Combi water flow?

[Gareth]

Could someone explain why the flow rate of hot water from a combi boiler to a tap open at a constant rate reduces as the water heats up?

[John]

The tap washer was compressed - the heat swells it and reduces the flow.

[John Stumbles]

I'd guess it's the metal works of the tap rather than the washer itself, but I've noticed the phenomenon too and agree it's thermal expansion of some sort acting within the tap.

[EricP]

Mine doesn't.

[R D S]

Mine does.

[EricP]

Weird.

I just tried it in case I had missed something.

Still doesn't.

[Rod]

Doesn't it happen regardless of type of boiler?

I am fairly sure that many taps do this - with consequent changes to the sound the water makes as it flows through them - even when fed by gravity. I know that I have often opened a bath tap at cold, left it running at a modest rate and on returning found it barely dribbling bit hot. Of course, the sound aspect is most noticeable on a tap that is making quite a bit of noise, typically when not fully open, but some always make a noise.

Agree with other posters - simply must be mainly thermal expansion of tap components. I'd hazard a guess that ceramic disc taps would be less likely to exhibit this behaviour. Wonder if changes in physical characteristics of water as if warms up also have an effect?

[John Rumm]

It seems more noticeable on main pressure hot water systems - presumable since the tape needs to be far less "open" to get a flow and hence any reduction is more significant.

[Rod]

And I thought I'd be the only silly bugger posting at this sort of time. :-)

Yes - that makes sense. And thinking about it, at least some experiences of such taps have been with fairly high hot water pressure - whether combi or not.

[dennis]

My fathers has a flow limiter and a temperature stabiliser on it.. when you first turn it on it has some stored hot water and the flow is quite fast, after a few seconds it slows down as it hits the maximum flow rate for the heat output. If your pipe work is just the right length it would just be getting hot at the tap when this happens.

[Dave Plowman (News)]

If that's happening on a gravity system something's wrong. Possibly an airlock somewhere.

[The Medway Handyman]

Ah, but what if the OP has ceramic taps?

[John]

My combi has a flow limiter on the input side of the boiler - used to come in with a 'clunk' but I think it has seized up now as I never hear it. As thermal expansion is quite small, I thought that heat softening a tap washer and making it swell to a pre-compressed state was quite a bit more likely. As others have said, with mains pressure, the tap would not be open very far so the effect would be more noticeable. Also - could suspended air have an affect? My hot is quite milky in appearance until it stands a while.

[Doctor Drivel]

What make and model?

[Ed Sirett]

Mostly the effect is in the tap, but as you say there a few models where some feature of the boiler comes into effect.

Those with a store of HW will last more than just enough to warm up the tap.

Another model which has this feature is the Pott Puma combi which did this the other way around, restricted the flow of HW until the boiler was working steadily.

[Ed Sirett]

I'll second that observation.

[Ed Sirett]

This is (IME) air coming out of solution, if possible try to turn the boiler down on the HW setting and/or increase the flow for DHW. This effect means the boiler is over heating the water and you may also get lime scale build up of the water is hard.