Eliminating air in central heating system

airlock(s) lower down?

JimK

What is boiler pressure, do you fill it cold till water comes out, then you have a leak, do you have an auto fill valve. Look for a leak.

You say that there's no header tank, so presumably it's a non-vented (pressurised) system?

Where is the filling loop and pressure gauge - and what is the system pressure when cold? If the gauge is at the lowest level, and there are three floors, the static fill pressure should be 1 bar or more to ensure adequate pressure on the top floor (there will be at least half a bar static pressure drop between ground floor and top floor).

I don't think the pump's replacement is relevant because you shouldn't bleed the system with the pump running, anyway.

Is there any inhibitor in the system? If not, there's a distinct possibility that the 'air' which is collecting in the radiators is, in fact, hydrogen - resulting from corrosion which is taking place inside the radiators.

When you say you have no header tank do you mean you have a "sealed" system or is it using a Primatic (spit!) cylinder. Also does your pump "push" the water round from the boiler or does it "pull" it back from the radiators? Did the problem present itself after your new pump was fitted?

The pump does not create pressure - think of it more correctly as a "Circulator"

Does your boiler have a filling loop and pressure gauge? If so, follow the instructions to bring it up to pressure after bleeding - then repeat.

The gas within .. Hydrogen caused by the water breaking down. See if you can get some of the bleed gas in a paper bag then set light to it if it goes !! PoP !! then its Hydrogen..

All the usual disclaimers apply abaht not blowing yourself up etc;!...

With no header tank I'd say the system pressure has dropped. Top up time with the filling loop.

If it persists, suspect a minor leak.

Indeed. Thanks for all the replies - plenty to go on although I think hydrogen is unlikely.

E.