2009 IRC P2803.6.1 Requirements for discharge pipe.
The discharge piping serving a pressure-relief valve, temperature relief valve or combination valve shall: 1. Not be directly connected to the drainage system.
2. Discharge through an air gap located in the same room
as the water heater. >>> AN OFTEN OVERLOOKED REQUIREMENT]
3. Not be smaller than the diameter of the outlet of the
valve served and shall discharge full size to the air gap.
4. Serve a single relief device and shall not connect to piping
serving any other relief device or equipment.
5. Discharge to the floor, to the pan serving the water
heater or storage tank, to a waste receptor or to the outdoors.
6. Discharge in a manner that does not cause personal
injury or structural damage.
7. Discharge to a termination point that is readily observable
by the building occupants.
8. Not be trapped.
9. Be installed to flow by gravity.
10. Not terminate more than 6 inches (152 mm) above the
floor or waste receptor.
11. Not have a threaded connection at the end of the piping.
12. Not have valves or tee fittings.
13. Be constructed of those materials listed in Section
P2904.5 or materials tested, rated and approved for
such use in accordance with ASME A112.4.1.
Commentary: The discharge pipe from a water heater temperature
and pressure relief valve (T&P valve) is an extension
of the potable water distribution system. Because Section
P2902.1 prohibits cross connections between the
potable water supply and any source of contamination
(such as a drainage system), a T&P valve discharge
pipe that discharges to a drain system must connect
indirectly to that drainage system.
Regardless of where the T&P valve discharge pipe
terminates, an air gap is required to protect the potable
water supply system. Section P2902.3.1 requires a
minimum air gap dimension of twice the diameter of
the effective opening of the discharge pipe. There are
three reasons for the air gap to be in the same room as
the water heater: (1) it prevents a direct connection to
concealed discharge piping that might be bent, flattened,
plugged, reversed sloped or inadvertently
capped off; (2) it provides a location for observing discharge
when testing the relief valve; and (3) it provides
a readily accessible location to observe valve leakage
indicating a defective T&P valve, a water distribution
system overpressure problem, or a water heater operation
The size of the discharge pipe must be no less than
the size of the T&P valve outlet to ensure that the valve
can discharge at its full capacity. The pipe size must
not be reduced as this would create a restriction that
might prevent full-capacity discharge in an emergency
The discharge pipe cannot be combined with any
other discharge pipe or connect with any other piping
before terminating at the required air gap. Connection
of other piping could introduce flow that would interfere
with the relief valve discharge flow. Also, full-capacity
discharge could damage other connected equipment
or cause the discharge to exit at other points where
persons could be injured by the escaping hot water.
Water discharged from the T&P valve must be directed
to one of four locations: (1) the floor below the
water heater; (2) the water heater or storage tank pan,
if present; (3) a waste receptor, such as a floor drain;
or (4) the outdoors. The choice of discharge location
must consider the potential for personal injury and
structural damage that water discharge might cause.
For example, a floor discharge might be suitable in a
concrete-floored and curbed garage, but where the
garage walls are of wood and rest directly on the floor,
this discharge point would be unsuitable. Another suitable
floor discharge example might be the tiled and
sloped floor of a laundry/utility room that has a floor
drain. Discharge to laundry trays/tubs and sinks would
not be a suitable location as it violates the intent of
Item 6 of this section, which is to protect the person using
the fixture from hot water and steam that could
come from the discharge pipe.
As discharge from a T&P valve is an indicator of a
problem, the discharge point should be readily observable
by the occupants so they can take action to correct
T&P relief valve pipes must not have traps and must
be so installed to cause water to completely flow out of
the pipe by gravity. If a trap was in the line or the line
was not sloped to completely drain, hot water drying
out in the trapped location could cause deposits to
build up and eventually block the flow. Where the pipe
is exposed to freezing temperatures, an ice block
could form in the pipe and block flow.
Where the T&P discharge pipe discharges to a floor
or waste receptor, such as a floor drain, the opening of
the pipe must not be any higher than 6 inches (152
mm) above the floor or receptor. This precaution is
necessary to keep any hot water discharge from
splashing and possibly causing injury to someone
Threads on the end of a T&P discharge pipe are an
invitation for someone to install a threaded cap to stop
a nuisance relief valve drip. What would be perceived
as a fix would actually be creating a very serious hazard.
Because the installation of valves or tees in the relief
valve would only invite someone to close the valve
(thus blocking the flow) or connect another drain line to
the discharge pipe (creating another hazard), their installation
Relief valve discharge piping must be one of the piping
materials listed in Table P2905.5. Although the
pipe materials in this table have a pressure rating of at
least 100 psi (690 kPa) at 180°F (82°C), they have the
capability to survive the limited time exposures (e.g.,
15 minutes) at a temperature of 210°F (99°C) to carry
a full output relief valve discharge.
A frequently asked question is how this section
should be applied to a relief valve discharge pipe termination
that, originally, terminated just above the floor
without a drain or waste receptor to capture the flow,
regardless of the potential for damage. The water
heater might be in a basement where the building
drain is above the elevation of the water heater T&P
valve or the water heater might be in the middle of a
slab-on-grade building, without a nearby floor drain
and not adjacent to an outside wall. Where water
heater replacement installers are confronted with obstacles
that appear to prevent strict compliance with
this section, they should consult with the code official
before performing the replacement work.