The boiler is a Glowworm 30ci plus, which has a 5 litre store.
If I turn on the shower or taps at a low flow rate (hot), then I get
water for a bit, and then the flow cuts off. There's no problem
with higher flow rates. Curiously the monobloc mixer I just
installed in the bathroom doesn't appear to have this problem.
Any ideas? Is it a case of 'they all do that sir'?
Your flow switch may be a little sticky and only turning on above a certain
flow rate. It runs hot until the 5 litre store is exhausted. (This is just a
guess, and the scenario depends on how the specific boiler is plumbed
Even better. I would expect the combi internally to be like a tiny heat
bank. I suspect that this keeps the heat exchanger to mains hot. However, I
would expect the heat bank only to be circulated when the flow switch
activates. This means that you will only get a short burst of hot water, as
the heat exchanger won't be replenished even from the 5L store.
Righto. So if I open the tap and the flow rate is below threshold (which
the manual says is 1.7litres/minute) then I'll get some water,and then
nothing. I'll do some measurements and see if that's right...
Nope, but it might have started when I did some other work on the
dhw pipes - that was very soon after we got the boiler in, and
I couldn't swear it did/didn't happen before that.
I installed a tee (for the shower) just below the boiler. I cut the cold
water supply to the boiler before I did this, and opened a hot tap.
Thinking about it, not much came out then either - not 5 litres worth, yet
this is supposed to be how you drain the DHW part of the boiler,
Then it sounds like an airlock to me. Do you have any inverted U shapes in
the pipework? Perhaps a manual bleed valve at a convenient location would
I'm a little surprised, though. Normally, mains pressure can blast through
airlocks (indeed the normal solution to one is to connect it to mains
water), so it would have to be a biggie.
There *could* be a slight airlock in one place if the pipe isn't quite
but it's downstream of the tee to the shower, so shouldn't cause issues with
Presumably the boiler ought to have some kind of automatic venting, given
all pipes lead down from it.
You know, it's just occurred to me that a compression joint doubles up
quite well as a bleed valve :)
I haven't run the hot water at full flow for a while, so I'll try that for
a few minutes tonight and see if anything changes. Failing that I guess I'll
be having a nose inside the boiler to see what's it looks like.
You may also have a valve set to choke off the hot water to prevent you
drawing much more than can be heated from the boiler. If you can remember
the position any such valve is set to, then open it fully before attempting
to clear the airlock. These valves can either be the isolating valves for
individual outlets (particularly bath taps), or one for the entire lot, near
But it's not the boiler. It's the tap (some seem to be worse than
When you switch on and adjust the flow down the tap washer is only a
very short distance off the seat. As the boiler heats up the water the
hot water expands the spindle in the tap. This causes the washer to move
closer to the seat decreasing the flow; at a certain point the boiler
flow switch turns the boiler off.
Having said that; I'm not sure about the shower. I didn't think their
internals were made that way.......
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