Pressure is measured in "Bar". One Bar is about equal to the atmospheric pressure on Earth at sea level. In "old money" it's equal to 14.5 pounds per square inch (PSI).
3 Bar = 43.5 PSI (The normal size used for a relief valve in a central heating system)
6 Bar = 87 PSI (Common size for unvented water heaters)
7 Bar = 101 PSI
8 Bar =116 PSI
The outlet from a pressure relief valve should be positioned in a way so as to not scald people or animals with escaping hot water or steam.