Air in central heating system.

14 Jan 2013
Reaction score
United Kingdom
I have just done a bit of work on my central heating system.
I re-routed some pipework and added some new radiators.
When the system was refilled there were a few problems with air locks.
I bled the system at the highest points and this seemed to cure the problem.
All the heaters gained their desired heat and hot water tank heated to correct temperature.
After a few days I noticed gurgling sounds coming from some of the radiators.
I bled them but the same problem came back after another few days.
Air seems to be getting into the system from somewhere.
Could anyone tell me if air is getting into the system because of the way the system is set up?
Could it be that the pump is set to high? (Grundfos UPS 20-60 on setting 2 at present)
I have attached a drawing of my central heating system.
Sponsored Links
I seriously hope that your diagram is wrong.

First of all you have a pump and a motorised valve between the boiler and the vent. The vent must have an open route from the boiler, what you show is very dangerous.

Secondly, what do you mean by "vent through tiles" does the vent pipe go through the roof to outside? Surely not.

The feed and vent connections are also wrong, they should go in the same pipe to the pump and be located within 150mm of each other. Looking at your diagram you could be pumping water up and out of the vent, and then drawing new water which contains air into the system via the feed tank.

The connections from the F&E tank should be as per the diagram below (no more than 150mm apart) where they join the pipe which should be on the suction side (inlet) of the pump, so that water is not pumped up the vent.

Also notice how the vent is connected straight from the boiler without any valves or pumps in between. There have been explosions where pipes are obstructed, so that the boiler cannot relieve the pressure from the expanding water as it heats.

A frozen pipe can cause the same effect. House damaged by explosion
There is a Flamco Flexvent fitted to the top right hand connection of the boiler out in the shed. Would this work as a vent for the boiler?

The vent goes out the roof tiles, should this be venting back into the header tank in the loft?

This is the original set up of the central heating system when I bought my house.

The only changes I have made to the system other than add a few radiators is:

1) Fit the magnaclean filter in the return side of the system next to the boiler(in shed).

2) The pump used to be in this location on the return side, pulling the water back into the boiler.

3) I changed this so the pump is now located on the flow side pumping the water out of the boiler (in shed).

4) Fit the isolation valves to each of the radiator circuits to make the system easier to partially drain for maintenance.

If I changed the pump back to the original location and relocated the magnaclean filter somewhere different in the system would this be any better?

With the boiler being situated in the shed I could not easily route the supply and the vent pipes into the system proir to the pump.

The header tank is situated in the loft space directly above the the hot water cylinder which is where the supply and vent pipes conect into the central heating system.

The supply pipe connects into the return of the hot water coil just pior to the connection into the tank.

The vent pipe connects onto the flow of the hot water coil just prior to the connection into the tank.

All comments much appriciated.

Any suggestions on how to remedy problems with my central heating system?

I do not want to run a system which Stem tells me is very dangerous……….

Any suggestions would be much appreciated.

Sponsored Links
CHeck first whether the boiler is suitable to be fitted to a sealed system. If so, it means the "explosion" risk can be dealt with by having a £5 pressure relief valve by the boiler. They open to outside, at 3 bar.

The risk is only there is stuff seriously blocks up. It can happen.
Hi ChrisR,

My system is a fully pumped system fed through an F/E tank in the loft of the house, as shown in my drawing.

I've looked into it and my boiler can be fitted with an expansion vessel, pressure gauge and pressure relief valve and be turned into a sealed system.

I cannot do this as there is no mains cold water feed out in my shed where the boiler is situated.

So what you are telling me is that if for some reason I get a blockage in the return leg of the main pipework back to the boiler and both control valves are shut off then there could be a chance of an explosion of the boiler if it keeps heating under fault conditions, is this correct?
I cannot do this as there is no mains cold water feed out in my shed where the boiler is situated.

a hose pipe will suffice - that`s all a filling loop is ( with check valve )
Secondly, what do you mean by "vent through tiles" does the vent pipe go through the roof to outside? Surely not.

Sometimes it just has to be done - but a good reason to go for a sealed system . ;)

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links