Apparent leak on CH system

Is it a vented or sealed system? If the latter, is the expansion vessel doing what it ought to, or is it knackered and the pressure relief valve is leaking?

But that would show up as a regular need to repressurise, which hopefully the OP would have reported.

Might it not suggest a system with no inhibitor that is consequently still corroding?

Hydrogen, liberated from the water as iron takes up the oxygen.

Does the system have inhibitor?

On the basis that for something to get in something needs to come out, yes but it's not the whole story.

If a vented system is it pumping over? Even a tiny little bit just as the pump starts (or stops) will circulate oxygenated water into the system.

If the former is the system pumping over? Pump running too fast and the water is circulating up the expansion pipe and into the open expansion tank. This can aerate the water and when drawn back into your CH pipework the air has to go somewhere.

You may only see a pump-over as some of the system closes down such as when thermostatic radiator valves close down once temperature is reached etc.

That's a possibility.

Another is that its not air, but hydrogen accumulating from corrosion processes. Do you know if there is adequate corrosion inhibitor in there?

(you could test by trying to ignite the gas as its bled from the rad).

Such products still exist. Probably best only used as a last resort though.

Very small leaks can evaporate away before you see liquid water. In hard water areas you may see a small verdigris or scale stain though.

You could add a small quantity of Fluorescein dye to the primary water, and then go round with a UV torch to see if you can fine a leak location then.

No they haven't.

Probably not even worth doing. Mine has similar, but two years after the plumber noted a weeping gate valve on the primary of the boiler ' not worth fixing mate, it will scale up if you keep refilling ' the frequency of bleed and recharge is down to about 6-monthly

It's a sealed system. Only 7 month old from new. Peter

Yes there us, I watched the Engineer run it into the system Peter

It's hydrogen from the water in the system attacking the new steel. Chemistry: Fe + 2H2O > Fe(OH)2 + H2 Add an inhibitor. Something like this

By that I assume you mean the whole system is new, not just the boiler? In which case you may find it is just the air coming out of all that fresh water used to fill the system. I have just (2 years ago) gone though a similar experience and had to be bleeding the towel rail for the biggest part of 9 months or so. Not needed bleeds or any top ups in the last year.

It's odd. My towel rail also needs bleeding from time to time. It's not the highest rad in the system either - by a long way. System is pressurized and doesn't seem to have any leaks to speak of - I'd guess I add some water to it every couple of years.

There is plenty inhibitor, and the rad is stainless steel. Never have to bleed any other rad in the system.

Is it the first inverted "U" from the boiler? Air released from the water by the heating will collect there. This doesn't have to be the highest point in the system 'cause air won't travel down wards very easily.

That could well be it - boiler is in the bathroom too. It's no big deal - certainly not worth the effort of investigating why. There could well be a tiny leak somewhere - but doesn't show.

Skillbuilder posted this only yesterday:

Zoflora would probably work for this