will have the full G3 works but the question interests me because of the problems with routing the G3 vent pipe which are forcing me to put the cylinder in the loft (I don't particularly mind this, but am curious nevertheless). The regulations (as opposed to the guidance on what the secretary of state thinks) say only: '(3) A hot water system that has a hot water storage vessel shall incorporate precautions to: (a) prevent the temperature of the water stored in the vessel at any time exceeding 100˚C; and (b) ensure that any discharge from safety devices is safely conveyed to where it is visible but will not cause a danger to persons in or about the building.' Thoughts?

Yes and no. The flow of energy to the rads is only one part of the equation, it's the flow from the rads that matters and is the limiting factor. So if I heat the rads half time I do need a 70pc higher delta T between rads and room to get the same energy from rads to room. I am assuming that the rads respond quickly to turning off the flow, which typically they do. I think it might. This is almost the opposite, as you say, to the rad situation. You can get all the energy out of the slab that you can put in. So if you can put more in by a faster flow at the same deltaT across the flow/return, then you will get more out. Because the slab stores energy the on/off cycles have less effect. Not sure if it helps op and @JohnMo makes a very good point, but as another variable to play with its interesting.

I can't speak for others, but for me it wasn't a case of not accepting it, the fact that a TS will yield less dhw is clear and the reason obvious. I originally did the maths/physics because someone on the openenergymonitor forum challenged me to explain the physical principles, and it piqued my interest because I realised I couldn't quantify the benefit of stratification, which clearly has the potential to make a big difference as many here have pointed out. Having done so it gives a quantitative argument not just a qualitative one, which makes any design choice easier/more certain.

PS to the above - perhaps you mean 'not currently cost effective' as opposed to 'not suitable'. That then becomes a temporary and political problem to solve not a permanent and technical one, given that the price of energy is politically determined.