I had to drain the system, but now having re-filled it, I am not sure what to do with the red expansion vessel thing I have.  It has a bleed cap on the top of it and is plumbed into the return side of the CH pipework.

 

I am assuming I should just bleed thing? the same way as you bleed a rad, or am I meant to leave it full of air?!  Pressurised systems are new to me - have always had a header tank (and an easy way of life!)

 

Thanks.

Speaking from experience on a Worcester Bosch boiler, the expansion vessel should be pressurised to approximately 1bar and can be done with a bicycle hand pump and suitable tyre pressure gauge.

Don't take this as a recommendation, I'm only recalling vaguely what I had to do to a friend's system after consulting a WB engineer and checking boiler manual.

I suggest you look elsewhere for a more definitive answer.

as micmac says, it is there to add air pressure to a rubber bag. When you fill the water side and radiators, the bag allows the water in the system to expand and compress the air, when it cools the the water shrinks and the air keeps the pressure in your system.

 

 So, if you did not touch it, then just fill and bleed your radiators as normal, if you have let the air out of it, pump it back up and you will be ok.

 

 I have a WB boiler with the same thing.

Check this video out for a general guide, but consult your boiler manual for recommended settings.

 

http://youtu.be/KeO2gswQQe4