Air in central heating system

[viking99]

Help!

Any comments would be greatly appreciated.

My central heating has the following problem:-
-When only the heating is calling for heat, the mid position valve directs the water to the heating, all works well, no noise, warm rads, great.
-When heating and water are calling for heat, mid position valve sits in the middle, all works well, slight amount of noise in the pump area, like tiny bubbles of air, but seems to cause no problem.
-When only the water is calling for heat, the mid position valve directs the water into the coil in the cylinder, all sorts of horrendous air noises, like huge bubbles going through, occasionally the pump sounds dry momentarily. After this happens 2 or 3 times there is a terrific noise as the air rushes back up the pipes and up the vent!!!

I have descaled and flushed the system recently (problem was occurring before this) and added Inhibitor. The header tank has about 4 to 5 " head of water. The vent pipe curves over about 12" above the header tank.

Any ideas, please!!!

 

[ChrisR]

Probably bad system design, ie pipe layout causing pump-over or air ingress. If this has started after long period being ok, it's probably due to scale build-up altering flow resistances, hence pressures. Try Fernox "Heavy Duty Descaler".

 

[Rincewind]

May be worth fitting an air separater, check pump speed, does it look like pump may drag air in via vent pipe? Does this affect your boiler in any way?

 

[viking99]

Okay,

The problem has been occurring since we inherited the system.

I have just treated the system with Fernox DS40, neutraliser and then inhibitor.

Pump speed is at its lowest, faster speeds aggravate the problem.

There is no noise in the boiler, no air getting into the rads. The noise/air movement seems limited to the pump / valve / cylinder area.

I am as sure as I can be that air is not being pulled down the vent pipe.

Having just looked long and hard at the arrangement in the airing cupboard, with a view to possibly fitting an air seperator, I have noticed the following:-
The return pipe comes up through the floor, to a T junction. The 90 degree turn leads to an elbow and into the downward facing pump. Straight on through the T is the vent pipe. This seems like a good arrangement for air removal. However, if I follow the vent pipe, there is a length of about 600mm, which runs horizontal, possibly even slightly downhill. Is this a potential air trap? Should I replace this section with a gently rising section? Presumably this could be holding a small reservoir of air which keeps getting drawn in and finding its way back.

 

[ChrisR]

"I am as sure as I can be that air is not being pulled down the vent pipe.
"
Hold a glass of water up to the end.


"there is a length of about 600mm, which runs horizontal, possibly even slightly downhill. "
Get hold of it and bend it up! Even if you can hold it only while the noise is being made.

You may just be getting flow noise. Over certain speeds you DO hear things which is why we use 22mm etc on heating instead of microbore and a big pump. Try a gate valve in the cylinder loop.

Pumps aren't supposed to be positioned that they pump downwards because of air entrapment. A lot do without problems, though.

 

[viking99]

Thanks for the advice.

"Try a gate valve in the cylinder loop."

From the diverter valve, pipe is 22mm into, and out of cylinder, then reduces to 15mm, with a 15mm gate valve. This is fully open. Should it be so? I guess if it is fully open, whats the point in it being there!