Air getting into central heating system

[ceecjc1]

Hi guys,

This topic must have been discussed a lot but I can't find anything specifically answering my question, so hopefully you boys can help me out.

I've just moved into a house which has a brand new (one month old) oil fired combi boiler with a built-in expansion vessel, connected to a very old central heating system. One new radiator was installed along with the boiler.

The highest radiator in the house was full of air when we moved in (there was about an inch of hot at the bottom, the rest stone cold) so I bled it and all was good. Unfortunately the air returns after 3-4 days.

When the central heating turns on, which it does frequently because the house has poor insulation, there is a loud purring in the pipes which seems to start near the boiler, then can be heard following the central heating loop round into the living room and then finally upstairs into the bedroom where the radiator in question is. It's very noisy but after a minute or so it's quiet and the system runs as normal. After the heating goes off for a time then comes back on again, the same thing happens.

So how do I tell if air is getting in or if it's gas? It seems like a lot of gas to be produced in a short time and it seems that there was inhibitor introduced into the system a month ago. The system pressure is good at 1-1.5 bar and isn't going down. If it's air, how could it be getting in so fast? There are no leaks that I can see.

Thanks for any input! The house is rented so we can't call an expert in ourselves and have to have a good reason to get the landlord to do it.

Cheers,
Chris

 

[ukrobj]

When bleeding the rad, hold a glass upside down and if it gas (nitrogen) you will capture it in the glass, now hold a (long) match to the glass, if it is indeed gas it will burn rather quick. If not, then its air.

 

[D_Hailsham]

When bleeding the rad, hold a glass upside down and if it gas (nitrogen) you will capture it in the glass, now hold a (long) match to the glass, if it is indeed gas it will burn rather quick. If not, then its air.

If it is gas, it won't be nitrogen, it will be hydrogen!

 

[45yearsagasman]

If it's nitrogen we're all in trouble

 

[ceecjc1]

Do you think gas would form this quickly? It's a very old system as I said but there is inhibitor in it, I've still got the bottle from when the plumber did it. Sounds more likely that it would be air given how quickly it forms and it sounds like it's getting in near the boiler judging by where the noise starts (boiler) and where it finishes (at the top rad).

Am I safe to attempt to light hydrogen?!

If it helps, the dog was very interested in the smell of whatever was venting from the radiator, but that could have been the unusual noise or it could have just been the stink from an ancient radiator system.

 

[45yearsagasman]

Just an inverted glass or an old tin and then apply a match. All you'll get is a small POP. With a sealed system such as you have, and pressurised to say
1bar (15 psi approx) it is virtually impossible for air to get in.

 

[ceecjc1]

Cool, thanks Pete. I'll give it a go. It's a while since I did the hydrogen / nitrogen / oxygen tests with a splint at school!

 

[ukrobj]

Well spotted, that will teach me to drink and type.

 

[ceecjc1]

Just an update - it seems like it's air that's in the system, I couldn't get anything to happen when I tested the released gas.

The pressure in the system is also going down now, from about 1.25bar to 0.25bar in the space of a week, so I have to keep refilling it or the boiler refuses to start once it gets below that. The engineer says to keep an eye on it but if we make a fuss the agent will send him out to take a look.

Someone said there was no chance with a pressurised system that air could get in, so maybe my gas test didn't work and it is a lot of hydrogen in there, but what would explain the pressure drop?