Water level in header tank question.

[pcr]

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 ?

Thanks, pcr.

[Dave From Leeds]

Which header tank do you mean? A system as you have described should have two headers. The larger one tops up the hot water cylinder when hot water is drawn through the taps and the smaller one is to keep the central heating system primed with water. A slight rise and fall in the level in the smaller tank due to expansion of the water when the boiler is running is quite normal. Turning on the pump could possibly cause such an effect by drawing some water down the expansion pipe from the CH top-up tank. If this was severe it could empty the expansion pipe drawing air into the CH system. If your pump has different speed settings, try reducing it.

[pcr]

Thanks for replying Dave. I am refering to the F&E tank. The rise in level is not due to expansion as it happens in a second or two when the pump is switched on. There is no pump-over. I will check to see what is happening to the level in the vent pipe at this time.