A dripping T&P (temperature and pressure) relief valve is usually releasing pressure from thermal expansion in a home with a pressure-reducing or check valve and no working expansion tank, or from supply pressure that is too high. A worn valve seat is the next most common cause; overheating is rare but serious. Never cap the discharge pipe; find the cause and fix it.
- The T&P valve is the tank's last line of defence against dangerous pressure and temperature. Never cap, plug or remove it.
- Periodic drips during heating usually mean thermal expansion; constant dripping points to high supply pressure or a worn valve.
- Heated water expands by a few litres per tank; in a closed system that pushes pressure up until the valve opens.
- A gauge with a maximum-reading needle on a hose bib tells you which cause you have.
- Steam, very hot discharge or scalding taps mean the tank is overheating: turn off the heat and call now.
01 /What does a T&P valve do?
A temperature and pressure relief valve, or T&P valve, opens to release water if the tank's pressure or temperature climbs past a safe limit. It is the brass valve with a lever on the top or upper side of a storage tank, with a pipe running down the side. Natural Resources Canada's Water Heater Guide notes that new water heaters must have one, and that it lets water that is too hot or under too much pressure, from a faulty temperature control, escape from the tank.
Residential T&P valves are commonly rated to open at about 150 psi (1,034 kPa) or about 99 °C (210 °F), and their energy rating must match or exceed the heater's input. The ratings are stamped on the valve's tag. The temperature element matters as much as the pressure spring: water above its atmospheric boiling point flashes to steam if the tank ruptures, which is why a disabled relief valve is so dangerous. Can a water heater explode? explains the chain of failures involved and why it is rare.
02 /Why is my T&P valve dripping?
The timing of the drip is the best clue. Drips that appear while the tank is heating, then stop, point to thermal expansion. A steady drip at any time points to supply pressure that is too high or a valve that no longer seals. Hot water or steam rushing out points to overheating.
| Pattern | Likely cause | Fix |
|---|---|---|
| Drips during or just after heating, then stops | Thermal expansion in a closed system | Add or recharge an expansion tank |
| Started after an expansion tank was installed years ago | Expansion tank waterlogged (failed diaphragm or lost air charge) | Check the air charge; replace the expansion tank if it holds water |
| Steady drip at all times | Supply pressure too high (failed or missing pressure-reducing valve) | Measure pressure; service or replace the pressure-reducing valve |
| Began after someone tested the valve | Debris or mineral on the seat; worn seat | Lift the lever briefly to flush the seat once; replace if it keeps dripping |
| Old valve, steady seep, crust at the outlet | Valve worn out | Replace the valve |
| Hot water or steam discharging, taps scalding | Tank overheating: failed thermostat or gas control | Turn off the heat now and call a technician |
The Water Heater Troubleshooter has a T&P-dripping path that ranks these causes and lists the stop-and-call triggers.
03 /How does thermal expansion make the relief valve drip?
Water expands as it heats. Heating a tank from about 10 °C to 60 °C increases the water's volume by roughly 1.7 percent, which is about 3 litres in a 189-litre (50 US gallon) tank. In an open system, that extra water simply pushes back into the city main. In a closed system, it has nowhere to go, so the pressure in the whole house climbs with every heating cycle until something gives, usually the T&P valve.
A system becomes closed when something on the supply stops water flowing backward: a pressure-reducing valve, a check valve, a backflow preventer, or some water meters. Many Canadian homes have one. The City of Calgary, for example, notes that homes built after 1990 in areas with pressure over 80 psi are equipped with a pressure-reducing valve, and that a pressure-reducing valve more than five years old may need servicing if water pressure is high.
The standard fix is a correctly sized thermal expansion tank on the cold supply near the heater, with its air charge set to the house's static pressure. Plumbing codes generally call for thermal expansion protection in these situations (confirm the requirement where you live); the details and sizing are in expansion tanks and water heater permits and code.
04 /How do I find out which cause I have?
An inexpensive water pressure gauge that screws onto a hose bib or the water heater's drain valve, ideally one with a red maximum-reading needle, separates the causes in a day.
- Fit the gauge
Screw it onto a hose bib or laundry tap connection, or the tank's drain valve, and open that valve fully.
- Read static pressure
With no water running, note the pressure. Many homes sit between roughly 40 and 80 psi; readings well above that suggest a failed or missing pressure-reducing valve.
- Watch a heating cycle
Run hot water until the burner or elements start, then close all taps and watch. If pressure climbs steadily while the tank heats, you have thermal expansion.
- Check the maximum needle
Leave the gauge overnight. A maximum reading near the valve's rating (commonly 150 psi) confirms pressure is opening the T&P valve.
- Check the expansion tank
If you have one, tap it: the top should sound hollow. Briefly press the air valve (like a tire valve) on its end; water rather than air means the diaphragm has failed.
- Feel the discharge
Carefully, is the water from the discharge pipe hot or scalding? Hot discharge with normal pressure points toward overheating.
05 /What are the rules for a T&P discharge pipe?
The discharge pipe carries released water safely to where it cannot scald anyone. Requirements come from the plumbing code adopted in your province and the heater's installation instructions. The National Plumbing Code of Canada 2020 text on relief valve discharge pipes, as quoted in a National Research Council public-review proposal, requires them to be at least the size of the valve outlet, rigid, sloped downward from the valve, terminated with an indirect connection and an air break of not more than 300 mm, with no thread at the outlet, and capable of operating at 99 °C or more.
| Check | Why it matters |
|---|---|
| Same size as the valve outlet, no reducers | A smaller pipe restricts the valve's flow |
| Rigid pipe rated for hot water, sloping down | Soft or sagging pipe can kink, trap water or freeze |
| Ends near the floor or over a drain, with an air gap | Releases where it cannot scald and where you can see it |
| No thread, cap or valve at the end | Nobody can close it off |
| Not piped outdoors where it can freeze | Ice would block the outlet |
Your province may have amended the national code, and older installations may predate current rules. If yours looks wrong, have a plumber correct it; a replacement water heater is also the moment to bring the discharge pipe up to current requirements. Always confirm local requirements with your municipality or provincial safety authority.
06 /Should I test or replace my T&P valve?
Many manufacturers recommend lifting the test lever periodically, often once a year, to make sure the valve opens and to flush its seat. Place a bucket under the discharge pipe, stand clear, lift the lever for a few seconds and let it snap shut. Water will be hot.
Testing a valve that has not moved in years can leave it dripping, because mineral deposits stop it reseating. That is not the test's fault; it has revealed a valve that probably would not have opened properly when needed. Lift and release once more to flush the seat, and if it still drips, replace it.
- Replace it if it drips after testing, is crusted with minerals, will not open, or is as old as the tank on a tank past mid-life.
- Match the ratings: the replacement must have the pressure, temperature and energy ratings required for the heater, typically shown on the heater's rating plate or in its manual.
- Who does it: replacing a T&P valve means draining the tank below the valve and working on a pressure vessel's safety device. Many owners leave it to a plumber, and on gas units a technician can check the controls at the same visit.
If the valve is dripping because of pressure, replacing it alone will not help; the new valve will drip too. Fix the pressure or expansion first. For recurring pressure problems or an old tank, see signs your water heater is failing or request quotes.
FAQQuestions people ask
Is it dangerous if my T&P valve is dripping?
A drip usually means the valve is working and relieving pressure, so the immediate danger is low, but the cause should be fixed. The dangerous situations are a valve that has been capped or plugged, and a tank discharging steam or very hot water, which signals overheating.
Why is my hot water tank leaking from the overflow pipe?
The pipe down the side of the tank is the T&P valve's discharge pipe. Water from it means the valve is opening on pressure, often thermal expansion or high supply pressure, or that the valve seat is worn.
Can I cap a dripping T&P valve?
No. Capping or plugging the valve or its discharge pipe disables the tank's protection against over-pressure and overheating. Find and fix the cause instead.
How often should a T&P valve be replaced?
There is no single interval. Replace it if it drips after testing, is corroded or crusted, or does not open when tested. Many owners replace it when the tank is serviced in mid-life.
Does a tankless water heater have a relief valve?
Yes, tankless units are installed with a pressure relief valve, usually on the hot outlet or its service valve, and it needs a proper discharge pipe too.
Can high water pressure make my water heater leak?
Yes. Supply pressure that is too high, or pressure that spikes from thermal expansion, can open the T&P valve and stresses the tank and fittings. A pressure-reducing valve and an expansion tank control it.