Bleeding central heating....

[GMM]

Working in my 'study' (ie junk room where I work) on the second (top) floor of the house, every now and then I can hear a bubble making its way into the radiators. Easy enough to bleed from time to time but I'm starting to wonder a) where the air is coming from and b) if it indicates something amiss with the CH that I should be looking into.

The only other feature of the system that comes to mind is that when I drained it down, the flow from the drain point seemed pretty sluggish considering it had 3 storeys of head on it. Do these two symptoms indicate a blockage on the way in from the header tank? (It can't be completely blocked or it wouldn't have refilled when I drained it).

It's an open vented system with tandem boilers. Overall, it churns out a lot of heat (which might be a good thing or not - I haven't had a bill yet since we moved in!) so I don't think there's any major problem with function overall at present. The header tank is on the same floor as my study, so only a few feet above the level of the rads in question. I have drained it down twice since I bought the house but the last time (bled carefully on re-fill, added inhibitor etc) was about 10 weeks ago, so it seems unlikely that it is still pushing through air that was trapped somewhere from the last drain. I'm sort of guessing (though it's hard to be certain) that the air comes through each time the pump cuts in.

[Roger Mills]

Have you checked whether it's pumping over? Watch the fill & expansion tank whilst the system is running, and make sure that there isn't a constant stream of water coming out of the vent pipe. If there *is*, it needs dealing with - because that will introduce air into the system.

If it drained slowly, it *could* be sludged up - what colour and thickness was the stuff that came out? Or it could just be that there was no easy path for air to enter to replace the water as it came out. Make sure that any motorised 2-port valves are opened manually before draining or filling, and that 3-port valves are at the mid position.

[NT]

I'd look at pumping over, lack of corrosion inhibitor

NT

[GMM]

I'll have to watch the expansion tank when I get a moment. The water coming out was completely clear but I wasn't aware that the pumps had to be opened, so that probably explains the sluggish draining.

[GMM]

Certainly the boiler was swapped not long ago (before my time, but I've only had the place a few months). I'm not sure I understand why smaller waterways in a new boiler should cause this sort of problem, not that I doubt that you are right (!) I don't think the tank can feasibly go any higher than it currently, so perhaps it does need removing. Can't help feeling that could create some new problems though....

[GMM]

I hope that the inhibitor was still OK, a couple of months after adding it, although I do notice that the water is sometimes black when I bleed these radiators, even though it's clear when I drain off downstairs. I was thinking they were still holding a bit of historic sludge.

[Dave Liquorice]

That would be very bad pump over, more often there can be just a little dribble when the pump starts and it is pushing against the inertia of the water in the pipework. Both need sorting. A liitle dribble may well be cured by simply making the loop of the vent pipe higher.

Some one else said that air could be drawn down the vent pipe could be, not sure you'd see the water level in the tank rise though. Check where the vent pipe is connected relative to the pump. The risk with before is sucking air down, after pumping over.

Has the OP tested what this "air" is? Is it air or hydrogen?

[Chris J Dixon]

It is just possible that the system has been arranged in a way that causes certain sections, when pumped, to be below atmospheric pressure, so that leaks in those areas would be air in rather than water out.

I leave it to others to explain the detail, or how you would cure it.

Chris

[GMM]

That sounds like an interesting test.would it be a match to the gas being bled? Hydrogen should give a satisfying pop if it's at high concentration or pure - or is there a more sophisticated test that's easy to do?

[GMM]

I get the idea of that - presumably such locations would still leak water when the system is off though, wouldn't they? I haven't found any evidence of a leak anywhere (yet!).

[Roger Mills]

Yes, hydrogen will burn with a blue flame, air will blow the match out!

I didn't suggest this because you said that inhibitor had recently been added after a drain-down. But I suppose there could be some pockets of the system in which corrosion is still taking place.

[Dave Liquorice]

Water molecules are bigger than air molecules... With corrosion the material is likely to go porous at the molecular level before bigger holes develop. I doubt that this is what is happening with your system though.

[Dave Liquorice]

that's

Try an set fire to it, not sure if you need to collect the gas the light it or fi it will light from the bleed screw. I've never had a system that needed regular bleeding.

A very faint blue flame, spotting a hydrogen fire is not easy. Impurities will add their own colour(s).

They system has bee drained an filled twice in short succession and it appears to be a large system. Check that enough inhibitor has been added.

[GMM]

Well, I struggled with the question of how much inhibitor was 'enough': It's a large (6-bed) house and the layout is a bit complicated, so I assumed that the total capacity would be pretty large. I didn't have any inhibitor on the first drain (a bit of a panic when I screwed through a CH pipe!) so had to refill without. This revealed that the main drain c*ck seal was perished so I drained again a week or so later and added 3 litres of inhibitor (allegedly

100x concentrate) which I thought would do the job and a bit more. Of course, I suppose the inhibitor I used could have been rubbish/old/ etc. I was wondering whether I should test it, or if it's just simpler to put some more in. Oddly, although the water coming from the main drain-off ran clear each time, the key always winds up with black splatters when I bleed the top floor rads. I had assumed this was the result of historic sludge not having been cleared from the top floor when the boilers were changed (approx 2 years ago I'm told).

[thirty-six]

I'm starting to wonder a) where the air is coming from and b) if it

It's leaking, the air is coming from the boiling off of the top-up water.

If you did it recently, you have to fire the boiler on high to drive off the gases from the water. With a good design, these should exit from your vent pipe without having to play with the system but you could go around opening one radiator at a time to drive of the gas's from eac on an individual basis and appropriately vent at each radiator.

Which is all it needs.

Do as I suggested earlier firing with only a single radiator open at a time and go around the whole system.

[Dave Liquorice]

So enough to protect a 300l system. If you were really keen you'd find your rads on the web and the capacity of each and the capacity of the boiler then work out the total tube lengths, work out the volume for those etc.

Or count how many rads you have and multiply by 10 for each single convector and 20 for each double for radiators of "average" size say

1.5 to 2kW. I guess there are few small ones in downstairs loos or the like and some larger ones in bigger rooms. It'll give ball park system volume.

Test is simpl run off a little of the primary water into a jar and submerge a bright soft iron nail in it. It's shouldn't rust. I think you can get proper test kits, yep you can at a price. Though Sentinal have a two use kit for about =A33, donno if that would work with other makers inhibitor or only Sentinal X100.

[GMM]

Well, counting each double as two, there are 31 radiators, most of which are on the large size (as will be the gas bill when it arrives!). I doubt very much I could find these one the web as they're a few years old but, with a lot of pipework, I thought 300 litres might be a reasonable guess! Have just bought another couple of litres today, so I'll throw that in too, since I understand that it's difficult to over-dose. The last house had hot air heating. I didn't like it then, but I'm starting to look back on it fondly now !