Firebird combi oil fired boiler

[Downtown]

I was called out to look at a problem on one of these today and i am stumped. When the hot water switch is turned on the burner fires up and heats the rads and hot water.
Both the pumps on it work accordingly so i have no idea what this could be as there is no three way valve or anything on these from what i can see. Just 2 flows and a common return.
Anybody any ideas?

 

[kimbo]

You dont say what combi it is, need more info on model etc but it sounds like a check valve is stuck open.the check valves can either be on the pump or where the t piece is at the top of the boiler. The pump check valves are plastic and easy to fit, the ones on the flow pipe can be a real pig as they are in brass and I believe from memory there are two types. Nothing new with a two pump system better than a diverter valve and more reliable in my opinion