Central Heating boiling over into header tank

[Stonebarn]

Hello,

I have a slight problem. I have a two zone system, with the lower zone controlled via a wall thermostat and the upper zone via TRV's. When the lower zone is not calling for heat and the HW is satisfied, the system boils over into the header tank. I have turned the upper zone off from the programmer and the lower stat controlled zone and HW then seems to work ok.

There is one rad on the second zone that is not controlled with a TRV. How does the boiler know to shut down, when all the TRV's are not calling for heat and the rad in the bathroom is very hot. I had assumed that there would be some form of control on the boiler that measures the return water temperature, which would shut the boiler off and thus prevent the boiling.

Can I have some advice

Stonebarn

 

[jackecole]

if no room stat onj your second zone it will be controlled by the boiler stat (if older tpy ewill cycle on / off boiler stat) - when you say boils over, do you mean like steaming and then you hear a gush of water going down the cold feed to replace the lost system water?

if so sounds like the boiler stat has failed - but would not just do that for one zone.

 

[Stonebarn]

There is no wall stat for second zone.

I can hear water rumbling in the house (very loud) followed by water flowing out of the overflow. I then hear, what sounds like a value closing accompanied by a screeching noise. The water then stops overflowing and for the next few days (once/week it happens) all seems ok.

During times when we have just the downstairs zone on (wall stat controlled) and the HW the problem doesn't arise.

 

[jackecole]

do you mean overflow pipe outside or vent pipe over the header tank?

 

[Stonebarn]

Hi,

The only thing I am aware off is the external overflow, I had presumed that it vented into the tank and filled it to a point where it then overflowed outside.