Vokera linea 28

It could be some "eco" setting on the hot water programme that fails to heat a store of boiler water sufficient to maintain hot water output until the boiler has come on. This is especially likely if the central heating is running at a relatively low flow temperature.

I presume this is a thermostatic shower? If so then it might be worth opening a tap with a slow flow while the temperature is being stabilised and the HW flow varies.

It might also be worth checking the flow rate where the boiler doesn't sense enough flow to fire up.

I had an issue where the gas valve was "sticky" and wouldn't modulate at low gas rates, where I'd get a flame-out.

I had a similar problem (with a different combi stilly boiler).

Combi boilers have a safety feature; if the flow/pressure gets too low the water will get too hot so it stops heating so that you don't get burned/scalded. In my case a partial solution was to run another hot tap at low flow whilst having a shower. Then I changed the shower head and the problem went away because the old shower head was blocked by "fur".

Our Vokera has done that since day one. I put it down to the crazy pipe runs in our old house but it does seem to have got worse in the last year or so. Damn thing's only about 20 years old; does nothing last these days?!?

Our mains water pressure is quite high, so I don't believe it's cutting out because of a limited flow of water.