System constantly needs bleeding of air.

[Griffo]

I have a Worcester/Bosch Danesmoor 20/25 on 'conventional' diameter piping (ie; not micro-bore), feeding through a Honeywell mid-position valve and a Grundflos Superselectric pump, feeding 2 hot water cylinders and 16 radiators, over 2 storeys. The boiler is about 12 months old and the Honeywell valve has just been replaced (after 20 years), pump is about 5 or 6 years old (I think).

My problem is the system constantly has air in it. There's a screw bleed valve on one hot water cylinder and that always releases air and the highest rads usually have air in them. The system has been like this for some years now and I previously thought it was the old boiler that was starting to corrode/die. However, it's the same now we have the new boiler. Any ideas please?

As an aside, the last 2 rads on the system will only get hot if I carefully restrict the flow to most other rads. Is this a sign of poor design or too small a pump or what?

Hope someone can shed some light on this please?

 

[bster]

Its probably down to poor system design, check the positions of the cold feed and vent pipe in relation to one another and the pump ( it is a tank fed CH circuit?).

 

[Griffo]

It is fed by a small header tank in the loft and there is a vent into that. The pump is directly below the 3 port valve, right by the boiler. As I cannot see that there is a leak anywhere (it would surely show somewhere after all this time?), I am puzzled how air can displace water in a full system?

 

[bster]

air can be sucked in through the open vent above the tank, this can occur when the positioning of the feed and vent pipe are wrong in relation to the position of the pump

 

[wilhelm]

you say mid pos valve changed after 20 years...

lets post a pic. ? are you sure its a mid pos?

 

[bster]

wilhelm

what difference, regarding the problem with air, will it make if it is\was a 3 port mid or a 3 way diverter (DHW priority) valve?

 

[wilhelm]

iam trying to ascertain whether the bleedin' pump is on an old set up. ie pump is on the return for ch. maybe a c plan. not a y plan .

.......

so i suspect a two port.......

 

[Griffo]

Just in case I've misled anyone, it's a Honeywell V4073, mid position valve. I'm confident the valve and pump are on the feed side not the return.

 

[Griffo]

Returning to the positioning of the vent/feed - surely as the pump is on the ground floor, right by the boiler, the vent must come off the system somewhere upstairs, so quite a way from the pump?

 

[Slugbabydotcom]

System constantly needs bleeding of air.

System is constantly sucking air in then!

Try turning the pump speed down a notch. While it will reduce the speed of the flow around the system it will drag in less air if its getting in through the vent.

In the case of ingress through a vent then fitting a de-aerator and connecting both vent and feed to it will guarantee to stop the problem

If your pump is quite old and noisy it could be sucking in through the pump itself. Not common but I have seen it on more than a couple of grundfos pumps.
If you were to remove the pump and inspect it and found that the outlet is rusty red and the inlet is normal black gunge coloured this would confirm that the pump was the source.


A leaking rad valve under negative pressure [suction] from the pump can also let air into the system. Again rare in occurence but it does happen. You suss this out when you find a valve that leaks when the system is off and doesnt leak when its on.