Expansion vessel in gravity fed central heating system

[zagadka]

Hi guys,

I have an unusual heating system I have inherited and I am looking for advice. Below (and attached) is a diagram of the system. It is gravity fed but at some point an expansion vessel and non-return valve have been added converting it into (what I would call) a semi-sealed system. I have seen pipework that suggests there used to be a back boiler attached to the system but it is now disconnected.

The system seems to be running reasonably well. The boiler locks out occasionally and frequently there is air in the radiator system. When the boiler dies I plan to have a new boiler fitted in the kitchen as it is currently situated where we want to build an extension (in the medium/long term). In the short term I just want to keep it running.

Trying to cure the air in the system, I checked the expansion vessel pressure and discovered that the valve was leaking and there was no air in it. When inflating the expansion vessel water and air were forced into the header tank. I should say I don't think the air is caused by the air valves near the pump as I have tested with them closed.

My options as I see them are to replace the expansion vessel (or valve - which has proved difficult to source) and check valve or remove the expansion vessel and check valve and fit a return like a standard gravity fed system. Does anyone have any opinions of the better route to go? Also, if I replace the expansion vessel would I set it to 5psi or will it require a different pressure?

Thanks for the help... Mark

 

[Agile]

It is a sealed system and I would leave it exactly as it is.

The air pressure in the EXV needs to be set to about 90% of the pressure at that point.

But only set with no water pressure at all at the EXV.

Tony

 

[zagadka]

Thanks for the reply Tony, that is what I will do.

Re. the EXV pressure; the expansion vessel is right next to the header tank in the attic so, correct me if I am wrong but wouldn't there be very little water pressure at the expansion vessel when cold. Maybe a few inches of water. Or should I be thinking about the pressure at the lowest part of the system which would be about 3m of water head = 4.5PSI?

Thanks... Mark

 

[Agile]

Needs very careful thought!

I stated it like that because so many read here for advice which often relates to different situations!

As you rightly say, the pressure at that point is going to be virtually atmospheric. So the pressure needs to be very low ( but not lower than atmospheric ).

So I would suggest about 0.2 bar or 2-3 psi.

Tony

 

[zagadka]

Great stuff, thanks again Tony... Mark