My brother has just moved into a new house and guess what.... the heating is
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!
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
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.
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.
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.
Checked by AVG anti-virus system (http://www.grisoft.com ).
Version: 6.0.552 / Virus Database: 344 - Release Date: 15/12/2003
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
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
Hope this helps.
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!
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