Help with C/Heating

[jcampbell]

Hi..

Please confirm if i'm understanding this correctly. If I want to drain down a sealed system (c/heaating), I turn off boiler, then drain down from drain off cock which I run a hose from to a outside drain.... Once i'v finished altering pipe work and adding replacement rads, I then fill system back up. To fill up I use the filling loop.

Question: Boiler is in the downstairs kitchen and expansion vessel is on top of hot water cylinder upstairs in a cupboard. Kinda confused how I check system pressure?? If its a combi do I ignore the expansion vessel upstairs and go by whats on the boiler pressure gauge?

Can anyone help?

Thanku in advance any help would be appreciated..

 

[MANDATE]

The gauge on the boiler is for the water pressure which is what you follow for filling the system.
The expansion vessel has a membrane separating water on one side and air on the other side. Before filling the system with water, the air side of the expansion vessel needs to be checked with an air pressure gauge and adjusted if needed. For this you will need a cycle pump or car tyre pump.
The vessel will be all 'air' before you fill and probablely 90% air after filling. When the heating is on the volume of the water will increase as it gets hotter. The extra volume has to somewhere and that is in the expansion vessel. The 'air' gets compressed and its pressure increases so the pressures on both sides of the membrane are equal.
This explains why your boiler gauge increases from about 1bar to 2bar when you put heating on. Of course it will fall again as the heating cools

 

[jcampbell]

Cool!

Thanks for reply. To clarify, The process for draining down is correct though?

And when I refill sytem is when I need to worry about checking the expoansion vessel to insure its at nominal pressure? I understand that the system pressure will increase when its heating up. So it should all be done (expansion vessel pressure) when system is cold? Sorry last question (I hope): If its a combi boilerwill it have an external pressure vessel, if not does that explain why its there and the process discribed needs to be followed??

 

[MANDATE]

A combi will have the expansion vessel within the boiler casing. It's when they need replacing (being faulty or a larger vessel needed) it can be a problem. So the easier way is to leave it in situ and fit another one external to the boiler, I believe the best place is in the return pipe as near as possible to the boiler.
Also should you go over pressure when filling with water you can always lower the pressure by bleeding a radiator.
Also there should be a automatic air valve which allows air in the system to escape.

 

[jcampbell]

ok thanku again.

Its a baxi solo 2 pf. Not a combi i dont think but will check when see it. Would explain the vessel being outside the boiler and in the upstairs cupboard. Will double check system and post to let u know the score.

cheers

 

[seco services]

baxi solo 2's not a combi.