Problem with Thermal Store?

[Mike Hibbert]

Hello all,

My brother has just moved into a new house and guess what.... the heating is playing up!

The situation is this.

If you turn the CH on and leave the HW off, you can hear the pump start to spin, but the boiler doesn't fire. If you turn the HW on, the boiler fires, and if you leave the HW and the CH on, the radiators will get hot. But if you then turn the HW off the boiler turns off aswell. It sounded to me like an electrical fault at either the programmer or the boiler.

British Gas came out today and told him it was perfectly normal, because he has a thermal store. He said that you cannot have the CH on without the HW, as the CH pipes run through the store.

Is this right?

Any advice or thoughts would be gratefully received!

Mike

[Set Square]

I don't know a lot about thermal stores - but here's my five penneth!

AIUI, one type of thermal store (there may be more?) has a large tank of hot water which is heated by the boiler - and the boiler and primary(?) pump are controlled by a cylinder stat in order to keep this tank hot. Inside this tank are 2 separate heat exchanger coils - one for domestic hot water and the other for central heating. The hot taps are connected to the mains cold water supply via one of these heat exchangers - so that hot water is heated in real time as it passes through the heat exchanger, and is delivered to the taps at mains pressure. Water for the central heating is circulated by a second pump through the second heat exchanger coil and through the radiators. This second pump is typically controlled by a programmer and room stat.

If your brother has this type of system, he won't have any heating or hot water until the thermal store is heated up - so he needs to turn on whatever is required to make the boiler fire and the primary pump run.

My guess is that by just turning on the heating, he was running the secondary pump - which would simply circulate cold water through the radiators unless the store was hot.

I think there's some info on thermal stores in the DIY FAQ which may shed more light on the subject. The important thing is that you need to understand how it is *supposed* to work before deciding whether it's working properly or not.

[Christian McArdle]

This depends very much on how the central heating is plumbed. It could either be fairly conventional, where the thermal store is just used for hot water. In which case, something odd is going on with the controls.

However, if (as BG seem to think) the central heating goes through the thermal store, things get more interesting.

You say the pump runs. However, there should probably be two pumps. One will drive the boiler circuit to replenish the store. Another will drive the radiator circuit. Depending on how the system was installed, I'd expect you would need to run the "HW" circuit 24 hours a day, to keep the store hot. The "CH" circuit only needs to be run when you need heating.

However, there are a million and one ways of plumbing and controlling one of these systems and without some photos of the thermal store, zone valves, pumps and a nice diagram, it will be hard to diagnose.

Christian.

[IMM]

When either DHW or CH is called the boiler should fire. The boiler only heats the store (assuming a normal integrated thermal store. BG are silly, as usual, and probably don't understand it.

[Christian McArdle]

The controls may have been wiring with a conventional programmer with DHW on the boiler circuit and CH on the radiator circuit. If they didn't remove the appropriate jumper, then the programmer will allow the CH circuit to operate even when the DHW is off.

However, this can't be the whole story, as the central heating would work for a considerable period before the store depleted. I suppose there could be some sort of interlock preventing the radiator circuit operating when the boiler circuit is off. This would give the results that the OP appears to be seeing.

Christian.

[IMM]

It's probably an off the shelf unit, pre-wired etc. What make, model, etc?

[Lawrence]

I have a grant external combi which has a thermal store.

The bolier is fired off the hot water circuit. The pump on the central heating circuit and a flow switch for the hot water.

The programmer I have has a link which nominally tells the controller that the hot water is gravity fed. What it means is that the boiler is powered up when the central heating is switched on.

What I see if the pump starts on the thermostat some heat appears in the radiators and then 1-2 minutes later the boiler fires as the store temperature drops.

H>

Lawrence

usenet at lklyne dt co dt uk

[Mike Hibbert]

Well, actually, it looks like BG were right. He has a secondary pump on the CH system and the store does need to be up to temperature for the CH to work (although the HW doesn't need to be "on").

Thanks again for all your help!

Mike

[Peter Taylor]

IMM wrote

OMG! Tsk tsk! You can't say that publicly! At least, not unless you have extra special immunity from prosecution under the Data Protection Act. Better keep a lookout for brown envelopes.