As with any hot water or heating appliance, a hot water boiler system requires a constant pressure flow to maintain the optimum temperature. Hot water boiler systems are equipped with a pressure gauge and an automatic pressure reducing valve that will add water to the system if the pressure drops. Despite the pressure reducing valve, owners should be familiar with the standard PSI of their hot water boiler system and should check the pressure reading regularly.
Standard Cold Operating Pressure
The standard cold water operating pressure of most U.S. and Canadian hot water boilers is 12 pounds per square inch (psi). If the boiler pressure drops below 12 psi, the valve will allow new water into the unit to bring up the pressure reading.
Standard Hot Operating Pressure
Hot water boiler systems also have a high-limit temperature setting that triggers an automatic shutdown if the water temperature rises above that set point. If the high-limit temperature setting is too high, generally over 200 degrees Fahrenheit, the system pressure will reach an excess condition as well and start leaking. The high-limit temperature setting should be around 190 degrees and the standard hot water operating pressure around 20 psi.
The pressure gauge on most hot water boiler systems is located on the front of the unit, sometimes near the top. This gauge shows the current internal pressure and temperature inside the boiler. Reading the pressure gauge, before and after any troubleshooting efforts, as well as any time the boiler is turned on or shut off, can help determine if there is a problem with the boiler.
Other than temperature issues, there are other reasons why the hot water boiler system pressure may be too high or too low. Some of these problems include a leak at the baseboard or radiator unit. In addition, if the boiler has corrosion or is leaking, pressure readings will change. The expansion tank can also become waterlogged, which can cause excessive pressure readings. If the pressure reading on your hot water boiler pressure gauge has changed, especially before or after operation or troubleshooting, contact the boiler manufacturer or an authorized service professional for assistance immediately.