Posted by Harold Kestenholz - Hydronic Network on July 27, 2001 at 08:28:37:
In Reply to: Boiler water pressure posted by Nickj on July 27, 2001 at 06:41:18:
The boiler pressure should be 4 psig higher at the highest point of the system than the pressure would be if the system was filled to that point. So if a pressure reading was taken at the highest point it would be 4 psig. If the top of the system is 10 feet higher than the boiler gauge, the gauge would read about 5 psig (10 divided by 2) plus 4 equals 9 psig minimum fill pressure.
As most systems are filled through a pressure reducing valve set from the factory at 12 psig, that pressure is satisfactory for systems with the highest point no higher than 20 feet above the gauge. You can leave the fill pressure setting at the pressure reducing valve setting if that is the case.
As the water heats up it expands about 4% at the high limit temperature, so 25 gallons of water becomes 26 gallons. As the water is in a closed space with an expansion tank, the pressure will rise as the water heats (it should not go over 25 psig on a 30 psig relief valve boiler) and the water presure will drop as the water cools (not lower than the cold fill pressure.)