Pressure Relief Valve

Perhaps the most critical piece of information on a hot water boiler is its pressure rating. This number—30 psi for most residential boilers—designates the maximum allowable water pressure for the safe operation of a boiler. A boiler’s pressure rating must be higher than the static pressure required to fill a water-based (hydronic) heating system—about 12 psi for a typical two-story house—and high enough to handle minor pressure fluctuations.

Who sets boiler pressure ratings? The American Society for Mechanical Engineers (ASME), a group made up of volunteer professionals who set and review engineering codes and standards. The Boiler and Pressure Vessel Code (BPVC) is one of ASME’s most significant contributions. In the nineteenth century, boilers—especially on steam engines—would explode regularly. Don’t underestimate the explosive power of a home water heater.

Another step forward was the introduction of the relief valve. A modern relief valve uses a spring-loaded diaphragm to hold the valve closed. The spring pushes down against one side of the diaphragm with a pre-set amount of pressure, which should match the pressure rating of the boiler. (Taller buildings require longer piping and therefore boilers with higher pressure ratings.) When pressure rises above that point, the valve opens and water is dumped out of the boiler.

In addition to the pressure rating, manufacturers rate relief valves to release the boiler’s full British Thermal Units per hour (BTUH) load. This is important because when there’s an emergency, the boiler’s “exit door” must be large enough to safely relieve everything that wants to rush out. If that energy can’t get out, it can lead to a dangerous situation, even though the relief valve is wide open. Make sure your relief valve’s rating meets or exceeds the boiler’s US Department of Energy Heating Capacity rating (which translates into BTUH—a 110 heating capacity means 110,000 BTUH).

Xylem RCW’s ASME-rated safety relief valves have a diaphragm with five times the area of those in “pop-type” relief valves. This larger diaphragm offers greater operating force, helping to overcome any fouling that might occur on the valve seat. Xylem RCW’s valves also feature a unique fail-safe disc that allows it to work even if the diaphragm ruptures. Durable, they can be opened for periodic testing and still last for many years.

Below are some do’s and don’ts concerning the safe installation and operation of your ASME-rated safety relief valve. If you have questions about boiler safety, ask your Xylem RCW dealer, distributor, or installer.

  • Don’t use a relief valve that has a pressure rating higher than the maximum working pressure of the boiler.
  • Don’t plug a relief valve to stop a leak, even if it’s only for a few minutes.
  • Don’t pipe a valve’s discharge to the outdoors. If you do, and the valve discharges, water that stays in the line can freeze and plug the pipe. Even if the pipe is pitched downward, a partial vacuum can keep water in the pipe. (To observe this effect, stop up the end of a straw immersed in water with your finger and pull the straw out—water will remain in the straw!)
  • Do connect the relief valve directly into the boiler, and never put it up on a nipple. You want a safety valve to be close to the boiler so it can sense and react as quickly as possible.
  • Don’t reduce the size of the boiler tapping that feeds the relief valve.
  • Don’t add valves between the boiler and the relief valve because if these close, the relief valve cannot do its job.
  • Do treat relief valves as though they were as fragile as raw eggs. Never drop them on the floor or bang them with a tool.
  • Don’t recycle an old relief valve by installing it on a new boiler. The risk you take is just not worth the money you think you’ve saved.