What is the advantage of generating DHW by (over) heating the primary water and generating the DHW on demand via heat exchanger? Surely it is simpler and more efficient to to heat and store the DHW at the 60 degC recommended to avoid legionella?
It may be, it might not be. Definitely 6 of one, half a dozen of the other.
In one, you heat a large cylinder of water (a little warmer than you need the DHW), and transfer the heat via heat exchanger. In the other, you heat the primary water hotter than you need the DHW and transfer the heat via heat exchanger. What is the return temp from the indirect coil when heating the cylinder ? I'd wager that for a lot of the time it's not that cool - does it stay cool enough to keep the boiler condensing ? There are many variables in that - the temp of the DHW cylinder round the coil, the capacity of the coil (which reduces with reducing delta-T, the required flow rate in order to keep the boiler "happy", and so on.
I can answer those questions for a thermal store (if properly designed and implemented). The boiler return should never need to be more than the return temp from the rad loop(s) - and can be considerably lower (but note what I said earlier about heat banks being different).
Note again, I'm NOT
saying that a thermal store IS
the right option for him, just that I believe it isn't automatically the wrong option either. Once you actually look at it, I find it hard to believe your preferred option of an indirect unvented DHW cylinder is more efficient.
And that's only considering the DHW.
Once you consider the CH then it gets even less clear. A thermal store will very effectively decouple the disparate requirements of the CH loops (variable flow rate) with that of the boiler ("quite high" minimum flow rate). If there weren't this difference in requirements, then there wouldn't be all those heating systems with bypass valves required to patch the two together. Once you have a bypass in operation, then the boiler return temp is going to be higher than the return temp from your rads.
I'll throw in another reason why I believe that a buffer tank of some sort makes sense - as a neutral point. The OP has a very
wide range of heat load. At one extreme, CH only during warm weather, at the other extreme, reheating the DHW while the CH is coping with cold weather and the swimming pool is in use.
Designing the hydraulic circuits for 2 boilers*, 3 CH zones, DHE, and swimming pool is almost trivial when they don't interact - somewhat less trivial when they do.
* I'd consider 2 boilers rather than one big one. Gives some redundancy for breakdowns for one thing.
Really ? Check a dictionary then.
You're having an argument about engineering with a professional engineer.
An own goal there I'm afraid. Read the very first line of the page you link to, it says "A professional is a person who is engaged in a certain activity, or occupation, for gain or compensation as means of livelihood"
. Yes I am fully aware of the other meaning of professional, but it is wrong to say that "'Professional' (as against amateur) used in the context of sports has a different meaning to 'professional' used in the context of work."
have a different meaning, but it is not true to say that it does
have a different meaning.
PS - I have postnominals to my name as well, I make a point of not mentioning them here. As I'm not professionally (as in paid for it) engaged in plumbing or electrics, I don't want to risk someone thinking I might be (hence my sig).
Well yes, I can see why he suggests that the Thermal Store is not necessary, so let me confirm my objectives.
A] CH at 40C – 70C as needed
B] DHW at about 50C
C] Pool, taking as needed when available or timed for off-periods
Currently I have a 51kW plus a 30kW boiler which take about 8cu/mtrs if operating at the same time. This comes off a standard 6.4cu/mtr meter. It does still work however.
Brit Gas tell me that I need a new supply pipe and a commercial meter – this is unacceptable.
Out of interest, are BG telling you that you need to reduce your demand, or change the supply, "or else we'll cut you off" ? And is whoever services and maintain your boilers/system happy with the situation ?
I suspect that BG will be concerned that if you "overload" the meter then it might not record correctly. It'll only be designed to go up to a certain flow rate, and it's not guaranteed to operate properly above that. From the safety POV, it's a case of whether you still have an adequate supply that will run the boilers properly.
The objective of the Thermal Store is to build up a buffer store of hot water at a temp from 60 -80C as proven needed in practice.
This is something worth exploring. What do you have now - just rads and an indirect DHW cylinder off the boilers ? Are you saying that you heat the DHW up to that sort of level so that you can (for example) run several baths and after mixing it down still have enough hot water to go round ? There's a tradeoff to be made here - if you can store more water, then it can be cooler. I do agree with Onetap here - there is little point heating water hotter than needed.
The CH will draw via TMV a temp 40 – 65C as needed. Possibly this would be modulated by weather compensation controlling the TMV.
I suspect there are many way you could control the TMV. Weather comp would be one of them.
The return temp could be expected to be 30 – 50C which I am told suits the condensing boiler.
The DHW can draw at 50C from the HX in the store. [Not compromised as it would be without the heat store, and needing to draw from the boiler temp.]
OneTap is suggesting an unvented cylinder for heat storage - he's not suggesting a system without storage (though you will find plenty of "combi for all" people around). An unvented cylinder is very much like the vented one you probably have now - but pressurised to mains pressure† so you still get hot water at mains pressure.
† To a point - there will be a pressure reducing valve to restrict cylinder pressure, as well as other safety valves. The regs for unvented cylinders are quite strict in that the cylinder must come with all the valves as a package and it's not allowed to "mix and match" parts. Without the safety features, an unvented cylinder can be highly dangerous, with them they should be fairly safe (AFAIK we never hear of accidents involving properly installed ones).
As a point regarding TRV’s, the theory is fine but their effectiveness is not frequently possible when the rads are oddly positioned or then doors are left open, as in my case. [?]
Then you have another problem if your system isn't really controllable.
Ps What does SFAIK mean ?
So Far As I Know ?
AFAIK, As Far As I Know