Hope someone can advise.
I am trying to find where air is getting into the central heating system.
The system is semi pumped, pump is in return pipe next to boiler at ground level. Feed and expansion pipes are each side of the cylinder coil.
On turning the pump on and off repeatedly, the level in the header tank rises by approx' 8mm when the pump turns on, and drops back when the pump goes off.
I assume that the position of the pump attempts to force water up the gravity HW pipes, hence the rise in header level, but don't understand what displaces the water elsewhere.
Anyone know if this is normal ?