What does a temperature and pressure valve do?
Of all the parts on a water heater, the temperature and pressure relief valve – the T&P valve – is the one that matters most for safety. A sealed tank full of water being heated can build enormous pressure, and without a relief path that pressure has nowhere safe to go.
The T&P valve is the fail-safe. If temperature or pressure climbs too high, it opens and vents water, protecting the tank and your home. It’s a small, inexpensive part doing a very serious job. HeatLand, licensed and serving Metro Vancouver and the Fraser Valley since 1999, checks the T&P valve and its discharge line on every tank inspection because a stuck valve is a genuine hazard.
How the T&P valve protects your home
Water expands as it heats, and in a sealed tank that expansion creates pressure. Normally that stays within safe limits, but a failed thermostat or control could let the water keep heating unchecked. As temperature climbs past boiling under pressure, the tank could, in a worst case, rupture violently. The T&P valve exists to make sure that never happens.
The valve has two triggers built in. A temperature probe extends into the tank and opens the valve if water exceeds roughly 210 F (99 C). A pressure element opens the valve if internal pressure exceeds about 150 psi. Either condition causes the valve to release hot water through its discharge pipe, dropping temperature and pressure back to safe levels. It’s an automatic, mechanical safeguard – no electronics required – which is exactly why it’s trusted as the last line of defence.
The discharge pipe and why it matters
The T&P valve is only half the safety system – the discharge pipe is the other half. Code requires a pipe running from the valve down to near the floor (or to an approved location), so that if the valve opens, scalding water is directed safely downward instead of spraying outward. The pipe must not be capped, threaded at the end, or reduced in size, and it should terminate where a discharge would be noticed.
A missing, blocked, or improperly routed discharge pipe turns a safety valve into a hazard. If the valve ever opens, water at over 90 C could injure someone or cause water damage. During any water heater service, HeatLand’s technicians confirm the discharge pipe is present, correctly sized, unobstructed, and properly directed – a detail that’s easy to overlook but important for a safe installation.
Testing, leaks, and safety warnings
T&P valves can be tested by briefly lifting the test lever, which should release a burst of water through the discharge pipe and then reseat cleanly. If the valve won’t release, doesn’t reseat, or drips continuously afterward, it needs attention or replacement. A valve that leaks steadily often signals either a failing valve or excessive tank pressure – sometimes pointing to a needed expansion tank.
Important safety notes: if the T&P valve is actively discharging hot water or steam, or you hear the tank rumbling or the water is scalding hot, treat it as an emergency – the tank may be overheating. Shut off the heat source if you safely can and call a licensed technician immediately. Never plug, cap, or block a T&P valve to stop a drip; that removes your protection. HeatLand offers emergency service for exactly these situations.
Quick Checklist
- Confirm the T&P valve has a proper discharge pipe to the floor
- Ensure the discharge pipe is uncapped and not reduced in size
- Test the valve periodically by briefly lifting the lever
- Watch for constant dripping (valve fault or high pressure)
- Never cap or block the valve to stop a leak
- Call for emergency service if the valve vents hot water or steam
| Condition | What it means | Action |
|---|---|---|
| Valve opens occasionally | Thermal expansion | Consider an expansion tank |
| Constant slow drip | Failing valve or high pressure | Have valve/pressure checked |
| Valve won’t reseat | Debris or worn valve | Replace the valve |
| Venting hot water/steam | Tank overheating | Emergency – shut off, call a pro |
Frequently Asked Questions
Is it safe if my T&P valve is leaking?
A slow, occasional drip may just be thermal expansion, but a constant leak means either a failing valve or excessive tank pressure that should be diagnosed. Never cap or plug the valve to stop the drip – that removes a critical safety function. Have a licensed technician inspect it promptly.
Can I replace a T&P valve myself?
Replacing a T&P valve involves shutting off power or gas, draining the tank, and correctly fitting the new valve and discharge pipe. On gas units and older tanks especially, it’s safer to have a licensed HeatLand technician do it, since the valve is a life-safety device that must be installed exactly right.
How often should the T&P valve be tested?
Many manufacturers suggest testing the valve about once a year by briefly lifting the test lever and confirming it releases water and reseats. If it fails to release, doesn’t reseat, or drips continuously, it should be replaced. Include this in your annual water heater maintenance.
What is the discharge pipe for?
The discharge pipe directs any water released by the T&P valve safely toward the floor, so scalding water can’t spray outward and injure someone. Code requires it to be full-sized, uncapped, and properly routed. A missing or blocked discharge pipe defeats the valve’s safety purpose.
Reviewed by the HeatLand Heating & Cooling team — licensed HVAC contractor serving Metro Vancouver & the Fraser Valley since 1999. This article is for general guidance only. If you smell gas, suspect a carbon monoxide leak, or are unsure about any electrical or gas component, stop troubleshooting immediately and call a licensed technician or 911.