Air in radiators but no sign of leaks

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I have a pressurised heating and hot water system, recently I discovered alot of air in the radiators but no sign of leaks anywhere such as damp patches on the walls or anything.
I recharged the system adding some more protector fluid.
About 4 months later I have the same problem again with a lot of air in the radiator system.
I have again recharged the system, estimate around 3Litres including adding in 1L of internal sealing liquid.
I am concerned that the problem could be the hot water cylinder coil leaking as I am using a lot of water to recharge the system with no signs of leak patches anywhere

My boiler is an external type boiler located outside the house.



 
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If you are absolutely sure there are no leaks then see if you have an aluminium heat exchanger and it may be creating hydrogen, hydrogen will burn. Safest way to check is to see if the system is creating hydrogen is test the system water PH.

You really don't want to use leak sealer, nasty stuff.
 
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and it may be creating hydrogen, hydrogen will burn.

It also creates oxygen which together with the hydrogen creates an explosive mixture. Hence testing the "air" coming out of the bleed nipple with a lighted match is a bit hazardous. If the flame burns back through the nipple into the radiator then the gases in the radiator will explode.

Collecting some of the "air" in a small plastic bottle and then putting a match to the bottle is a less hazardous/destructive way to test for hydrogen
 
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Thanks fir the replies, I didn’t think about the heat exchanger, I’ll give that a check
 
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MJN

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I don't have the practical experience to back this up but from my recollection of physics at school I would expect the pressure in a sealed system to *increase* if hydrogen was being produced, not decrease.
(Edit: Or have I just assumed the pressure has been decreasing itself, not just as a result of you bleeding the radiators?)
 
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