Pumping over

[wd.t]

My heating and hot water system pumps over into the small tank in my loft. I had a new pump fitted about six months ago and it was after this that I first noticed the problem. I had the pump setting turned down and this appeared to solve the problem only for it to return again. It is hot water that is being pumped over for the duration of the time that the system is on. Yesterday I had a full power flush. The system was very dirty and the flush has rendered the system quieter. The guy tested the system and there was no pumping over when he was here. When the system kicked in this morning, I went to check the tank in the loft and I was horrified to see that the pumping over of hot water water was back. Anyone any idea how to solve this problem?

[AJH]

Was the position of the pump in relation to the vent pipe changed when the new one was fitted?

Could a piece of crud be causing a restriction somewhere after the vent pipe? Is there anything else (part closed vale) causing back pressure?

Taller loop in the vent pipe over the feed and expansion (header) tank?? Better still a pressurised system.

I am not a plumber but have experienced this problem, it was the pump being moved the wrong side of the vent pipe.

AJH

[Grunff]

This is most commonly caused by incorrect positioning of the vent pipe branch, too close to the pump. It is rarely fixable without relocating the vent branch further away from the pump output.

[PM]

The diagrams I've seen show the poisition of the vent pipe on the input side of the pump, so the order is

1) Flow from boiler 2) Vent pipe 3) Inlet pipe from F&E tank, no more than 150mm from vent pipe 4) Pump inlet

Is this an acceptable configuration or have I got it wrong?

[Grunff]

This works well - as long as you have the feed pipe *in between* the vent and the pump (as per your description). Placing the vent immediately before the pump often leads to air being sucked down it, which is as bad as pumping over.

You can get away with placing the vent on the pump outlet side, but if you do it needs to be further away from the pump - past the radiator + tank branch offs.

[wd.t]

The new pump was placed in exactly the same position as the old one, ie just above the boiler.

[PM]

Thanks Grunff - when the feed and vent are between the boiler and pump is it better to have them nearer the boiler, the pump or is it immaterial? I am about to plumb in a new boiler and can adjust to suit.

[Fred]

Better still just a 22mm feed rather than the combination of a vent a feed. Far easier to fill, put in inhibitor etc than a pressurised system - particularly if there's already the header tank etc.

(As long as the boiler is designed for a pressurised system!)

[Grunff]

I think this usually gets decided based on physical pipe locations (whatever is easiest). Logically, the closer to the boiler the better.

[Roger Mills]

It doesn't really matter. The important thing is that the vent and feed pipes must be on the *same* side of the pump, and as close together as possible - so that the flow along the main pipe doesn't cause any appreciable differential presssure between the vent and feed pipe connection points.

[PM]

Thanks Roger

[PM]

Thanks

Pete

[Andy Hall]

The order doesn't really matter.

The critical aspect is that the vent and feed pipes join the circuit as close together as possible and certainly not with either the pump or boiler between them.

A good way to achieve this is with an air separator like the Myson Aerjec which apart from preventing this problem facilitates easy venting while filling the system.