boiler air pocket in a Baxi

[brian108]

Can anyone explain the cause of this problem?? We have a Baxi Bermuda boiler, gravity hot water and an indirect cylinder. There are seven radiators on a pumped circuit. On occasions, when the programmer switches on the hot water at 5 a.m. there is a large air pocket which leaves the boiler, goes up through the cylinder, and deposits with an almighty splash into the cold water tank in the loft. It splashes over the top of the tank, wetting the plaster board and staining the bedroom ceiling. I suspect this air pocket is building up over a period of time until it is released by the heat of the burners, but what causes it, and how can it be rectified?
brian108

 

[namsag]

Hot water gravity? no pump. HW only on
If it is sending water up and out the open vent then this is usually caused by the boiler stat being shot and allowing the water in boiler to get too hot and boiling over

 

[brian108]

thanks for your reply. it doesn't seem to boil, as the problem is only evident when the boiler first lights up. I was wondering if this could be caused by an indirect cylinder? brian108

 

[BingoBongo]

just think of the indirect cyl as being another rad.

are you sure its gravity hw

no pump on flow to cylinder before the motorized valve?

 

[brian108]

I'm not an expert, but I sure its gravity HW and a pumped CH system. Don't know anything about motorized valves, and I'm sure there isn't one. thanks brian108