Thermal store questions

17 Aug 2015
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United Kingdom
So the plan is this. Currently I have all rooms individually controllable through programmable TRVs allowing me to heat only the rooms that are being used on different profiles during the day and night. So far so good.

But I have three main problems with my boiler.

1. there is no boiler interlock with my TRV system so I have one rad permanently on and the room stat turns off the boiler when that area gets too hot. That is not efficient and might mean other areas are too cold. This issue is solvable but I might not need to if I go for a thermal store.

2. it is a combi boiler which, despite promises when it was installed, is not capable of running a bath and a shower at the same time and

3. it's not working properly anyway in spite of several maintenance visits so I'm sick of it.

The question is what do I replace it with?

Ideally I'd like a thermal store system. This would solve a number of issues:

- Boiler interlock is no longer a problem that needs to be dealt with because the boiler heats the store and the store pumps to the radiators.

- a system heat only boiler is simpler and presumably therefore cheaper and

- I get mains pressure hot water - yippee!


I am constrained in that the boiler (and where the hot water tank used to be many years ago) is in an airing cupboard so I am limited by how big the thermal store can be. The house has five bedrooms and three reception rooms and is old (and therefore drafty) It is adequately heated though by the current 33kW boiler. I think a 250l thermal store is as big as I can fit without potentially relocating it to the cellar, which is a bigger job. Is that anything like big enough for my needs?

So far I've seen (if I remember correctly) thermal stores that pump the stored water through the boiler or heat the store indirectly via a coil and also ones that pump the stored water round the rads or heat that indirectly via a coil. I understood stratification was a key element of a thermal store. Is one type better than the other?

I've also seen systems that heat the DHW using an external heat exchanger (which will fail if the power goes off) and
ones that pass DHW through a coil. The manufacturers of each type of course swear theirs is best.

Finally vented or unvented, sealed boiler systems or not? Frankly the one thing I do like about my current setup is doing away with the header tank although the system does very gradually lose pressure and has done so while we've been away on holiday so we've come back to a cold house. If both the boiler and the rad system were sealed then I guess I'd be looking at two pressure vessels although one might be integral to the boiler.

I am confused by so much choice. Do I (can I?) go for a thermal store or do I go for a regular system boiler and go back to having a hot water tank for DHW only or do I go for something like a Viessmann Vitodens 111W, which has an integrated storage tank and promises that it can indeed fill a bath and run a shower at the same time (I only found out about this today!).

So many choices. I'd be very grateful if the experts here could shed a little light on what's good and what's bad
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