water backflushing to feed/expansion tank every AM after pump change

[jollybeenz]

Need some help. I recently had a plumper replace an old pump on the central heating. New pump seems to work fine, but now I am getting back flushing problems to the feed tank in the loft. Only seems to happen at the end of the first heating of the day. After the boiler switches off at approx 7.30, there is a great gurgling sound and rush of water up to the loft. It is coming out with such force and volume that it overflows the tank and spills onto the floor of the loft.

Feed tank is normally slightly less than half full at any given time, so a fair amount of water is coming back up the pipes to overflow it. Have bled all the radiators and the pump. To be honest, hardly any air at all comes out (maybe half a second of air on a good day....usually just water).

Boiler does not sound like it is struggling to heat and all the radiators feel evenly heated.

Boiler: Glow worm space saver 75
Pump: Grundfos Selectric

 

[Howsie]

Get him back. Very strange.

 

[Bronze_tap]

Does that new pump has an overrun timer set to 3 or so min? - If yes, than the heating valve should be opened, while the the pump is in the overrun.

Is its operation synchronized with the valves? (looks like the heating valve is closing while the pump is still running).
More info about the system layout and expansion (heating) tank connections is needed. Esp note if there are any devices between the two pipes that go to the loft from CH system (one from the bottom of the tank, another ending above the tank).

You can also try reducing the pump speed setting (if it has that speed selector).