drain cock will not drain sealed system

[fonyod24]

Hi

I would appreciate some advise on this please.

I had a new central heating with Vaillant EcoTec Plus Combi installed 3 months ago. One of the plastic push fit elbows started leaking the other week. I tried to drain the system to replace the faulty fitting according to your very helpful instructions on this site: Turning off valves to boiler and all but one radiator valve, attaching hose to drain cock at lowest point in the system.
But when I open the drain cock no water comes out at all although the system is pressurised at 1.5 bar.
Thanks.

 

[powell30]

Has the rubber seal stuck in the drain cock? if so, use a small screw driver to prise free. If not then you have created a vacuum and you will need to allow air in to the system from another point......open a rad bleed nipple??

 

[fonyod24]

Thanks Powell30 for the quick and very useful tip.
It was the stuck rubber seal.