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