Hot Water Flow Decreasing.

[Simmo]

Hi,
I have an Ideal Logic Combi C24 boiler in a house which I've recently bought and I noticed the other day that when I turn on the hot water tap to a 'steady flow' that the flow starts off OK but then rapidly decreases to a trickle. If I open the tap a bit more it seems to settle down and run OK. Just to prove a point I went to my partner's house to see what her boiler did and the exact same thing happens there too. My concern is that I intend to fit a mixer shower but am a bit nervous about buying one and piping it in only to find that the boiler won't serve it adequately. I'm told by the 'techie' guys at Ideal that the C24 boiler will be fine for a mixer which is fine of course but the diminishing flow issue does concern me. Is it 'normal' for this to happen please?

Thanks.

 

[Scott_D]

At all taps?
What type of taps?

 

[Simmo]

Tap on my system is just a 'bog standard' sink tap in the bathroom. Does the same with the mixer tap in the kitchen too though. The tap on my partner's sink in her kitchen is a mixer which has the same diminishing flow issue. That's why I wondered if it was just a thing which happened with combi-boilers and wasn't anything out of the ordinary.
Thanks for the reply.

 

[Chuck]

There are other possible explanations, but if the tap has a washer or o-ring seal in it, then this may have degraded or been replaced with one made of the wrong material/grade. Anyway, if it's too viscous and/or soft it'll take several seconds to recover its relaxed shape when decompressing as the valve is opened and, in doing so, will partially close the valve again.

Interestingly, a brand new cartridge I recently got from Ideal Standard is behaving in exactly the way you describe. Not a brand I've had any trouble with in the past. I suspect that there may be a factory somewhere producing cheap but out-of-spec washers...

@YorkshireDave - Any thoughts?

 

[Best]

Ordinary rubber tap washers will expand slightly when hot water reaches them and close the water flow down.
When your tap is turned on to just reach a steady flow, the actual rise of the tap washer will be minuscule and therefore a slight expansion on washer will reduce the gap and flow

 

[Chuck]

Tap on my system is just a 'bog standard' sink tap in the bathroom. Does the same with the mixer tap in the kitchen too though.

Okay, my idea that it could be the washer in the tap looks a lot less likely if it's happening on more than one outlet. Seems like the dynamic pressure in the system is significantly lower than the static pressure. There may be a strainer on the inlet side of the boiler that needs clearing next time the boiler is serviced.

 

[sammathias]

What best said

 

[Simmo]

Ordinary rubber tap washers will expand slightly when hot water reaches them and close the water flow down.
When your tap is turned on to just reach a steady flow, the actual rise of the tap washer will be minuscule and therefore a slight expansion on washer will reduce the gap and flow

Ah that makes perfect sense, never thought of that!
Obviously wouldn't expect that problem with a mixer shower then.