Heating Header Tank filling up dilemma!

[Modus8]

I have a problem with water flowing out of my overflow pipe from the central heating header tank. Ballcock is not the problem as I isolated it; nor is it coming from the vent pipe. The excess water only flows when the water and heating comes on. It is coming in from the bottom pipe into the tank, the one that should fill the system. I isolated the hot water cylinder (HWC) by putting a bung in the feed from the cold water tank (CWT) but left the heating on. The isolation stopped the header tank filling up. So my heating engineer, who also checked the boiler and thermostat, concluded it must be a pinhole or split in the coil in the HWC. However, here’s the dilemma: after removing the bung and restoring the hot water, the header tank is no longer filling up. If it’s the coil I would have expected it to immediately start filling up again. What’s happening?

 

[Billy Bob Bob]

is the system on?could be aproblem with the coil only when its hot

 

[Modus8]

Yes, the system is currently on and it is programmed to fire up in the morning and evening.

Come on guys, someone must have some ideas about this?

 

[oldplumber]

prehaps you had a blockage in your heating circuit and blocking the feed and expansion pipe!!(very risky) cleared the blockage?

 

[WHPES]

Pressure test the coil in the cylinder using a test pump.

 

[Modus8]

Thanks guys for the replies. Oldplumber, why was that risky? My heating eng. guy did it?
WHPES, would my heating eng. guy know how to do this. How do you pressure test the coil?

 

[newbie1]

if the coil was split it would pump into the main cold water tank not the feed and expansion and youre hot water would be sludge coloured..sounds more like an obstruction in the heating flow circuit

 

[WHPES]

Do a powerflush - it will clean out any rubbish and also identify if your coil is leaking.

 

[Modus8]

Thanks all for the thoughts and advice.