If the water from your taps has suddenly become hot enough to sting, treat it as a safety issue first. Scalding water injures young children and older adults fastest. In most homes the cause is one of a few things: the temperature setting, a thermostat or gas control that no longer shuts off, sediment in the tank, or a failed mixing valve. This guide covers what you can safely check and what needs a licensed technician.
Common Causes of a Water Heater Running Too Hot
The temperature is set too high
The simplest cause. Someone turned the dial up, or a new unit was installed at a higher setting. Health Canada recommends keeping the tank at about 60C to control Legionella bacteria, then using an anti-scald mixing valve so water at the tap stays at or below 49C. A tank set well above 60C with no mixing valve will deliver water that can scald.
Failed thermostat (electric)
Electric water heaters have an upper and a lower thermostat. If one sticks closed, its element keeps heating past the set point until the high-limit cut-off trips. Water that gets hotter and hotter, then suddenly goes cold, often points to this.
Grounded heating element (electric)
An element that has shorted to the tank can keep heating even when the thermostat has switched off. This needs a technician with a meter to confirm and replace the element.
Faulty gas control valve or thermostat (gas)
On a gas water heater the temperature is controlled by the gas valve and its sensor. If that control fails, the burner can keep running past the set point. Gas valves are not a homeowner repair; a licensed technician must replace them.
Sediment buildup
Minerals settle at the bottom of a tank over the years. The layer makes the burner or lower element work harder, causes rumbling or popping sounds, and can make water temperature swing. Flushing the tank helps; heavy buildup in an older tank is often a sign it is near the end of its life.
Failed mixing (tempering) valve
If your home has a mixing valve at the water heater or at a tap, it blends cold water into the hot. When it fails or is bypassed, taps get full tank temperature. This is a common cause when the tank setting has not changed but the taps suddenly run much hotter.
Tankless Water Heater Too Hot
Tankless units heat water as it flows, so a very low flow rate (one tap barely open) can push the output temperature up. Check the set point on the control panel first. If the temperature still overshoots at normal flow, the flow sensor or temperature sensor may need service.
What You Can Safely Check Yourself
- Check the temperature setting on the tank or control panel and turn it down if it was raised. Give a tank a few hours to settle before judging the result.
- Test the tap temperature with a kitchen thermometer after running hot water for a minute. Above 49C at the tap is hotter than recommended.
- Ask everyone in the home whether the setting was changed, or whether a plumber recently worked on the system.
- Look at the temperature and pressure relief valve discharge pipe. If it is wet or dripping, shut the unit off and call for service.
- Do not open the electrical panels on an electric water heater with the breaker ON; the terminals carry 240 volts.
When to Call a Technician
Call EcoFrost if the water stays too hot after you turn the setting down, the temperature swings from scalding to cold, the relief valve is discharging, or you hear loud rumbling from the tank. A runaway thermostat or gas control is a safety issue, so these repairs need a licensed technician. If you have no hot water at all instead, see our no hot water troubleshooting guide. We service and replace gas, electric, and tankless water heaters across the GTA.
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Book Water Heater Repair?Frequently Asked Questions
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