Air getting drawn into sealed heating system

[marc1977]

Hi,

Ive posted this before but still not found the cause of the problem. I have a sealed S plan system with expansion tank that is drawing in air. I have to bleed one of the rads every week for 5-6 seconds. Details below:

1. The system is NOT loosing any pressure until of course I bleed the radiator. I left it alone for 3 weeks and the pressure when cold remained at 1 bar.
2. There is sentinal inhibitor in the system and i have also added sentinal leak sealant.
3. There are NO visible leaks anywhere. I have checked all rad valves and connections.
4. The expansion tank appears to be fine and has 1 bar pressure.
5. No visible leaks on the pump.
6. Worcester Bosch have been out and inspected the boiler and found no issues.

What the Damn is going on?! Have I got a magical heating system that defies the laws of science?

Many thanks

Marc

 

[Harvest Fields]

How many radiators do you have? are they small or large? Are they doubles or singles and how much inhibitor did you put in to the system?

 

[marc1977]

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.

 

[Harvest Fields]

Sentinel x100 is a good inhibitor but for a system your size I would use 2 x 1 litre bottles. As a 1 litre bottle only does 8-10 rads have you have the condition of the water in the radiators tested?

 

[marc1977]

No. I should mention all the rads are new and the system was drained down and refilled 6 months ago. Most of the pipe work is new too. Can you buy testing kits for the water?

 

[Jay The Plumber]

did you check the expansion vessel pressure when the system pressure was at 1 bar or did you drop the system pressure to check it?

 

[marc1977]

i did it when the system was cold.

 

[Jay The Plumber]

i did it when the system was cold.

and the system pressure was at 1 bar cold ?

you normally need to drop the system pressure below the pressure that the expansion vessel is charged to.

 

[Last Plumber]

Where is the pump in relation to the expansion vessel ?
Can you post a photo of the installation of the pump and vessel?

What pipe material is it?
If it is plastic, is it the correct one (barrier) ?
Most of it is these days but you never know !

 

[marc1977]

Last Plumber,

Photo attached. All pipework is copper.

Jay,

So the expansion vessel has to be below the pressure of the 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

 

[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

 

[Tim Cremin]

Most people in the heating business do not know that the air is formed in the boiler when temperatures exceed 60°C. Therefore air/gas is present in all water based heating systems.
One of the reason you are witnessing air accumulating in this system maybe the boiler stat is faulty or set very high.