Possible Airlock?

3 Jan 2013
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United Kingdom
Hi Guys,

I have been redecorating one of our upstairs bedrooms and have had to replace the Rad valves in this room as they were corroded and would not lock off to remove the rad.

I drained the system down from a downstairs rad in the dining room as this was the only place I could get a bucket under the pipe. There was no drain off valve anywhere so whilst I had the system drained I decided to fit a drain to the rad in the living room. This rad is the nearest to the patio doors and best place to fit the drain.

I did find that this rad had not emptied and was still full of water despite draining the system from the dining room. So, I drained this rad from the valve into a bowl then fitted the drain, fitted the new rad valves upstairs and and re-pressurized the system (it is a combi boiler). I then bled all the rads, topped up the pressure and fired up the system. All rads got hot apart from the one that I fitted the drain to. So I concluded there was an airlock.

To fix this I locked of all rads apart from the cold one and fired up the system. There is a load of gurgling and the rad gets hot. I open all other rads and the system is fine with all rads staying hot. I have bled a lot of air out of one of the upstairs rads (about half the rad).

When the system fires up the following day the same rad with the drain is cold and wont start until I lock off all other rads to bring it to life. Does anyone have any ideas what could be causing this and how to fix it? Is it worth re draining the system and running some cleaner through it to stry and clear any sludge build up? I am wondering if there is a build up that needs some higher pressure to shift and get things going, but once it is circulating water it is fine until it cuts out and the sludge settles?


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Does the pipework from the lounge radiator disappear under the floor or has it been taken upwards? Sounds like you may still have an airlock on the feeds to this radiator so look for a high point in the pipes where gases could be trapped.
Non of the rads have been balanced. They weren't when we moved in 12 months ago and the rad was fine until now. happy to try balancing if it is likely to make a difference?

The pipes come out of the rad at the bottom then go 90deg into the wall
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I'm guessing you have solid floors and the pipes go up above the lounge ceiling. If that's the case there's probably an airlock up there.

If you can get that rediator to heat up when all the others are locked off give it a good run and keep bleeding it and repressurising the boiler.

Without knowing the system layout it's difficult to suggest anything else.

Make sure you also have plenty of inhibitor in there to avoid future gas/corrosion problems.
As said chances are a drop system and an air lock just above where the pipes go under floor, may be small enough to push past when other rads off but the extra resistance stops it working when all rads in use.

If all lock shield valves are fully open you are quite lucky it worked before, certainly worth checking down the rads that get hot first.

Two ways to balance, faffing around with flow and return thermometers to get the temperature drop correct, or "rule of thumb" start with the furthest rad fully open and those nearest the pump about 1/4 turn open with the others each slightly more. Then try it and cut down any that get hot much quicker and open those that are slow.

Truth be known that's the way most guys do it, although some will scream heresy! ;) ;)
Try turning all other rads off except the one that isn't working and leaving it like this for a while not just till it gets hot and it should eventually push all of the air out of the pipework and into the radiator. I wouldn't worry about balancing until the airlock has been sorted.
The boiler is upstairs, and all rads are fed from the walls. It is a 70's house with parque flooring under the carpet and I suspect that it is poured concrete under that.

It is weird that once it gets hot it will stay on until the system shuts down. I am still getting a bit of air out of the upstairs rads though
So I have managed to work out that it is one radiator that is causing the problem. If I lock off the rad that is upstairs and directly above the problem rad. the problem rad fires up fine. I have locked this one rad off to try and clear any locks. I am still getting air out of one of the upstairs rads but it is only collecting in one rad which I think is the first one after the boiler.

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