Bubling Radiators only when water is Hot

[ivanov]

I have a Keston boiler with sealed CH which I've recently drained and refill for treatment. After flushing the system, adding the restorer and I also added Fernox leak sealer, I'm experiencing the following problem. When the water is Hot (i.e. boiler working) after the pump starts I can hear the bubbling noise in the radiators upstairs. However if I switch of the boiler and circulate the water for a while there is not noise at all nor any air is coming from the bleeding points (Rads, pump, air vents etc.). After everything is looking nice I switch on the boiler again and after a while (i guess when the water heats up) the same old story start again: bubbling radiators and noisy pump when the CH starts for about 30 sec. Any idea what can cause this?

Any help will be very appreciated.

Thanks

 

[delmel]

sounds like an air lock somewhere in system