Air Stuck, Vent pipe blocked?

[Schmill]

I recently ran some cleaner around in my central heating system after doing some work, and then flushed and refilled.

As much as the system seems to be working fine now, and refilled ok etc, if I listen near to the pump in the airing cupboard it sounds like there is air in the water. Not an air-lock as such, but just as if there is air in the water, (if you know what I mean?!?).

I'm wondering if the vent pipe is blocked in someway and therefore not letting the air out of the system properly? I don't know what happened when I refilled the system, as I was downstairs, so for all I know the air may have exited the system by going back up the cold fill pipe.

When the system is running and up to temperature the cold fill pipe is hot for as high as I can reach in the airing cupboard, (presumably this is due to convection effectively heatnig the 'cold' water in the pipe?). However the Vent pipe feels fairly cool only about 1ft up from where the main flow T's off towards the fill & pump.

Other than cutting through the vent pipe to check it is there some other way I can check for a blocked vent pipe?
I was considering perhaps attaching a hose pipe to the cold fill & the outside tap (turned on slightly), and seeing if I could then cause the system to effectively overflow up the vent pipe, which would presumbly tell me that it's not blocked. My only issue with this is that if the vent pipe IS blocked then I would presumably be pressurising my system and I don't want to risk blowing any joints

I have run various cleaners through the system recently (X800, X400) in order to clean the system out and get heat to all the radiators (which is now working fine), but presumably these won't have touched the vent pipe as it's not in the main path of water circulation?

Any suggestions? Do I go for the hose pipe, or do I bite the bullet, drain the system AGAIN ( ) and take a cutter to the vent pipe?

Thanks

 

[TicklyT]

IME it takes some time to get all the air out of a system.

Something like bleeding everything on the initial fill,
Then bleed the rads again the next day,
Then the next week,
Then the next month.
Etc.
Etc.

Dissolved air and other gases slowly comes out of solution in the fill water as the chemistry settles down, and there are nearly always a few trapped air pockets that get shifted over time. Not every gas bubble gets trapped by the vent as it passes by ayway. As long as the intervals between the need to bleed radiators increases, there's not a lot wrong.

It's normal for the system to push a couple of litres of hot water up the cold feed as it heats up and the water expands, but the same amount of expansion will only push a few cubic centimetres of water up the vent pipe. It's to do with the relationship between the surface area of the F&E tank and the cross sectional area of the vent pipe.

The old jam jar full of water over the end of the vent trick can reveal if the vent is working. If the vent is open, the water level in the jar momentarily rises and falls as the pump starts and stops.

A partial blockage in the cold feed connection, usually at the tee where it joins the system, can cause air to be drawn down the vent during normal operation (or the water in the jam jar to disappear completely )

 

[moggridgeplumbing]

As above but also hold a magnet next to the tee where the vent pipe rises and up the vent pipe. If it feels like it's magnetic and the pipe is copper then you've got a build up of magnetite which you haven't cleared with the flush. Quickest way is to cut it out and replace.

 

[Schmill]

Ok, now I am confused

It appears that no-one has told my heating system how it is supposed to work - lol

I got fed up of the air in the system last night, so I did a partial drain down and then refilled it, all seemed to go well.

However, when refilling I noticed that there was air bubbling up through the FILL pipe into the FE tank. Not sure why it wasn't coming up the VENT pipe?!
So then I decided to test the Vent pipe (again). Placed a full jam jar of water over the vent pipe and got the wife to switch the pump on and off. I had bubbles in the jam jar, but then I also had the vent pipe suck a good 2/3rds to 3/4 of the water out of the Jam jar, AND I still had bubbles coming UP the FILL pipe. Why are the pipes working the wrong way around ?

This is the pipework in the airing cupboard;

Top of Heating Pipe Work

• Schmill
• 7 Apr 2011
• 3

White vertical pipe is the flow from the boiler, heading straight up to the loft as the VENT pipe which then hangs over the FE tank.
The copper pipe, including the angled section, is the cold fill from the FE tank.
The pump on the right of the picture pumps DOWNWARDS.

I believe all of this is correct, so I am confused as to why it's not working properly