Low flow rate of h/w cuts off after time (combi)

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'?
Ben
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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 internally).
Christian.
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I'd expect that a faulty flow switch would simply cause it to run cold. I don't get anywhere near 5 litres out of it before it cuts off.
Ben
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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.
Christian.
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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...
Ben
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Hang on, reading again, do you mean the water stops completely? The tap is dry?
If so, you may have some sort of airlock. Did this start happening when you installed the new tap?
Christian.
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Yes.
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, according to http://www.glow-worm.co.uk/epower/gw/manuals/pdf/c_products/30ciplus_UIS.pdf
Ben
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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 help.
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.
Christian.
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There *could* be a slight airlock in one place if the pipe isn't quite horizontal, but it's downstream of the tee to the shower, so shouldn't cause issues with that.
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.
Ben
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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 the boiler.
Christian.
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But it's not the boiler. It's the tap (some seem to be worse than others). 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.......
--
mark

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