Understanding Boiler Relief Valves and Appropriate Settings
The primary function of a boiler relief valve is to prevent a boiler from exploding -- a pretty important job! Relief valves moderate thermal expansion, which occurs when matter changes size in response to a change in temperature. In the case of a boiler, if the internal pressure exceeds what the boiler can manage, the relief valve opens and excess water or steam is released from the boiler into a relief pipe.
Pressure relief valves work by employing a spring-loaded diaphragm to hold the valve closed. In a dangerous pressure situation, the relief valve springs open and allows the water to pass into the discharge line. Bell and Gossett, manufacturers of A.S.M.E safety relief valves, advises to "treat relief valves as though they were as fragile as raw eggs. Never drop them on the floor or bang them with a tool." Additionally, they say to never re-use an old relief valve in a new boiler.
Relief Valve Settings
Most household boilers have a pressure capacity of 30 psi. Manufactures determine the amount of pressure a boiler system can bare based on guidelines established by the American Society of Mechanical Engineers (ASME). The pressure relief valve should never have a pressure-release rating that exceeds the maximum working pressure of the boiler. More simply, the system for measuring the pressure of the boiler should align and not exceed the maximum allowable pressure of the boiler. Manufacturers provide a second rating for the full BTUH load of the boiler. This measures the amount that the boiler can unload in extreme circumstances.
If the relief valve on the boiler measures up to 30 psi, a setting of 19-20 psi falls within normal functioning range. Consult the manufacturer to ensure that the boiler, in fact, may manage pressure capacities up to 30 psi. If concerns persist, consider contracting a professional to test the blow-off settings of your system. It is not advisable to adjust the instruments on your boiler without professional guidance.