Air in Rads... It's not what you think

[WHATSAHAMMER4]

Hello, I believe my central heating is sucking air in through the vent pipe. I am having to bleed 2 or 3 of my rads almost every week. Yesterday I decided to trace the source of the leak.
Each time the pump kicks in there is a rush of air that fizzes through it after a few seconds. Up in the loft I placed a half pint glass of water over the end of the vent pipe. When the pump kicked in the vent pipe sucked around a quarter to a half of the glass up an then dumped it back down. When the pump stopped it blew bubbles in the water. By the noises in the pipe it sounded as though the air was getting well below the loft floor.
So my questions are... Am I sniffing in the right area/is it possible? and could it be that the pump is to strong?
I'm using a SurePump SP65 with 12 rads in a four bed house and all of the radiators or in good condition and the water is clear.
P.S. It's been like this since I moved in last year.

 

[45yearsagasman]

You need to find where the feed and vent are connected to your system(usually just before the pump) and check for restrictions in that area.
Check with a magnet and see if you get any attraction to the copper.
This may help, but no idea of your system configuration. View media item 39000

 

[WHATSAHAMMER4]

Thanks for your response.

I see what you mean, if the pump draws equally from the boiler feed/vent 'T' then I see how the water/air in the vent would be sucked in (Least resistance etc)
I've been to Wickes & B&Q and neither sell Magnets, unless I want a kitchen. Also the pipes concerned in the diagram are in an airing cupboard and strangely they used steel piping!?! Cosmetics? Who knows? Anyway, I think the magnet would stick to them anyway.
I also drained the system last year, flushed and treated it at the same time. Would this have removed and fragments? I didn't see any in the deposited water.
It might also be worth taking into account that the end of the vent pipe is probably within 8 feet of the pump and is practically over the top of the pump with the pump running vertically. Might this indicate that the top of the water line in the rising part of the vent pipe is too close to the pump?

 

[dormermike]

I'm having a similar thing, but oddly only since i drained and refilled system.. i think my header water level is too high - could that be your problem?

 

[WHATSAHAMMER4]

That suggests that the water level is higher than before you drained it. Not sure how that can be unless you've moved the tank or changed the ball cock.
Can you elaborate on how the tank being fuller could cause it. I'm not taking the p*£%, I'm just new to the DIY scene.

 

[Terry7556]

whatsahammer4 quite a few people, well their wives, keep magnets on the side of the fridge or microwave

 

[WHATSAHAMMER4]

whatsahammer4 quite a few people, well their wives, keep magnets on the side of the fridge or microwave

Bingo!! Good thinking, Thanks for that

45yearsagasman. I have done what you said to do with a magnet and there was no more attraction in that part of the piping than anywhere else. Do you have anything else to try?

Thank you

 

[45yearsagasman]

The vent pipe should rise at least 450mm above the tank water level before bending down into the top of the tank. If you can achieve more then thats better. The magnet test will only detect iron based magnatite and will not detect other material.
Whenever I get this problem with a system, I almost always cut out the pipework and check. If you are not confident with soldering then compression fittings are entirely suitable to use when refitting pipes.

 

[dormermike]

That suggests that the water level is higher than before you drained it. Not sure how that can be unless you've moved the tank or changed the ball cock.
Can you elaborate on how the tank being fuller could cause it. I'm not taking the p*£%, I'm just new to the DIY scene.

]

Good point about it being higher since refill - not sure

If the water level is higher, then the level up the overflow/breather is also higher, so presumably more disposed to pumping over.

 

[WHATSAHAMMER4]

OK brilliant, I'll go for that. Should I take out the whole area or just at the junction with vent & pump feed?