Air getting into a sealed system

[marcon]

This is my first post so I hope I am giving enough info for someone to help.

On moving into our house 11 years ago we renewed most of the central heating system- new boiler (Glow Worm Ultimate 80FF), new pipework, new radiators, new controllers. We have a sealed system operating in a Y plan arrangement with a Honeywell 3-way motorised valve. There is inhibitor in the system which is changed every 5 years. The only part of the old heating system we kept was the hot water tank.

The system has worked perfectly until two years ago. At this time we had the bathroom refitted and changed the radiator in the bathroom for a heated towel rail. Since the towel rail was installed, we have been getting air in the system - enough air to fill the top one or two rails of the towel rail within a week. I have to bleed it every week.

The towel rail would seem to be the source of the problem but the bathroom installer can find nothing wrong with it - indeed if you shut the towel rail off air still gets into the system.

I have tried flushing the system out with a descaler/sludge remover - no effect. I have tried adding a leak sealer to the system - no effect.

We asked another heating engineer to look at the system. He found some corrosion in the coil of the old hot water tank, so we changed the tank. At the same time the system was power flushed - no evidence of sludge buildup. We still get air into the system.

I can find no evidence of leaks anywhere in the system. There are no brown stains on ceilings from pipes under the floor. I do not find a significant pressure loss either with these repeated bleedings.

I (and everyone else) is at a loss to explain this. The system operates at 1 bar pressure, so how can air possibly get in? The system is and has always been protected with inhibitor and the power flushing revealed no sludge, so corrosion looks unlikely.

Anyone any suggestions?

 

[Nige F]

Compression connections on the suction side of the pump eg. isolating valve

 

[marcon]

Nige F,

Thanks very much for your quick reply. The isolation valves on the pump were changed when the new hot water tank was installed - the engineer said that they were beginning to look their age i.e. 11 years old.

Your comments have set me thinking though - if we set the pump speed to max (3) the air seems to build up more quickly than when the pump is on a lower speed (2). Could air be getting in through any other part of the pump?

Does this mean that the installation of the towel rail was coincidence?

 

[chrishutt]

Can you tell us how high the system is in relation to the boiler (where your pressure gauge shows 1.0 Bar)? If the top of the system extends say 7 metres or more above the pump, you could be close to getting sub-atmospheric pressures at the top of the system. Only where sub-atmospheric pressures arise will air be able to enter the system.

The towel rail is probably merely the place where the air collects, not itself responsible for the problem. As far as the pump is concerned. you can get quite high negative pressures on the inlet side, but if the system pressure at the pump is 1.0 Bar, you should not get sub-atmospheric pressures.

 

[ollski]

Many moons ago at college we had a system which was set up and running and we were told to undo the compression fitting on the inlet of the pump. Was quite surprised to find the fitting bould be undone and the pipe pulled out almost to the level of the edge of the fitting and no water was spilt....the air being sucked in was phenominal though. I would imagine you have a micro leak on the negative pressure side of the pump.

 

[marcon]

Chrishutt - the boiler, the expansion bottle and the pump are all sited high up in my garage above a mezzanine floor. The pump is level with the mid point of the boiler with about 1.5m of piping between the boiler outlet and the pump inlet. The boiler had to go this high to allow the fan flue to vent vertically out of the roof. The highest point of the system is less than 1m above all these three items.

Before we installed the towel rail the piping in the garage was the highest point in the system. Exchanging a (low) radiator for a (high) towel rail now means that the top of the towel rail is just below the highest piping. I still don't understand why with the old radiator in place we did not get any air in the system for 9 years.

Ollski - thanks for your comments. As I only have 1.5m of piping between the boiler and the pump I will take a careful look at this area.