Dear Inspector,

This month's inspection tip is from Douglas Hansen, Managing Instructor of the new Kaplan Oakland, CA facility.

Inspectors often ask whether they should test the Temperature & Pressure Relief Valve (TPRV) on a water heater. In most of the country, inspectors usually do not operate the TPRV. Many inspectors have learned the hard way that a valve will sometimes keep dripping after it has been tested, and in a few instances valves they have been known to stick in a fully open position, resulting in an emergency call to a plumber when you wanted to be finished with the inspection. There are some inspectors who test valves anyway. After all, the Watts Regulator Company (the most popular manufacturer) recommends they be tested at least once a year to assure an unobstructed waterway. In Texas, the Standards of Practice require testing TPRVs unless there is a visible defect in its drain piping.

The reason a TPRV is required in a storage tank water heater is to act as a backup in case the primary thermostat fails. On either a gas or electric water heater, thermostat failure could cause the water in the tank to become superheated and lead to heat rupture of the tank. Superheated water flashes to steam with explosive force, and will expand to 1,600 times the original volume in the tank. The result is a water heater that becomes an unguided missile. A water heater that is part of an "open system" might balance its pressure back through the utility meter, but the high temperature and stored energy in the tank still causes an explosive condition.

The water temperature in a tank is hotter near the top, and TPRVs must be installed with their thermostat extending into the water in the upper six inches of the tank. Sometimes a TPRV is installed through a "T" fitting on the top of the tank, and might need a longer thermostat. TPRVs are available with either four inch or eight inch thermostats. In the most common system, the TPRV will open when the pressure exceeds 150 PSI or the water temperature exceeds 210 degrees F. The TPRV must have drain piping so there is no dangerous flash of scalding water and to drain the water to a location where it will not cause damage. Building codes require the TPRV drain to run continuously level or downhill, and to end outside the building or in another approved location. The end of the drain may not have threads, and must be within six inches of the floor or grade (6 - 24 inches if the UPC is the applicable plumbing code). The drain piping should be an approved material, such as any of the types of water distribution piping allowed inside the building.

Several things could cause the TPRV to periodically drip a small amount of water. It could be the result of fluctuating pressure, water hammer in the building, high incoming pressure, or thermal expansion in a "closed" system (one with a pressure regulator or check valve). Expansion tanks are supposed to solve the problem of thermal expansion. The most frequent cause of TPRV failure is "liming up" in the valve as a result of hard water. Deposits become lodged in the valve, and once the valve is tested, they can move and prevent it from reseating properly. The result is a dripping valve that was "OK" prior to the inspection. Because of this possibility that the valve will continue to drip, inspectors who do test the valves should always check the TPRV drain piping before operating the valve. Be sure that water leaking from the pipe will not cause damage or create a condition that would require immediate repair. If it does keep dripping, try opening the valve up all the way a few times to see if it will move the deposits through the valve and eventually reseat and shut off properly. If you decide not to test these valves (my choice) it is still good policy to recommend that your client operate the valve at least once a year, per manufacturer's instructions. You should also warn them that a valve that keeps dripping will need to be replaced. Even though replacement could be a nuisance, it is important to know that these safety devices are working properly. Mike Casey will be returning next month with the June 2004 Inspection Tip.

 

Douglas Hansen
Kaplan Professional Schools
Now You're Ready For Business!™