Boiler cutting in and out every 15s

[Johnking]

Is this normal behaviour for a combi when supplying hot water?

The burners light, it runs for about 15s, then the burners stop. 10s later it reignites, and the cycle continues. This is while supplying a shower two floors up. I'm guessing it's because there is insufficient pressure to give a big enough flow rate, so it's cutting out to manage heat. But is that right, and will it wear it out?

 

[John D v2.0]

It's not normal for a standard Combi in good condition.
Is the shower temperature consistent?
Ha it always done that?
Does it still do that if you turn the hw dial down to the lowest and use hot only?
Is the incoming cold water temperature already quite warm?
What boiler is it?
How old is the boiler? And is it a hard water area?

It could be as you say that the minimum heat input of the boiler is higher than is required to heat the hot water to the selected temperature. Or it could be that the heat exchanger isn't exchanger heat efficiently enough.

 

[terryplumb]

Do you have issue with hot water taps ? If you run a hot water tap does the boiler cycle as you described or is the problem only with the shower ?

 

[Johnking]

If I run the hot tap on the sink by the boiler, the burner runs constantly, no cutting out, even if I turn the temp to max. I suspect it's a combination of the warm weather meaning the cold supply is warmer than normal, and the shower being two stories up, leading to a lower flow. We do occasionally get problems with the hot cutting off completely in that shower, as it does when the microswitch on the diverter wears out, which would be consistent with the pressure problem. It's a Vaillant Turbomax 282, getting on for 20 years, last serviced about 18m ago.