when?

hi-spec is correct but there is bit more to it

if you only had one or two radiators then there is not a lot of water to expand so the smaller the volume of radiator water, the smaller will be the increase in pressure.
If you had say ten radiators then the inbuilt pressure vessel may not have suffient internal volume to accomodate this larger amount of water expansion. Sometimes you may need a second vessel. Read the instructions for the appliance it will be tell you about maximum system volume. Average about 60 -80 litres. (from memory)
average increase is from 1 to 1.5 bar --but it does vary

dave broadbent
June 2007

only to about 1.2 bar

hi-spec plumbing
June 2007