Frequently Asked Questions
Q: How to size/select a hot water boiler pressure relief valve?
A: There’s a multitude of factors which together determine the right relief valve type and size for a hot water boiler, but most commonly they are:
1. Boiler BTU size (see nameplate or specs) - should be lower than the relief valve rating (also in BTU).
2. Boiler max. operating pressure or MAWP (Maximum Allowable Working Pressure - found on nameplate or in specs) - must be at least the same or greater than the relief valve, so that the valve would open at or below the MAWP.
3. System operating/working pressure - determining system working pressure is especially tricky, since it is uniform neither in time, nor in place. Most hydronic heating systems operate in the 12-25 psi range and thus a 30 psi relief valve is adequate for most applications.
Q: How to determine system operating pressure?
A: In general, one of the main factors which determine the system operating pressure (considering a proper size expansion tank has been selected) is the boiler "cold" pressure, or the setting on the pressure regulating valve which supplies makeup cold water into the heating system. It, in turn, is primarily dependent on the height of the building. To overcome the gravitational pull, cold water pressure must be sufficient to deliver water to the top-most floor. For very 2.3ft of water column, 1 psi pressure is needed, and at an average floor height of 12ft, we can determine the "cold" pressure by the following formula: Pcold
x 12ft / 2.3
: in a 5-story building with 12ft floor height, the minimal pressure to deliver the water should be 5 x 12 / 2.3 = 26 psi. In given conditions, a 30 psi relief valve may not work considering the increase pressure from expanding hot water and 5 psi added safety factor. At 4 floors, the equation would produce ~21 psi, which is well within acceptable range of a 30 psi relief valve. For a 2-story home, the result is ~11 psi, which is basically the same as 12 psi cold/fill pressure found in most 2-story residential homes in the USA.
Needless to say, the relief valve setting should always be below the boilers maximum rated pressure.
Q: Why the different price for valves with seemingly the same specifications?
A: The difference is in the construction materials and especially the relief washer, seat and spring. The higher quality valves would incorporate better materials, ensuring lasting trouble-free performance, while the less expensive models use basic materials and, while meeting the industry standards, will have a shorter life span and may require maintenance or replacement (read, expenses) sooner than the better quality ones.