Combi boiler query

[Rupster]

I have an old Vaillant combi boiler that's not a condensing boiler.

When it's heating up the water for the radiators, it correctly gets the water temperature to around 63 degrees.

Every so often though, the burner turns off for a few minutes, and the temperature down to around 50 degrees. During this time, the pump is still working.

The burner eventually starts up again and the water returns to 63 degrees.

Then after a while, the burner turns off again, and the cycle repeats itself.

Is this normal? It means the radiatiors are getting hot, then cooling, then getting hot, then cooling, and so on.

It may be that the radiators are not able to disperse as much heat as the boiler delivers, so it turns off to avoid overheating. This is more common with an older boiler that can't modulate (reduce) its flame size to try to match output to demand.

If any of your rads are turned off, or have cold patches due to sediment or gas, this will be worse. Or if the rads are too small to heat the rooms adequately.

Thanks.

An update:

Last night, I turned the radiator temperature dial up on the boiler, and it now heats the radiator water up to 68 degrees.

More interestingly, the burner no longer turns off. Once the temperature reaches 68 degrees, it's held there with the burner still running.

At one point, I turned it up to 72 degrees, and it worked fine there too.

So it seems the burner is now able to modulate its flame size to keep the temperature steady, but why it wasn't working previously like this remains a mystery to me.

It might have been a colder day and the heating load greater, especially if you have TRVs which would have automatically opened themselves. Even a modulating boiler has a max and min (e.g. mine goes down to 8kW minimum)

Though perhaps there is some other cause I haven't thought of.