no hot water - marathon myson

[colorado]

My Marathon Myson boiler has a problem. It seems that when the hot water is on but the heating is off, either because I have it programmed for hot water only, or when the heating is on but the ambient temerature is warm enough not to trip the room thermostat, I get no hot water. However if I turn up the thermostat until the heating comes on, I also get hot water.
I'm not sure what kind of system I've got, but there is a hot water tank in the loft.
I'd appreciate any advice before I contact a plumber.

 

[namsag]

Check the ball valve on the little tank in the loft it may have stuck causing not enough water in the system

 

[Agile]

I suspect you may have a faulty motor valve!

Tony