combined feed and expansion pipe problem

[wonderboy]

Hi all

I have a heating system with a combined vent feed pipe. The vent has been teed off just as the feed pipe leaves the tank and the vent pipe loops over the tank. I am having the problem that the vent pipe discharges water when the pump stops this in turn increases the level of the water in the tank. When the pump starts again the levels drop dowm again and so the process goes on. This I assume is drawing air into the system which is not good. The combined pipe is connected to the boiler flow about 100mm before the pump ( boiler - combined vent - pump- system ). Why does the vent pipe discharge water when the pump stops? The system has been power flushed recently and a new pump fitted at the same time.

Any help gratefully recived

 

[tonybhoy]

Probably pump speed set to high or a blockage in the feed pipe.

 

[superdupergasman]

What speed is the pump at?? and is it a tall house or small house??

Its the force generated by the pump sucking/pushing the water into and around the system and when it switches off the valves will close and stop the flow of the moving water and loss of suction will cause it to over flow a little.

 

[heatingman]

What boiler have you, how much dry vent, water level in tank--top of vent pipe = dry vent.

 

[wonderboy]

tried pump at speed 2 still the same. The system incorporates a 3 port valve and system by-pass after the pump so there is alwsy a circut open for the pump to push round. The pump is located on the ground floor and the header tank in the attic and house is 3 storys.

 

[wonderboy]

Keston c80 and about 20 inchs

 

[heatingman]

Have you got the MI's for the boiler, have you the correct minimum operating head. Odd boilers, they are much happier on sealed system

 

[wonderboy]

the manus say to have a min head of 8 feet and is okay to have a combined feed and expan.