Air getting drawn into sealed heating system

[Jay The Plumber]

@marc1977 - yes the system pressure needs to be dropped below the pressure inside the expansion vessel otherwise you are just reading the system pressure.

 

[Last Plumber]

No leaks around the boiler ?
Look at pipes and fittings from tee of exp vessel to rear of pump via Boiler,
When it's cold and not working.

To work on the boiler you need to be gas safe registered so if you cannot see the pipe joints at the back, you'll need a Gas Safe registered Engineer.

 

[Jay The Plumber]

check all the drain offs, if they aint tight they can pull in air. check for them around the boiler and also on the rads, tighten the valve (square nut) and the outer knurled 'nut'
let us know if any were loose

 

[marc1977]

ok, checked all of those and cant see ANY leaks or loose nuts.

I drained the system down to release the pressure in order to properly check the expansion tank pressure. It now looks like the expansion tank was probably slightly under pressure before so have pumped it up to 1.5 bar.

I let all the air out of the expansion vessel first and there was some moisture in the air that was released..it was only spitting a small of water though. i wonder if this is normal?

 

[JCplumb]

You mention you have drained down and also that the radiators are quite new so the system was drained down when they were installed. Air is common in systems that have been drained down, you have a big system too which increases the potential for air remaining in the system after refilling.
How many times have you bled the rads after the latest drain down? Depending on your pipe runs, it could be quite normal that you need to bleed half a dozen times or so after work has been carried out or system has been drained/refilled.

 

[Chuck]

I have to bleed one of the rads every week for 5-6 seconds.

There is a possibility that you are bleeding hydrogen not air.

If you are talking about bleeding off ca 100ml of gas per week I suspect it is more likely to be your new radiators and pipework generating the 'passivating layer', which is a process that liberates a certain amount of hydrogen. Inhibitor slows this process, so it can take several months to complete, but doesn't stop it completely. Another possibility, if the system or pump is noisy, is turbulence due to excessive circulation speeds damaging the passivating layer, which will then liberate hydrogen as it tries to reform.

I would check the pump speed is correct first. Then wait until the system has had several (my system took four) months to settle down. If it's still a problem get the installer to take a sample of the gas and test its composition (i.e. air or hydrogen). Don't do this yourself as most amateur methods are dangerous. (Actually, I've seen installers use methods that IMO are dangerous in the past but they are trained and insured so it's their problem!)

Sentinel sell a System Water Check kit that retails for about £50, which involves you taking a sample of water and posting it to a laboratory for analysis. The report will tell you what's causing the evolution of the gas. Most likely cause, IME, is high chloride levels due to flux not being washed out properly by the installer. There are more exotic reasons possible in principle, but I've never heard of anyone having them in a domestic system.

Useful further information here:

How To: Control Corrosion in Central Heating Systems | Sentinel

C.

 

[marc1977]

its been about 6 months and ive bled them every week but it does seem to be very slowly improving so Im coming to the conclusion you may be correct. either that or there must be a very small leak i cant find..

 

[Harvest Fields]

I am sure it is because the system is new. It is important that you have the correct amount of inhibitor in the system.

 

[marc1977]

Yep, ive bought a test kit to check the levels. thanks all

 

[Harvest Fields]

Your welcome