DIY thermal store?

[evs2]

Hi all, I've been reading the various old discussions here about heat banks and thermal stores, and would appreciate some advice for my own situation.

I'm renovating an old cottage. It had a coal fire with back boiler heating a direct cylinder in the loft. I've replaced the fire with a stove, and the cylinder with an indirect one suitable for gravity operation (160L, the biggest that would fit), and separate header tank for the boiler circuit.

So far so good. I've also put in a radiator circuit (only 5 radiators, it's a small house and increasingly well insulated), intending to pump that off the other pair of stove boiler connections, using a pipe-stat.

However I've just discovered that I have mains gas after all, and so have bought a Glow-worm 18HX, intending to join the whole lot up with a Dunsley manifold and loads of pumps and temperature sensors. Or an H2panel.

But I know that a thermal would also do the link-up job nicely, and it would allow some buffering of the stove and the boiler, and provide mains pressure hot water. And may even be simpler / cheaper than the gravity system plus whatever linkup method.

But I can't find an affordable store tank in the size I need. So I'm after any advice about whether it's sensible to pursue the thermal store route, and are there any clever ways I could convert this tank to a TS? Can I reuse the coil in it for the DHW take-off, or won't it transfer enough heat?

I'm not particularly keen on the heat-bank idea with pumped heat exchanger as it sounds expensive and maybe overkill for a small property, but any suggestions are welcome.

Thanks

[robgraham]

I hope this thread gets some take up as much of what you are talking about is parallel with my own situation except I'm a little further down the time road than you. I have a wood burning stove running in parallel through a neutraliser with an oil boiler.

My understanding is that the heat bank is directly heated by all the sources hence the neutraliser would not be needed. The one problem that the heat storage system faces is that the tank does have to be somewhat bigger than that for a normal DH system.

I'm not sure quite how you would get round not having a HW exchanger as one of the problems with single storey properties is that the gravity feed is inadequate and mains pressure HW solves all the inadequate pressure in showers, taps, etc.

And please, please, please ... let's not have the combi boiler suggestions as that is not what is wanted in this thread.

Rob

[YAPH]

I recently put in a system with a solid fuel stove with back boiler (12kW output) combined with a gas combi boiler. Rather than have the gas boiler waste energy and store space heating up a store when it can produce heat on demand I connected them up so as to have the thermal store (a pre-loved direct cylinder) act as a buffer for the solid fuel stove when it was burning (since you can't turn it on and off) and the gas boiler running when the stove and store were cold. And since space heating elsewhere in the (quite big) house was likely to be required when the stove was on (you wouldn't run it in the summer!) the system was arranged to direct the stove/store to heating the rest of the house. Eventually it should do DHW as well, with the combi taking over as necessary, but that's icing on the cake (and Round Tuits away).

The system required several pumps, some motorised valves, a relay and some head-banging logic and was quite laborious to build but it does actually work ;-) Though there are still one or two wrinkles to iron out.

[robgraham]

John - what is the size of the cylinder you are using ?

Yes - you do need the head-banging logic for the control system, perhaps even more using a store and a combi. Mine is effectively two heat sources in parallel with an adjustable hysterisis temperature sensor on the wood burner output, that shuts off the oil burner when the stove is hot. I get away with 2 pumps and one valve into a traditional DHW and CH system.

The idea of a buffer tank for the stove is appealing and introduces the thought of perhaps two tanks - one from the stove and one from the oil burner. Interconnection will be a headache but with limited space I have it might be the solution. My target is mains pressure HW and instantaneous CH - yes, I know that's what combi gives but I am not going down that road, and anyway to my knowledge there aren't oil combis.

Rob

[evs2]

Hi Rob,

That's correct, as I understand it. Well, the store doesn't *have* to be bigger but it probably helps.

Of course, you do need a heat exchanger for DHW. In a thermal store this is just a copper coil in the top of the tank, through which mains pressure cold water is fed and is heated by the store "on demand". What I've been wondering is whether the typical boiler coil in a normal (cheap) hot water cylinder would work for this. Of course, it tends to be in the wrong place anyway, but I've been wondering about unsoldering it, fitting new bosses higher up and moving it..

I got a quote for a similar sized thermal store tank to my =A3150 DHW cylinder. 800 quid + VAT :(

Of course the TS comes with a mixer valve and expansion vessel for the hot water, but those do not justify that price premium IMO.

[dom]

Not unless you were passing water through the coil a few drips at a time.

When I've drained down my thermal store, there's a lot more water in the coil than I envisage - that coil is very, very long (and finned I think), to get the heat transfer.

[evs2]

OK, thanks for the info!

[robgraham]

The info that I have picked up is that external heat exchangers are really not that expensive and are extremely efficient. This does add the complication of pumping the hot water from the tank through the HE; the question is whether this is more expensive in complication (less in outlay I think) than the expensive special tank ?

The one particular advantage of an external HE is that it can be serviced more easily. Rob

[<me9]

Any sources for such a heat exchanger at reasonable price?

[robgraham]

Oops, put myself on the line there, haven't I. The comment came from a memory of reading a post somewhere which gave a price for something like a 10KW one that may have been

[dom]

You can buy combi boiler heat exchangers on ebay - but you might need to use 2 (or even 3) to get a decent heat transfer rate (e.g. to fill a bath at a reasonable rate). You might be better getting the proper part.

Power cuts also mean you are instantly without hot water - whilst themal stores will at least ride-through until exhausted.

[<me9]

What I'm particularly looking for is a heat exchanger to use with my system boiler to produce mains pressure hot water just for a shower. For other uses I want to retain the gravity fed hot water. Combi heat exchangers mostly seem to have non-standard connectors. I'm looking for heat exchangers that will handle mains pressure at ~15 kw at £50 or less, and connect with standard parts.

[Tim Watts]

[e-mail] ( [e-mail]) wibbled on Thursday 10 February 2011

00:36:

Here:

got one from them for the one I will eventually build - I suggest aiming for the order of 100kW - you'll need to know your cylinder normal operating temperature and assume the mains water is somewhere around 5-8C.

Flows:

The most I can get from a 1/2" alkathene incomer (10m long) is around

55l/min (I *have* measured it!) so ideally you would like hot water at that rate - but less may be sufficient (eg sum of a bath tap and shower or shower and sink).

Specific heat capacity of water is 4200 J/kgK (joules per kilogram per kelvin heating) and a kg of water is a litre near as dammit. So you have enough numbers to do a few calculations.

You may conclude the PHE is oversisized but this compensates to some degree for when either the incoming water is mega cold (melting snow in winter round my way maks for exceedingly cold water as my pipe is only 2' down) or the store being a bit under max temperature or everyone trying to use HW at once. So a rule of thumb is a 100kW PHE for a 2-3 bed house isn't as mad at it may first seem - and also explains why small cheap combi boilers suck.

There are some useful data and claculators here too:

Tim