I have a Worcester Danesmoor Utility 26/32 oil fired boiler on a vented CH/HW system. When the boiler is running it vents hot water through the "heating vent" outlet pipe to the expansion tank in the roof space - causing condensation problems in the roof space. Otherwise the boiler and CH/HW system appear to be working properly. The boiler is only a few years old and has been recently serviced.

Gordon, December 2009

Boiler stat is working - that's been checked. As has the stat on the hot water cylinder.
Pump speed is set to low - any higher and that does cause an overflow but via the vent from the primary circuit in the hot water cylinder.

Gordon, December 2009

Sounds like the boiler thermostat isn't working - allowing the water to boil or the pump speed is too high.

John, December 2009