Air in CH system options

[Ronob]

OK so for a while now I've had one upstairs radiator that seems to always need bleeding, it's gone from once or twice a month to once or twice a week so far this season. It is an old single rad and I am pretty sure corrosion is a major problem as there is now a very distinct smell when the air is released which doesn't happen with the other rads.

A new ground floor rad was added earlier this year and the system was flushed with the Fernox F3 cleaner (left in for 48hrs) prior to drain down and flush.

My Readers Diguest guide tells me to hold a jam jar full of water under the vent pipe in the loft (with its end in the water) and see if it sucks in water when the heating is switched on, so I just tried that and the result confused me more......

As soon as the pump started up there were bubbles (biggish bubbles) in the water, suggesting air was being pushed from the pipe, then a small amount of water was sucked out. I didn't have a jam jar so used a kitchen jug which has a measure scale on it and about 4oz of water was sucked out.

So I'm thinking is that 4oz loss due to a fault or is it a siphoning effect of air blowing into the water, which then stops and thus a vacuum created and water sucked on?

Either way, can anyone diagnose a fault, if indeed there is one, from what Ive experienced?

Thank you

 

[HERTS P&D]

How high is the expansion pipe work above the expansion tank?

Andy

 

[Ronob]

The 22mm vent pipe is about 18" above the header tank then turns downwards with the end about 6" above the water level in the tank.

If important, the pump is under the bathroom floor directly below but to one side of the tank, so 9'6" to 10" above the pump.

 

[Armo74]

Air could be being drawn in where the cold feed tee's into the flow from the boiler due to a blocked cold feed.
Or it's just a build up of magnatite giving off hydrogen resulting in the radiator being taken off and flushed out with a garden hose.

 

[Ronob]

Air could be being drawn in where the cold feed tee's into the flow from the boiler due to a blocked cold feed.
Or it's just a build up of magnatite giving off hydrogen resulting in the radiator being taken off and flushed out with a garden hose.

So if it's a blocked cold feed, how would I check this?

I assume you are referring to the pipe leading out from the bottom of the header tank. When I cleaned the system out with Fernox F3 earlier in the year water flowed through that outlet quite freely on refilling the system

 

[Ronob]

Anyone?