Boiler stop starting on hw

[mrshr]

Hi guys,
Quick question:
I've noticed that when boiler is heating up hw cylinder, it will run for about 40 mins until cylinder stat reaches temp and shuts boiler off (all normal and as expected).
But a few mins later the boiler will start up again to heat up the cylinder. Then after say 10 mins it will shut off again. Cycle repeats.

Why would the boiler fire back up again if I haven't opened any hot taps?

System is an unvented Megaflo with Vaillant system boiler.

Hope someone could shed some light?
Thanks.

 

[mrshr]

Nobody come across this before?

 

[45yearsagasman]

It is possible that the motorised valve controlling the HW is faulty, either sticking or has a faulty micro switch. A better description of your system and a pic would help.

 

[Agile]

The stat is quite close to the heating coil.

The stat has some hysteresis which normally covers your problem.

I suspect that the stat is reacting too much to the heat output of the coil and as soon as heat is turned off the lower part of the cylinder cools and the stat turns on again.

But to confirm this the operation of the stat needs to be electrically checked.

its also possible the hysteresis has reduced as the stat has aged.

Having a too high boiler flow temp will encourage this problem. It should be as low as possible commensurate with enough heating of the rads ( as well as having ta least 10 C boiler flow above the hot water temp set which means the boiler needs to be at 70 C if the water is at the usual 60 C).

Tony

 

[dcawkwell]

This is perfectly normal.
The boiler cannot modulate down enough to heat the water to the final temperature so it has to shut off it will still be pumping hot water around the tank. When the temperature in the piping to the cylinder cools a little it will fire again to bring the piping back to temperature.
It might have to cycle 3 or 4 times until the tank thermostat finally switches off.

 

[45yearsagasman]

Take your pick from, Hysteresis, a control fault, or its operating within its capabilities.