Radiator bleeding valves suck air in when pump is on

[cap_]

Please help me

I have an open system (i.e. with expansion tank in the loft) powered by Ideal Mexico Super CF55 boiler which seems to work fine. I have replaced some of the old radiators and radiator valves, and now after refilling the system I am stuck with the following problem:
1. When I filled the system with water it all seemed fine, I could bleed the radiators both upstairs and downstairs and after some air there was water coming out (from every radiator)
2. When I switched the boiler on it seems to work fine - the water got hot in hot water taps (i.e. in the hot water cylinder upstairs) and in the radiators downstairs. But radiators upstairs stayed cold.
3. So I assumed that there is air in each of them so I tried to bleed them but instead of air (and later water) coming out I've discoverd that each one of them sucks air in! I run downstairs and switched the boiler off (I was afraid to damage it), but left pump on to see what would happen.
4. After a minute or so of sucking air in from one of the bleeding valves upstairs I heard water going through the safety (overfill) pipe of the expansion tank in the loft.

I played around with it a bit, draining the system and refilling it again several times to see if that helps but with no luck. Still when pump is on the radiators upstairs suck air in! Switching the pump off makes the system "normal" - I can bleed radiators upstairs and after a little air water starts coming out. I also discoverd that I don't seem to be able to drain the system completely any longer - radiators downstairs stay full (I open the bleeding valve and water comes out) even after water stop coming out from the draining tap. But radiators upstairs get empty as normal.

Any suggestions on what the problem is and how to solve it will be greatly appreciated.

Andrey

 

[scatmanjohn]

Idea.... Don't bleed the rads with the pump running.

 

[cap_]

Idea.... Don't bleed the rads with the pump running.

Do you mean this is a normal behaviour? It did not do it before. And anyway the radiators upstairs stay completely cold so something is wrong...

 

[scatmanjohn]

If you turn all the downstairs rads off what happens to the upstairs rads?

 

[cap_]

If you turn all the downstairs rads off what happens to the upstairs rads?

This has no effect. The ones upstairs still suck air in.

 

[scatmanjohn]

If you turn all the downstairs rads off what happens to the upstairs rads?

This has no effect. The ones upstairs still suck air in.

Thats not what i meant.

I mean if you turn off the downstairs rads and then run the heating, do the upstairs rads heat up.

If they still don't heat up try turning them all off apart from one.

 

[cap_]

If you turn all the downstairs rads off what happens to the upstairs rads?

This has no effect. The ones upstairs still suck air in.

Thats not what i meant.

I mean if you turn off the downstairs rads and then run the heating, do the upstairs rads heat up.

If they still don't heat up try turning them all off apart from one.

I will go and try to do that now.

 

[pannierstan]

Oh dear, there is nothing wrong with your system!!!

Merely that when the pump is running then the system is under negative pressure possibly because the pump is on the return there fore sucking on the system rather than blowing if it were on the flow.

Who ever told you to bleed the rads with the system running anyway!

Turn off the system then bleed all the rads

If the upstairs rads are still cold it could be an airlock due to the fact you have pulled in so much air by doing what you were doing.

 

[cap_]

Oh dear, there is nothing wrong with your system!!!

Merely that when the pump is running then the system is under negative pressure possibly because the pump is on the return there fore sucking on the system rather than blowing if it were on the flow.

Who ever told you to bleed the rads with the system running anyway!

Turn off the system then bleed all the rads

If the upstairs rads are still cold it could be an airlock due to the fact you have pulled in so much air by doing what you were doing.

Thank you! That's true - the pump is on the return. Sorry for being so silly, but my DIY book says to bleed the rads twice - once "before" and once "during" (or "after" - it's not clear). They probably meant "after" but I understood it wrong. Thank you very much for your post(s) - I will try to bleed the system when it's off and then run it again and see whether it solves the problem.

 

[corgiman]

blocked cold feed meebbe

 

[cap_]

I don't know if it was always working and it was just me panicing or it's the system cleansing agent I put in last time I refilled the system but it works now! The radiators upstairs (and downstairs) got hot so it's *almost* sorted. It's "almost" because I discovered that I put the thermostatic radiator valve in the bathroom wrong way up - the little arrow sign goes against the water flow (judging by the order in which the pipes heat up).

Anyway, thank you guys for your help.

 

[Nige F]

That`s an old Thermo valve then , they`ve been Bi for years.directional that is Irespective ..as said you shouldn`t bleed rads with pump running but a properly designed and installed system shouldn`t have that much negative pull in any rads .........you`ve probably got one of the thousands that aren`t right but still work ....trouble is me and the other guys on the forum can only install a certain number in our lifetimes........but I`ll bet we`ve never done a wrong`un