Quick answer
A pilot light that is on with no hot water means the main burner is not firing or its heat is not reaching the taps. First check that the control knob is on ON, not PILOT or VACATION. After that the usual causes are a failed gas control thermostat, a weak thermopile, a blocked burner orifice, a broken dip tube or heavy sediment. Gas control replacement is a licensed job.
When your pilot light is on but there is no hot water, the pilot has done its part and the fault is further along. A lit pilot proves gas is reaching the heater and the safety circuit is satisfied. What it does not prove is that the main burner lights when the tank cools, or that the heat it makes ends up at your taps. One test splits those two branches: turn the temperature dial up and listen. If the pilot is out as well, start with why a water heater is not producing hot water instead.
The two-minute test: does the main burner fire?
Run a hot tap for a few minutes, then turn the dial on the gas control toward its hottest setting. A working heater answers with a click and a low whoosh as the main burner lights, and through the sight glass you will see a ring of blue flame instead of a single small one. Turn the dial back to where it was afterward.
No whoosh means the burner is not firing, and the next section applies. A burner that lights and runs while the water stays cool sends you to the section after it.
Pilot lit, main burner not firing
- Knob left on PILOT or VACATION. After a relight, the knob has to be turned from PILOT to ON, and the VACATION setting keeps the pilot burning while holding the burner off. This is the most common cause and costs nothing to fix.
- Failed gas control thermostat. The gas control is the box at the front of the tank with the dial; a temperature probe on its back reaches into the water. When the probe or the valve inside fails, the control never calls for heat even with a cold tank. On the common Honeywell control, seven flashes of the status light mean the control itself has failed; the label on the control is the final word.
- Weak thermopile. On heaters with an electronic control, the pilot flame heats a thermopile that generates the few hundred millivolts the control runs on. A tired thermopile can make enough to keep the pilot valve open and too little to pull in the main valve. That same Honeywell control signals low thermopile voltage with two flashes.
- Blocked burner orifice. Rust scale falling from the flue lands on the burner and can plug the main orifice, so the valve opens but little or no gas comes through. Expect a weak, lifting or yellow flame, or none.
Burner fires but the water stays cool
- Broken dip tube. Cold water is meant to be piped to the bottom of the tank. With the tube broken, it enters at the top and leaves through the hot outlet, so the burner heats a tank whose hot water you never receive.
- Heavy sediment. A thick mineral layer on the tank floor insulates the water from the flame. The burner runs long, rumbles and pops, and the tank recovers a fraction of what it used to. Hard water in Halton and Hamilton builds this layer in a tank that has never been flushed.
- Crossover through a mixing valve. A thermostatic mixing valve stuck toward cold, or a worn single-handle cartridge elsewhere in the house, feeds cold into the hot line. The tank is hot; the taps are not. Touch the hot outlet pipe above the tank carefully: if it is hot there and cool at the taps, the heater is not your problem.
- Blocked flue. If the flue baffle or vent is obstructed, the burner cannot draw properly and exhaust can spill at the draft hood. Soot, a hot-metal smell or melted plastic at the top of the tank means turn the knob to OFF and call; how to tell if a water heater is backdrafting explains the signs.
Checks you can do, and signs to call today
- Knob position: ON, with the dial at its usual mark.
- The burner test described above.
- Status light pattern, counted over three or four repeats and written down.
- Hot outlet pipe temperature at the top of the tank.
- Age of the tank from the rating plate.
Call the same day if:
- You see soot, scorching or melted plastic near the draft hood or burner door.
- The burner flame is yellow, lifts off the burner or rolls out toward the door.
- A carbon monoxide alarm has sounded. Get everyone outside first.
- You smell gas. Leave, then call Enbridge Gas at 1-866-763-5427 from outside.
What the repair costs and who can do it
Gas control replacement is a TSSA G2 repair: the control is the heater's gas valve, thermostat and safety in one body, and swapping it means draining the tank, breaking the gas connection and leak-testing it afterward. A gas valve replacement runs $400–$600. A thermopile, burner cleaning or dip tube falls inside the $150–$800 repair band, and the $90–$150 diagnostic fee comes off the repair when you go ahead.
On a tank past 10 to 12 years, a failed control plus a floor of sediment is usually the point where a replacement at $1,200–$2,500 installed is the better use of the money. We will give you both numbers in writing.
ZK Mechanical's technicians hold TSSA gas licences and repair gas tanks across Burlington, Oakville, Hamilton, Milton, Mississauga and Brampton. Start with the water heater service page, or call (647) 801-1252 or request a visit online with the status-light pattern and the tank's age.