Air Bubbles in Gravity Fed Central Heating System

[paul_murphy50]

specs: - Potterton Profile Boiler - Grunfos Selectine pump - Hercal h/w tank

I have a 3 bed house, my central heating system used to be fine, bar the banging of an old boiler.

I got the boiler and the hot water tank changed. After this a small amount of air everynow and then used to get into the radiator in my small bedroom. So I'd bleed it once a month and everything was fine.

This year I got 2 radiators changed (a large one in the lounge and a small one in the bathroom). Now more air is entering the small bedroom radiator. The frequancy of the air is un-acceptable, not only making bubbling noises that wake people up but also requiring to be bled once a week or every other week.

What I want to know is 'why' is this air getting in ?

I have not seen any leaks in the system, my plumber says that it's through the vent and the pipework needs to be changed to 'fully pumped system'. BUT the question in my mind, is why is this happening? The pump hasn't changed, so surly the vacumn in the system hasn't changed, so why whould the water in the vent pipe now be getting sucked lower for air to get into the system? Also why would this be made worse once again by 2 new radiators.... it doesn't add up in my mind, any ideas?

- also tried to knock the speed of the pump down to test, by this has made no difference.

[John]

If you've no leaks, the only open place is the vent pipe. This is probably on the suction side of the pump with the make up pipe on the delivery side. When the pump runs, There's enough pressure differential to pull the water down the vent and then air follows. Can you raise the header tank?

[Ed Sirett]

Make your self a diagram of where all the pipes go. It sounds like the system was not right from the outset but operated tolerably for you, but now it doesn't.

Especially take note where the vent pipe goes and the feed pipe, especially if they are on opposite sides of the pump.

[John Rumm]

Did this radiator change require a drain down? If so, was new corrosian inhibitor added as the system was refilled?

If there is not enough inhibitor in the system then one possiblity is that they gas you are collecting is not air, but being generated by corrosian.

The new rads may have changed the flow resistance of the whole circuit...

When I had a (badly designed) vented system it suffered a similar problem due to the vent being on very close to the suction side of the pump. Everytime the pump kicked it it would suck in a gob of air via the vent. I fixed it by cutting into the vent pipe about a foot above the pump and inserting a tee, then connecting the feed from the header tank to the tee (blanking off the connection to the other side of the pump that it used to connect to).