Myson Valaire boiler keeps stopping and starting

[iamjames]

My parents old Myson Valaire boiler keeps stopping and staring every 5 to 10 minutes
The hot water side is gravity fed. The pump is just for the central heating.
The thermostat was replaced but made no difference.
The pump seems to run when the central heating is on.
There have been some problems with radiators not getting hot so the thought was that this could be some kind of flow problem.
I don't think there is a thermostat at the cylinder, just controlled at the boiler.

With the boiler switched just for hot water, the boiler heats the water fine but when it has been on for a while (i guess when the water is up to temp), the boiler then seems to switch on and off every 10 minutes, without any hot water being used. I could imagine the water cooling slightly but I don't think it should be switching so reguraly.
With the central heating going as well, the switching on and off happens more reguraly, say every 5 minutes.
Any thoughts as to the problem? How much does the water have to cool for the boiler to start up again?

 

[ChrisR]

The heat's getting away somewhere, possibly round the rads. Otherwise the water would boil.

 

[wilhelm]

got water in header tank?

..

 

[Agile]

It could well be just the heat loss on the gravity pipes.

A boiler stat should have a differential of about 4-5 degrees when new but this often reduces to more like 8-10 degrees when it gets old.

I suggest insulating the gravity circuit pipes.

Tony