Short definition
A pressure relief valve is a safety valve that opens at a preset over-pressure threshold to vent excess to atmosphere through a discharge tube. On a residential water heater the device is a combined temperature and pressure relief (TPR or T&P) valve that opens at 150 psi or 210°F, whichever comes first. It’s a fail-safe — not a regulating device — and a dripping TPR is always a symptom of an upstream problem, never the valve “going bad” by itself.
What it is
The T&P valve is a brass body with a spring-loaded disc and a temperature probe extending into the upper portion of the water heater tank. The set point is fixed at manufacture — typically 150 psi pressure or 210°F temperature for residential heaters — and cannot be field-adjusted. Above either threshold, the valve opens and dumps water through a discharge tube to atmosphere. When pressure or temperature returns below the threshold, the valve closes again.
Code rules for the discharge tube:
- Terminate within 6 inches of the floor (or to a code-approved drain).
- Not threaded at the end so a hose can’t be attached to defeat the safety function.
- Pointed away from foot traffic to avoid scalding bystanders during a discharge event.
- No upward bends or traps that would hold water in the line.
A pure pressure-only relief valve (without the temperature probe) is used on hydronic boilers, expansion tanks, and some specialty applications. Residential boilers in WA typically have a 30 psi pressure relief valve.
Why it matters to a homeowner
The TPR is the device that prevents your water heater from becoming a pressure vessel during a thermostat or pressure failure. When it’s working, you don’t notice it. When it’s discharging — drips at the discharge tube, water on the floor, or a full-bore vent during a high-pressure event — you have a real problem to diagnose.
A dripping TPR has three possible causes, in order of likelihood:
- High supply pressure. Static pressure above 80 psi during the day or above any threshold at night pushes through the heater, expands during heating, and weeps the TPR. Fix: install a PRV.
- Failed expansion tank in a closed system (a system already with a PRV). When the expansion tank’s bladder has ruptured or the pre-charge has leaked out, thermal expansion has nowhere to go and lifts the TPR each heating cycle. Fix: replace the expansion tank.
- Defective TPR. Less common; replace last after ruling out the other two.
A TPR that’s discharging at full bore (not weeping) signals a thermostat failure, sediment buildup defeating the temperature probe, or some other significant issue — call a plumber and shut off the heater’s gas or electrical supply.
Common variants and what it isn’t
- Pressure relief valve vs. pressure reducing valve. Pressure relief is a safety that opens at over-pressure. Pressure reducing is a regulator that sets steady downstream pressure. Same abbreviation “PRV” is sometimes used for both — pay attention to context.
- T&P valve vs. pressure-only relief. T&P responds to either temperature or pressure threshold. Pressure-only is used on boilers and expansion tanks where temperature isn’t the relevant trigger.
- TPR vs. PRV-relief. Same valve under different names depending on the source.
Common failure modes
- Weeping TPR. Three causes — high supply pressure, failed expansion tank, defective valve. Diagnose in that order.
- Stuck-closed TPR. Failed safety. Dangerous. Replace immediately.
- Mineral buildup at the seat after a previous discharge event — won’t fully reseat. Replace.
- Discharge tube terminated above the 6-inch maximum or threaded — code violation. Re-pipe.
Washington note
WA adopts the Uniform Plumbing Code under WAC 51-56, which requires a TPR valve on every storage water heater plus discharge piping per UPC Chapter 6. The TPR is checked at every plumbing inspection — improperly piped discharge is one of the most common minor violations cited.
Annual maintenance: lift the test lever briefly to verify the valve operates, then close. Some sources advise against this on older heaters because aging valves sometimes fail to reseat — a partial flush prevents the failure mode. If your heater is over 10 years old and you’ve never tested the TPR, plan to replace the heater rather than risk testing the relief.