Quick answer
On a gas tank, no hot water is usually a pilot that has gone out, a failed thermocouple ($150–$300 to replace) or a gas control valve ($400–$600). On an electric tank it is a tripped breaker, a tripped high-limit reset or a burnt-out element ($200–$400). Warm-but-not-hot water points to sediment or a broken dip tube instead. On a tankless unit, read the error code first.
A water heater not producing hot water announces itself in the shower, but the cause is usually one of a half-dozen well-known failures, and several are safe to check yourself before paying for a visit. Start by identifying what you have: a gas tank with a burner underneath, an electric tank with wiring into the top, a power-vented tank with a fan and plastic vent pipe, or a wall-hung tankless unit. Each fails in its own way, and the checks below are grouped accordingly.
Gas water heater: check these first
- Pilot light out — look through the sight glass at the bottom and relight it following the label instructions. A draft, a dirty pilot orifice or a brief gas interruption can blow it out.
- Pilot will not stay lit — the thermocouple has likely failed and is no longer proving the flame ($150–$300 to replace). This is the classic gas-tank failure.
- Gas supply — confirm the shut-off valve on the supply line is open and the stove or furnace still works. If nothing on gas works, call Enbridge Gas before calling a contractor.
- Pilot lights but the main burner never fires — the gas control valve may be done ($400–$600). On a tank past 10 years, that price usually points to replacing the unit instead.
Electric water heater: check these first
- Breaker — a tripped 240 V breaker is the most common electric 'failure'. Reset it once; if it trips again, stop and call, because that is a short, not a nuisance.
- High-temperature reset — the red button behind the upper access panel; press it once. Repeated tripping means a failing thermostat or a grounded element.
- Heating elements — elements burn out with age and scale; lukewarm-only water usually means the lower element is gone ($200–$400 replaced), while no hot water at all points to the upper one.
Tankless: no hot water and an error code
A Navien, Rinnai or Bosch tankless that stops making hot water almost always shows a code on its panel, and the code is the diagnosis: ignition failure, a flame-sensing fault, a blocked exhaust or air intake, a frozen inlet, or scale tripping an overheat sensor. Two homeowner checks are safe: confirm the unit has power and that its intake and exhaust terminals outside are clear of snow, leaves and nests, and check that the cold-inlet filter screen is not packed with grit. GTA hard water scales tankless heat exchangers, and a unit that has never had its annual descale ($150–$300) can fade to lukewarm long before it faults outright. A brief burst of cool water between draws — the cold-water sandwich — is normal, not a failure.
Warm but not hot, or runs out fast?
Partial hot water has its own culprit list: heavy sediment insulating the burner from the water, a broken dip tube letting cold inlet water short-circuit straight to the hot outlet, or a thermostat turned down (60°C at the tank is the safe standard). A tank that roars and rumbles while heating is telling you sediment is the problem — the water heater maintenance guide explains how an annual flush prevents it.
Power-vent tanks: the fan matters
A power-vented tank — plastic vent pipe, blower on top — locks out if the blower fails, the pressure switch cannot prove draft, or the vent terminal outside is blocked. It also needs 120 V from a wall outlet, so a tripped GFCI in the laundry room is enough to kill the hot water. Check the outlet and the outside terminal before concluding the tank is dead.
When to stop DIY and call
- Any smell of gas — leave the house and call Enbridge Gas's emergency line at 1-866-763-5427 before anyone else.
- Water pooling around the tank base — corrosion has likely breached the tank, and replacement is the only fix.
- Breakers or reset buttons that trip more than once.
- Anything involving the gas control valve or burner assembly — in Ontario that is work for a TSSA-licensed technician.
What these repairs cost in the GTA
Water heater repairs generally run $150–$800: a thermocouple at $150–$300, an element at $200–$400, a gas control valve at $400–$600, with a $90–$150 diagnostic typically credited toward the work. On a tank past 10 years, weigh any large repair against the $1,200–$2,500 cost of a new owned tank before approving it.
ZK Mechanical handles water heater repair across the GTA and Hamilton, with a 24/7 emergency line when there is no hot water and a houseful of people. If the unit is past 10 years old, you get a straight repair-versus-replace comparison before any work — contact us or call (647) 801-1252.