Quick answer
Hot water that is suddenly too hot means the heater has lost control of its temperature or the mixing valve has stopped tempering it. Causes are a thermostat stuck closed or set too high, a grounded electric element heating non-stop, a failed mixing valve, or thermal stacking on a gas tank. Above 60 °C water scalds in seconds; shut the heater off if taps steam.
When your hot water is suddenly too hot, treat it as a safety problem first and a comfort problem second. A tank is normally kept at 60 °C, and Ontario's building code limits the water delivered to fixtures in a home to 49 °C, which is why newer installations have a mixing valve on the hot outlet. Water that now stings your hand means one of those two layers has failed. Which one decides whether you need a valve or a heater repair.
Four causes of water that is suddenly scalding
- A thermostat that is mis-set or stuck closed. Check the simple version first: a gas control dial knocked toward its hottest mark, or an electric thermostat turned up during a repair. If the setting is normal, the thermostat itself may have failed with its contacts closed, so it never tells the burner or element to stop.
- A grounded element on an electric tank. An element with a cracked sheath leaks current to the tank and goes on heating with the thermostat open. The water climbs until the red high-limit button trips; if you have been pressing that button, read why a water heater reset button keeps tripping.
- A failed mixing valve. When the thermostatic cartridge seizes toward hot, tank-temperature water goes straight to the taps. This is the likely cause when the tank setting has not changed and the heater cycles normally. The fix is a mixing valve replacement.
- Thermal stacking on a gas tank. The thermostat reads the water near the bottom. A run of short draws, such as handwashing, a dishwasher filling or a quick rinse, brings in small slugs of cold that fire the burner again and again while the top of the tank is still hot. Heat layers at the top, and the first water out can be 10 degrees or more above the setting.
Stacking comes and goes with how the house uses water; the other three faults stay until they are repaired. If the scalding water appears only after a morning of short draws and settles by evening, turning the dial down a notch is a reasonable first step. Before going lower than that, read what temperature a water heater should be set to.
How fast water above 60 °C burns skin
Scald time falls sharply as temperature rises. Commonly cited figures for adult skin:
- 49 °C: several minutes of contact before a serious burn.
- 55 °C: roughly 30 seconds.
- 60 °C: about 5 seconds.
- 65 °C: about 2 seconds, and less above that.
Children and older adults burn faster because their skin is thinner. An overheating tank can be well past 65 °C at the tap, so until it is fixed, run the cold first at every basin and keep small children away from the taps.
A discharging relief valve is a warning, not a nuisance
The temperature and pressure relief valve on the side or top of the tank opens at about 99 °C or 150 psi. If it discharges while the water at the taps is also hotter than usual, the tank is overheating and the valve is the last thing standing between that and a ruptured tank. Never cap the discharge pipe or wedge the lever. A valve that weeps with normal tap temperatures is a different fault, usually pressure; see why a relief valve drips.
When to shut the water heater off
Shut it off now if any of these is true:
- Steam or spitting comes from a hot tap.
- The relief valve is opening or its discharge pipe is hot.
- The tank is rumbling hard, hissing or too hot to rest a hand on at the top fittings.
- The reset button on an electric tank has tripped more than once.
On an electric tank, switch off the double-pole breaker. On a gas tank, turn the control knob to OFF, or close the gas shut-off on the pipe beside it. Leave the cold-water supply open so the tank stays full, and let it cool on its own; do not drain a tank that is overheating. Then call.
What you can check yourself, and what the repair costs
- Read the dial or thermostat setting and compare it with where you left it.
- Measure the water at the nearest tap with a kitchen thermometer after a minute of running. Over 49 °C in a home with a mixing valve points to the valve; over 60 °C points to the tank.
- Note the pattern: constant, or only after a run of short draws.
- Check a tankless unit's wall controller. A setpoint changed by accident is the usual reason a tankless delivers hotter water from one day to the next.
Diagnosis is a $90–$150 visit, and that fee is folded into the repair if you go ahead. A thermostat or mixing valve falls in the $150–$800 repair band, an electric element is $200–$400 replaced, and a gas control valve is $400–$600. You get the price in writing before any part is ordered.
Gas control work has to be done by a TSSA-licensed gas technician, which is what ZK Mechanical sends. For overheating that needs someone today, the 24/7 line is (647) 801-1252 with no after-hours surcharge; for anything that can wait until morning, see water heater service or send a booking request.