Partially blocked cold feed

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I am sure this has already been answered but a quick trawl of the forum hasn’t found anything so hoping one of you lovely heating engineers can give me an explanation.

So many posts say if you have a radiator that needs constantly bleeding and lots of ‘whooshing’ noise near the pump it is likely there is a blocked cold feed. Pump is only 4 years old btw.

I have an open vented system and yes you’ve guessed it a bathroom radiator that constantly needs bleeding and loads of noise when pump starts. I have tried the magnet trick and yes it is sticking a bit at the tee to the cold feed but just want to understand why this would cause air in the system.
 
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Blocked cold feed can occur if the system is inadequately protected against corrosion, i.e. doesnt have any or enough Inhibitor added. The 'whooshing' noise is a separate issue I suspect, and needs looking at. Do you get hot water streaming out the overflow pipe when the system is running?

That would suggest the system is 'Pumping over', i.e. it's sending a load of water up the Vent pipe and dumping it in the F&E cistern, so then it will be constantly drawing water (and air) back in to top itself up again. This air then finds itself trapped in the system and the whole corrosion process inside the system is being accelerated with the introduction of the air.
 
It depends on the configuration of pipe work.
If the feed and expansion is blocked it can cause air to be sucked down the vent,
In some cases water is pumped up the vent.
 
So there is no water coming out of vent pipe when system is running (stood and watched it for a bit) but it does have condensation when I look in f and e tank. When pump starts it seems to draw a bit of water from f and e tank as can see movement and drips from cold feed for a while

Had inhibitor added the two weeks ago

If it is as pumping over surely the f and e tank would overflow?
 
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If the F/E was restricted enough it would overflow !! Again depending on config and where the restriction is.
How is yours configured ?
Is there negative pressure on the vent on start up..
If the feed /exp pipes were 22 mm same as vent this problem would be far less prevalent.

As an example "Myson" sell an air separator called an "Aerjec" which has a 22mm tapping for c/f/e. Virtually all are reduced to 15 on installation and suffer blockages.
Every one that I fitted Were 22 mm f/e and none suffered a blockage. ….Luck or fitted as per instructions !!! ???
 
The cold feed and vent is before the pump both pipes are next to each other up into the loft.
F/E pipe is 22m but converts to 15mm as it enters the loft
 

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