Shared Knowledge

"Hydronic Heating System - Normal Water Pressure"

28 October 2010

Summary: there are 2 normal water pressures in a hydronic heating system. Cold water pressure and hot water pressure. A normal range for water pressure for when the system is completely cold is between 12 and 20 pounds per square inch (PSI). Hot, or as hot as the water ever gets in your system, should never be more than 3 or 4 pounds above the cold water pressure for a maximum of 24 PSI.

Normal hydronic heating system water pressure (cold system):

The water inlet pressure valve has some default, manufacturer setting. In my case 12 pounds per square inch (PSI). So this is the lower, cold water, normal system water pressure.

The pressure relief valve has some default manufacturer setting where the valve will release pressure. In my case this valve is set at 35PSI. This valve will open when the pressure in the system reaches 35PSI no matter if the water is hot or cold.

So any cold water pressure between 12 and 34 is valid.

Now given that the water pressure in a well maintained system is going to increase 2 to 4 pounds when system water is heated to its maximum temperature, the upper end of cold water pressure is more like 30 PSI.

The default water inlet valve setting of 12PSI is probably fine for a single floor system where the heating system is on the same floor as radiators.

System water pressure should be increased about 3 to 4 PSI for each floor above the furnace floor. In my case, I have 2 floors above the furnace room and thus I use a system water pressure of 12+6 or 18 PSI. Again, this is cold water pressure.

It is intuitive to think that if you have gurgling noises in one or more of your system water zones or loops that increasing water pressure will help this situation and it will or will initially as increased water pressure will compress air in the system into the water but over time, noise in pipes or gurgling will return. Increasing water pressure is not a substitute for system air bleeding.

Normal hydronic heating system water pressure (hot system):

As stated above, in a well maintained hyrdonic heating system, the difference in water pressure between a system that is absolutely cold and one that has water in it as hot as the system control unit defines, should not be more than 2 or 3 PSI.

The expansion tank plays a big part in regulating system pressure as water heats. As there is air in the top of the expansion tank, as water is heated in the hydronic heating system, the water in the expansion tank compresses the air above it in the tank.

Although no hard and fixed rule, hot water system pressure should not be above 20 PSI.