Connecting combi boiler to thermal store

[Tuna]

So, we have a lpg combi boiler, which I'm planning to connect to our new house's heating system - a small thermal store.

The store has coils for solar, dhw and the boiler, as well as direct connections for UFH and a wood stove.

The boiler does hot water and central heating, though in its previous life (18 months in our caravan whilst we built the house) it just supplied hot water.

So my plan has always been to connect the combi's central heating circuit to the boiler coil on the store. Our solar controller (a Navitron TDC3) has a spare switched output for calling for more heat if the store gets low, so I was planning to combine that with a timer unit (with some magic electrical thingy?) so that the combi comes on when (a) the thermal store is low and (b) it's a sensible time of day.

However, talking to plumbers, they've asked about diverter valves, pumps and all sorts. My expectation was to just do a pipe run between the store and the combi, and that the combi would handle everything - no need for diverter valves, expansion cylinders or other plumbing gubbins - literally just flow and return pipes.

Does that make sense?

Here's a diagram of the internals of the combi boiler (a Glow-Worm Ultracom 24cxi) - looks to me like it has all it needs.


Here's some rubbish ascii art:

 +---------+
 |         |
 | Boiler  |       +-------+
 |         |       |       |
 |         |       |       |
 +---------+       | Store |
 CH |   |CH        |       |
Flow|   |Return    |       |
    |   +----------+       |
    |      Coil out|       |
    |              |       |
    +--------------+       |
           Coil in |       |
                   |       |
===================================

Edited by Tuna on Thursday 9th December 14:53

[ferg]

Yeah.
Sounds fine to me. You should keep some hot supply from the boiler to a tap rather then ignore it, but otherwise if you have just one 'zone' i.e. Thermal store and no heating, then diverter valves are pointless.

[Tuna]

ferg said:

You should keep some hot supply from the boiler to a tap rather then ignore it,

So plumb in a cold feed and put a tap on the wall for hot?

[ferg]

Yeah. Or maybe hot to the nearest tap?? Sometimes it can save pulling hot from the store across the house for a half a minute wash of the hands! Obviously I don't know the layout of your house, but it's a thought.

PS Don't talk to those plumbers again...

[Tuna]

ferg said:

PS Don't talk to those plumbers again...

We've fired our plumbers as they made lots of promising noises, then ballsed up the job. The basics are in place, but we're currently running off an immersion element as there's no way on earth I'd let them back in to connect the other heat sources up.

So now we're trying to find a reliable plumber that won't run a mile the moment we mention stuff like solar and thermal stores. Not surprisingly most are coming in to see a half finished job and are sucking their teeth and asking "what's going on here then?".

[Tuna]

BTW - Thanks Ferg, I appreciate you answer a lot of the questions around here.

We'd have you round like a shot but (a) you're not that close to Cambridge and (b) I'm sure you've got work right through to next spring right now.

[andy43]

If you've got tappings on the store at/near top and bottom it may be more efficient to hook the boiler up to the store direct, without a coil. Boiler fed from a tapping right at the bottom of the store.
We've done this, and the result is the boiler gets fed the coldest water from the bottom of the store - ideally it should have a tmv on the return to give the boiler the best return flow temp for condensing. We'll do that eventually...
I've also fitted two stats on our store, toward the bottom/middle of the store, with a hold-off relay, so there's no phantom boiler firing, plus I can set the stats so the store can be depleted of quite a bit of it's heat before the boiler is called in. It then runs it's nuts off to get the whole store warmed up again.
Via Navitron etc I have read of boilers feeding stores via coil, and they do have a tendancy to cycle as some coils can't transfer enough heat into the store. S'pose a dead high efficiency coil may be ok, but unlikely to be as good as a direct connection. Stick an average 20-30kw boiler through a coil, and it *may* struggle - only from what I've read.
Doing it direct solves that problem instantly, and frees up a coil that may be used as a dhw preheat/solar/whatever coil instead.
We've valved ours up so that switching two valves over means all the dhw is combi-supplied rather than via store - I do that maybe once every couple of months to keep the dhw side of the combi working - plus there's a backup dhw supply if the shunt pump or flowswitch on the store heat exchanger breaks. If your dhw is coil-supplied, I don't suppose there's much to go wrong there