Tank in tank thermal store with 2 primary connections -issue

[SimonH2]

I'm guessing there will always be a compromise in terms of what is sent back to the cylinder to prevent a couple of pipe's worth of water spinning around the boiler circuit until it reaches the sweet spot. Has to be better to get almost up to temp water into the tank than for the rads to keep on eating away at the thermal store.

Is there a generally accepted rule of thumb?

Sorry, I don't get what you're asking there

 

[Mustardshots]

I'll try another way. The point of the thermal mixing valve is to prevent the cold return from the radiators (+14degrees from elec boiler) going straight into the top of the cylinder and cooling the overall mass of water - yep? It works by circulating the water thru the boiler until it reaches a chosen temp-say 60degrees.

The question is-what should that temp be to make best use of boiler and thermal store bearing in mind that thermal store temp will be reducing all the time if the rads keep calling for more heat.

 

[SimonH2]

There isn't a right answer to that

Normally there is a gas boiler involved, these days a condensing one. For those, you want to keep the return below about (IIRC) 54˚ so it will always be condensing - but to get the store hot this does mean restricting the flow a bit and running them with a higher delta-T than the manufacturer specifies. In your case there's no such restrictions, and until you reach the upper setpoint limit on the boiler there isn't really any restriction.

So, from the point of view of storing the most useful heat, this is what I believe you need to think about :

1) The hotter the better. Obviously, the hotter the water, the more energy stored in it. There are some constraints (upper limit on boiler for example), and the downside is that the hotter the store, the higher the standing losses. Losses (ie the heat that leaks out of the store) can be minimised by insulation, and as long as the store is within the heated area of the building then the losses aren't really lost - except in hot wether then you'd prefer no heating.
So you may well want to adjust the system for the changing seasons. For the same reason, some people have several thermostats (or sets of stats) and switch between them - heating the full tank in winter, and only the top in summer when they are using little heating and less hot water.

2) Stratification is important.
If you can keep everything stratified then you can take more useful heat out of the system. ie, you can keep drawing off the hot water, and the returned cold water will push it upwards. In an idealised setup, you'd be able to draw off all the hot water at whatever you heated it to in 1) above, and end up with a tank full of cold water. In practice it's not as ideal as this.
On the other hand, if you don't design the system right and mix all the water up, then you cool the whole tank and end up with a tankful of tepid water that's not hot enough to be useful. This could cut the amount of available heat by half (or worse).

Also, if you keep things stratified, then your reheat is more effective. If you have run the store right down, then as you put hot water in at the top, you don't need much before you have useful heat available - once you've got hot water in the top of the store, then you can draw it off and use it. If you heat "bottom up" then you end up having to heat all the water in the store before you can draw off anything useful.

 

[Mustardshots]

So I guess the effect of the thermal valve really is to impose a lower limit on the top of the thermal store (in terms of the heated water that's added).

The elec boilers will go up to 90 degrees - pretty decent! Anything above 50 is going to be useful for hot water. I'm now wondering about adding or adapting your system further to 2 stages: 1) keep circulating the water around the boiler until it's close to 50-60 degrees and the pass to 2) if lower limit send water to mid point of tank, if closer to 90 send to top of tank.

It's all well and good but I think in reality I am dreaming. If I can figure out how to get your suggestion working I'd be a very happy man

Many thanks again

Pete

 

[SimonH2]

So I guess the effect of the thermal valve really is to impose a lower limit on the top of the thermal store (in terms of the heated water that's added).

Exactly, that way you put hot water in the top at the temperature you want the top end, and it displaces the cold water downwards.
For your setup, the good thing about that is you can heat just the top section (with a separate thermostat) if you run out of heat during the day - so for a minimum energy input, you can keep a small volume of hot water ready. Then when your cheap rate kicks in, switch to a different stat and heat the whole cylinder top-down.

The elec boilers will go up to 90 degrees - pretty decent! Anything above 50 is going to be useful for hot water. I'm now wondering about adding or adapting your system further to 2 stages: 1) keep circulating the water around the boiler until it's close to 50-60 degrees and the pass to 2) if lower limit send water to mid point of tank, if closer to 90 send to top of tank.

The trouble with that is it gives you bottom-up heating. It'll work if you are planning to heat the whole store (ie at night), but if you need a top up during the day you cannot heat just the top end like that. Since you have no efficiently issues about going straight to the higher temperature (unlike those using condensing gas boilers), there's no point in complicating things.

So to summarise, what I'd suggest is :

Rig up a TMV (or whatever) so that you only put hot water (at whatever temperature you decide) into the top of the store.
For night time/cheap rate, sun the boiler from a thermostat low down on the store, and set to a fairly highish temperature so you completely heat the entire store.
When on peak rate, use a second stat positioned and set to maintain a minimum quantity of water at the minimum acceptable temperature - just to avoid you running out completely.

It might be worth contacting the boiler manufacturer's technical department, explain what you are doing, and ask if there are any approved operating regimes that might help. In theory, it should be possible to reduce the flow rate through the boiler to get the right outlet temperature. They will have specified minimum flow rates/max temperature differentials to ensure that anything complying with the specs will work correctly and safely - but that doesn't automatically mean that anything outside of that is unsafe. I doubt they'd allow reducing the flow rate to such an extent that you could go from (say) 25˚ to 80˚ in one step !
Talking of which, are your boilers connected in series or parallel ? If each can do 14˚ at the nominal flow rate, in series they could do 28˚ in one step. You only need to get the inlet at 52˚ to 80˚ out then. The downside is that as the return temperature increases, the second boiler will turn off/reduce it's power and you'll reduce the heat output.