Air getting drawn into sealed heating system

[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

 

[Old Account]

Its a big house..15 rads with 2 very big doubles rest are singles. Lots of drops too if that helps as downstairs floor is concrete.

I put 1.5 bottles of sentinal in.

Also Ive tried igniting the gas but it didn't ignite so dont think its hydrogen.

As you say you've got quite few up and downs of pipework which means air will be collected there too. Have you got any dead legs ? As harvest mentioned you definitely need the right amount of Inhibitor ( 2 bottles x100 sentinel ) which covers your system.

Is just a thought could there be an AAV faulty and draw air in ?

 

[marc1977]

No AAV's..but i also think the up and downs wont help.

 

[Harvest Fields]

Ups and downs are rubbish but sometimes necessary unfortunately

 

[Old Account]

Why not trying to bleed the system with a hose connected onto the air vent ? It may help to get rid off more trapped air

 

[marc1977]

Matchless,

not sure what you mean?

 

[Chuck]

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..

I've never known a leak draw atmospheric air into a sealed pressursised system. I guess it might be possible right up close to a circulating pump that's working flat-out but even that seems to be a bit of a stretch to me.

Another possibility that hasn't been explictily explored so far is a small leak across the membrane in the expansion chamber. There's a lot of compressed air in those things and several litres of water get pushed in and out every time the system heats and cools. If you've noticed a reduction in the amount of 'air' in your system over the last month or two it could just be the warmer weather means the number of cycles of expansion/contraction has been reducing.

C.

 

[marc1977]

Yep,ive drained down the system and re pressurized the expansion tank and it seems to hold air ok. If i still have the issue in a couple of weeks i will drain it down again and check the expansion still has 1.5bar in it. sound sensible?

 

[Chuck]

Yep,ive drained down the system and re pressurized the expansion tank and it seems to hold air ok. If i still have the issue in a couple of weeks i will drain it down again and check the expansion still has 1.5bar in it. sound sensible?

Maybe, as long as the temperature is the same and you have a calibrated pressure gauge and remember that atmospheric pressure varies from day to day by about 5%.

I'd leave it a lot longer than a couple of weeks. It's easy to get obsessed with this sort of thing and cause more problems than you solve. Constantly draining and refilling the system will, for example, reintroduce air into the system every time. If you have to pump up the expansion tank once a year and bleed one radiator (because it's cold not from curiousity) twice a year I'd consider that par for the course. It's a domestic central heating system, not the space shuttle.

 

[marc1977]

Thanks again all.

 

[Harvest Fields]

Your welcome marc1977