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Central Heating

Air lock on system?

I have an old header tank system. I have moved a radiator, now the heating and hot water will not come on?

Do I have an air lock? and if so how do I solve this?

Cold water supply into the system?

8 Answers from MyBuilder Heating Engineers

Best Answer

Sounds to me like the cold fill to the sytem is blocked.

This is very common.

When ever I work on a system with a feed and expansion tank, I always leave the mains water on whilst drainning the system down. This way I can be sure that the system will re-fill.

I.E. by watching thw tank level fall, and the ball cock drop as the system is drainning,

Firstly check if there is any water left in the expansion tank.

If the tank is dry, then simply the ball cock is jammed. Free / replace as necessary.

If the tank is full, then clearly the system is blocked. It will be blocked at the point where the cold fill pipe joins the heating circuit, this can often be found in the airing cupboard possibly near the pump.

You could always follow the pipe down.

Options are to cut out the T completely and replace, a simple job for an experienced plumber.

A diy job could be that you cut the 15mm pipe close to the T and unblock mechanically with a piece of wire / coat hanger etc. Then re join the pipe with a compression fitting. Remember though that when you cut the pipe the water from the tank will come down as the upstream pipe is not blocked !

Be prepared, either manually drain the expansion tank, or block up the outlet, then have dust sheet / container / bucket handy.

Another 'Quick fix' we often do is simply join the fill pipe in the loft across to the open vent pipe, thus the system will fill via the open vent, not ideal in terms of automatically expelling the air from the system but will work perfectly well.

Arm yourself with a 22mm plastic T, a 22mm to 15mm spigot reducer, a 15mm plastic T and a short length of 15mm plastic pipe.

This would work happily.

If you dont want to get involved with all that, take your garden hose and attach it to a drain down vent and you coiuld reverse fill the sytem !

Hey preso, 30 Years worth of experienced gained tricks !!!

Alternativelt why not place the job on this site, should get plenty of change from a Hundred Quid !!

Adrian Hagger
Southsea plumbing


Answered 9th Oct 2011

If the boiler has locked down you most likely have air..... Have you fully bled the system? Is the header tank filling up?
If you have bled the system and theheader tank is full then there are a few things I would try.
In your cylinder cupboard look for a bleed nipple near the 3 way valve, see if you have air trapped here
Look in your loft to see if you have central heating pipes ... There may be one on any horizontal pipes
Failing this isolate every radiator with the valves and open 1 rad at a time and bleed them until you have about a pint of water come through the vent


Answered 9th Oct 2011

the heating and hot water are on different systems,"bleed the radiators" ie open the small valves on side of rad and fill up(until water comes out) as for the hot water (thermostat may be faulty) are you obtaining cold water through the taps?


Answered 9th Oct 2011

If it was only radiators not heating up I would say airlock, however the fact you have no heating or hot water suggests this is not case as the hot water cylinder is fed from the bigger tank ( cold water cistern) in the loft, not the small f & e tank so draining the rads would not effect the hot water. Here's what to do.

I know it sounds simple but have you definately powered the programmer to on for heating and hot water? If not do this!

Ensure room stat is turned up to desired temp.

Bleed all rads starting from downstairs

Bleed thumb vents in cylinder cupboard

Try now. Last resort to remov
e airlock in rads is to put a hose on radiator drain point and open it up for a minute or two just to pull through any possible trapped air.

If none of the above works it sound s like there is a problem with the 3 port valve that direct water through the cylinder and rads.

Let me know how you get on.



Answered 9th Oct 2011

Hi, on your Pipework there should be an air vent,this will have a little thumbwheel open that up and you should hear air rushing through as soon as water starts spitting out close the thumbwheel.

The location of the thumbwheel will be on a high point of your pipework.

If there is no sound of rushing air then you will have a valve closed, generally this will have a red wheel on it, open that up and try opening the air vent again.


Answered 9th Oct 2011

You could have an air lock. Try draining complete system opening all valves on all rads and manually opening any other valves such as 3way valve.
Then opening up all bleed valves on all rads and pipe work, pump ect from highest to lowest.
Then refill using inhibitor bleeding from lowest to highest.(remember to shut all bleed vents before re filling!)
If there is still air in the system there are a few tricks such as shutting down most of system so heat and pump is working on one area at a time or probably the fastest is to connect a hose from the main and blast air lock out.
But really unless your experienced or confident with the last two methods id call out a plumber/heating engineer as its a job best left to the experts.
Kind regards

George Ray


Answered 9th Oct 2011

Your air- lock seems to be in the boiler itself, the best way to remove this is to start and stop your boiler (heating side) and you should hear water gurgling through the system from the movement being forced through the central heating pump, and eventually it will get rid of the air-lock. Alternatively the feed to your heating system from the feed and expansion tank could be blocked which is a whole different problem.

Kind regards and good luck


Answered 9th Oct 2011

You wouldn't have hot water if your cylinder is heated by your boiler
Although you would still have cold water coming through the hot tap.
If when bleeding your system no water is coming out, look in your f&e tank and see if that is full
Which could indicate a blocked cold feed, or air lock (as above )

Open your motorised valve/s to the manual open position and then bleed all of your radiators and the air vent with a little screw cap on top normally by your hot cylinder.

Unlock your motorised valves demand hot water only on your clock as the hot water circuit is the shortest route to get air away from your boiler,(apart from ov) keep bleeding the air vent by your cylinder periodically until the pipes by your cylinder get hot and your boiler stays firing, you should hear some air moving in the pipe work

Make sure your room stat is turned up
After about ten minutes put heating and hot water on

Hopefully this will solve your problem if it doesn't try the process a couple of times.

As an after thought check your boiler doesn't need resetting

Kind regards



Answered 12th Oct 2011

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