What causes this please?

[WaterTight]

I have it on my kitchen mixer (hot side only) but it's never bothered me so never looked into it but I customer has mentioned she has it on a tap so would like to understand it.

I turn on the hot tap on my kitchen mixer to a reasonable flow and the combi kicks in. Very often - although I don't think always - the flow will then slow down considerably but stay hot. I will then open it up a little more and it will be back to original flow and it will sometimes do this a couple of times until it's good flow and hot.

I get the feeling it's a peculiarity/feature of the boiler rather than the tap? There's a flow restricting aerator fitted but it doesn't happen on the cold.

Maybe linked: To shower I just open my bath shower mixer tap on full hot on a good powerful flow. I'd say 65-75% of the time this just naturally produces the perfect shower temp with no cold needing to be mixed in.

Sometimes however I need to add a little cold because it's too hot. Sometimes it is scolding unless I add some cold. There is usually no fluctuation however. However it first starts so does it remain (more or less, sometimes moving shower head from high to low affects temp briefly)

Thanks

 

[WaterTight]

Oh and neither tap is thermostatic

 

[oz-plumber]

Long story short.

The combi is trying to adjust itself to the temperature you have set it too.
It doesn't want to over cook the heat exchangers nor does it want to send scalding water out of the tap.
The combi has to make sure it ignites and after that reduces the gas to the main heat exchanger until it understands how much gas is required to heat the hot water to the temperature you have set it to.
It also has to understand the flow rate you are asking it to provide

 

[JCplumb]

So you're not fully opening the tap?
Traditional pillar tap?
If so then the washer is expanding as it heats up and restricting the flow, totally normal and my basin taps do it, they're the only pillar taps in the house.
Not a great deal you can do about it although decent peglar washers tend to expand less.
Doesn't happen with ceramic discs or monos.

 

[Ric2013]

Also more likely to happen if whoever turned the tap off felt the need to turn the tap until water ceased to issue and then manhandle the head further clockwise just for the sake of it. They do it in the staff room at work and every time I wash up my soup bowl I have to open the tap at least twice grr.

 

[Welder]

So you're not fully opening the tap?
Traditional pillar tap?
If so then the washer is expanding as it heats up and restricting the flow, totally normal and my basin taps do it, they're the only pillar taps in the house.
Not a great deal you can do about it although decent peglar washers tend to expand less.
Doesn't happen with ceramic discs or monos.

Good point and I have to say can quite understand looking at the composition of some of the rubbish washers out there. Some, I have to say, are actually more like foam than solid rubber. Do agree that the Pegler ones were better. Not so sure now.

Just a question? Why not convert them to ceramic discs?

 

[WaterTight]

Thanks for all the info chaps, highly elucidating it were.