How to Replace a Temperature and Pressure Relief Valve

Why this matters

The T&P valve is the last thing between an overheated tank and a steam explosion, and it is also the part most often replaced for the wrong reason. A valve that opens because house pressure reached 150 psi, or because a thermostat stuck and the tank passed 210 F, is working. Swap it without finding why it opened and the new one weeps by the weekend, or worse, someone notices the drip and caps the discharge. This card covers the swap itself, and the one check that has to come before it.

Establish why it opened before you touch it

A relief valve is a safety device, so the rule from the rest of this shelf applies: prove the device is the fault before treating it as one. Three readings, ten minutes:

• Static pressure at a hose bib with no fixture running. Over 80 psi is a PRV problem first.
• Peak pressure through a recovery. Leave a gauge with a drag needle on the hose bib, draw hot water, and let the heater recover. A needle that climbs toward 150 psi means thermal expansion in a closed system, and the fix is an expansion tank, per How to Install a Thermal Expansion Tank and Set Its Precharge.
• Hot water temperature at the nearest tap after a full recovery. Water far above the setting points at a stuck thermostat or gas control, which can let the tank reach the valve's temperature setting; that is a dangerous fault, and the heater stays off until the control is replaced.

If pressure stays well under 150 psi and temperature is normal, and the valve still drips, its seat is damaged, usually by mineral left after earlier openings. That valve gets replaced.

Hazards before the first step

Scald. The tank holds water at its setpoint, and the water that comes out of the drain or the valve port during this job can be near that temperature. Cool the tank first: with the heater off and the cold supply still on, run the nearest hot tap until the water runs cool, which pushes cold water into the tank. Wear gloves and eye protection when opening the drain and when the old valve comes out.

Electric heaters. Switch off the heater's breaker, lock and tag it, and verify absence of voltage at the heater's junction box with a tester proven on a known live source before and after, per NFPA 70E (current edition), with work practices under 29 CFR 1910.333. An element that comes on while the top of the tank is drained burns out in seconds, and exposed element terminals under the access covers are live until proven otherwise.

Gas heaters. Turn the gas control to pilot or off. If you smell gas at any point, everyone leaves the house, nobody touches a switch or a phone inside, and the gas utility is called from outside. Gas work beyond the control knob is done only where your license covers it.

The swap, step by step

1. Shut the cold supply and relieve pressure. Close the heater's cold shutoff and open a hot tap upstairs. Why: the valve is threaded into a pressurized vessel; with pressure on it, the first turn sprays water. Skip it and the valve threads loosen against house pressure, and the last turn is a jet at your face.

2. Drain below the valve opening. Connect a hose rated for hot water to the drain valve, route it to a floor drain or outside, and open it. With the hot tap open to admit air, draining a few gallons drops the level below a top-mounted valve port; a side-mounted valve near the top of the tank needs more. Lift the old valve's lever: when it hisses air and no water, the level is below it. Why the lever check: it proves the level without guessing gallons, and on a valve that has not been operated for years, if the lever will not lift or the valve then will not reseat, you have confirmed the diagnosis on the spot.

3. Remove the discharge pipe, then the valve. Cut or unthread the discharge at the valve outlet. Put a pipe wrench on the valve body, not on the outlet or the lever housing, and turn it out while bracing the heater against the torque with your other hand or a second wrench on the tank fitting if there is one. Why brace: a 50 gallon tank sits on its own weight, and a wrench pulled hard against a stuck valve can rack the tank on its base and stress the gas connector or the water lines.

4. Match the new valve to the heater. Read the heater's rating plate, then the new valve's tag, and compare three things:

• Relief capacity. The valve's Btu/h rating (gas) or kW rating (electric) must equal or exceed the heater's input. A residential electric heater's elements usually do not run together, so its input is one element's wattage unless the plate says otherwise.
• Pressure setting. Not above the heater's rated working pressure, which on residential tanks is commonly 150 psi; the standard valve is set at 150 psi.
• Temperature setting. 210 F.

Then the probe. The valve's thermal element has to sit in the top 6 in of the tank, which is where the hottest water is. A valve in the top port takes the standard probe; a side port or a valve installed on a fitting needs an extension-probe valve listed for it. Never cut a probe to fit.

5. Thread the new valve in. Sealant on the male threads only, potable-listed, kept off the first thread and off the probe. Turn it in hand-tight, then with the wrench on the body until it is snug and the outlet points down or toward the discharge path. Why outlet down: a discharge that starts upward traps water in the first bend, and that standing water can block or scale the outlet.

6. Reconnect a full-size discharge. The discharge is the same size as the valve outlet, 3/4 in on a residential valve, in a material rated for the temperature, running by gravity with no valve, no trap and no threaded end at the termination. It terminates where the adopted code allows, which under IPC 2021 Section 504.6, as adopted, includes a point not more than 6 in above the floor or a waste receptor. The UPC 2021 and local amendments add their own terminations. The rules other trades break on this pipe, and the reasons for each, are owned by The T&P Discharge Pipe and the Rules Other Trades Break.

7. Refill before heat. Close the drain, open the cold supply, and leave the hot tap open until water runs without air. Only then does heat go back on. On an electric heater this is the step that protects the upper element: it sits near the top, and it is the last part of the tank to be under water.

8. Restore heat and watch the first cycle. On gas, return the control to its setting and watch the burner light. On electric, remove your lock, close the breaker, and confirm the element draws current with a clamp meter at the supply. Keep clear of the access covers and the valve outlet while it heats.

A worked replacement

The call. A 40 gallon electric heater, two 4,500 W elements on a non-simultaneous control, with a T&P dripping into a bucket. A thermal expansion tank went on two years ago.

Why it opened. Static at the hose bib: 58 psi. Gauge left on with a drag needle through a recovery after a 15 gallon draw: peak 66 psi. Hot water at the kitchen tap after recovery: 122 F, against a setting of 120 F. Neither 150 psi nor 210 F was approached, so the valve opened on neither of its settings. Its seat is leaking, and replacement is the right repair.

The comparison.

HEATER RATING PLATE         NEW VALVE TAG
Input   4500 W, one         Relief   kW on tag
        element at a time   rating   above 4.5   PASS
Working pressure 150 psi    Set      150 psi     PASS
                            Temp     210 F       PASS
Port: top of tank           Probe    standard    PASS
Old discharge: 1/2 in PEX   New      3/4 in      REDO

4,500 W is 4.5 kW, which is about 15,400 Btu/h at 3,412 Btu/h per kW, so on this heater the capacity check is easy to pass. The line that failed was not the valve at all. The old discharge had been reduced to 1/2 in through an adapter at the outlet, which cuts the flow area to well under half at the point the valve most needs it. It came out and went back as 3/4 in copper, ending 4 in above the floor beside the floor drain.

Drain-down. Tank cooled by running the bathroom hot tap for several minutes with the breaker off and cold still on. Cold shut, hot tap open, about 3 gallons out of the drain, lever lifted: air only. Valve out, new valve in, outlet pointed down, discharge rebuilt.

Finish. Refilled until the tap ran solid, breaker on, element current confirmed at the supply. After recovery, the outlet was dry for 30 minutes at 58 psi static. Then the manual lift: lever raised for about five seconds with the discharge connected and nobody near its end, a full-bore flow at the termination, and on release the valve reseated and stopped within seconds.

Finish tests, and what a fail means

• No weep at operating pressure for at least half an hour after recovery. A new valve that weeps with pressure and temperature normal has debris on its seat, often scale dislodged during the swap. Operate the lever once or twice to flush it; if it still weeps, replace it, because a valve that cannot seat is not one to leave.
• Full flow on a manual lift. A trickle on lift means a restriction in the discharge, an undersized fitting, or a partly closed termination. Find it before you leave.
• Before the lift, confirm the discharge is connected and its end is clear of people and of anything water at near-setpoint temperature can damage. The lift sends hot water out at full flow, and a valve lifted with no discharge attached discharges at the tech.

What changes the job

The old valve will not come out. On an old glass-lined tank, the fitting can tear out of the tank before the valve breaks free. If the valve resists hard with the tank braced, stop and talk to the customer about the heater's age before the tank is damaged in the attempt.

The valve is on a fitting, not in the tank. Some installations put the T&P on a tee in the hot outlet. That is only acceptable with an extension-probe valve whose element still reaches the top 6 in of the tank, per the heater maker's instructions.

The heater has no T&P port in use, or the valve is plugged or capped. That heater is not safe to operate. It stays off until a listed valve and a code discharge are installed.

References

• IPC 2021 Section 504.6, as adopted by your jurisdiction, and the UPC 2021 relief valve discharge provisions, as adopted
• NFPA 70E (current edition) and 29 CFR 1910.333 for the electric heater isolation
• Heater and valve makers' installation instructions, which are part of the listing
• See related: The T&P Discharge Pipe and the Rules Other Trades Break
• See related: How to Install a Thermal Expansion Tank and Set Its Precharge