Expansion Vessel Calculation ????????

[pauliepie]

I know if you install a sealed system boiler that there is a built in expansion vessel inside the boiler, but...
What if you are replacing an old 120,000 floor standing boiler conventional boiler for a 120,00 btu condensing sealed system type on an existing pipework/radiator system.

Surely the vessel in the boiler is not big enough to accommodate the expansion of approx 18 radiator system.
How do you calculate this......
Any Ideas...
cheers

 

[kevplumb]

18 radiator system.

it's not how many
it's how big
you need to calculate the amount of water in the system

 

[oilman]

The expansion vessel should be at least 10% of the system volume, but having it bigger is better. The ideal is so big that you eventually discard the vessel and use the earth's atmosphere, then you can call it a vented system. You need a header tank then so you can replace water that evaporates.