Quick answer
On a working appliance it is not dangerous if a pilot light goes out: a thermocouple or thermopile senses the lost flame and closes the gas safety valve, so gas stops flowing. It becomes dangerous if you smell gas, the appliance is very old or the safety valve has failed. Smell gas? Leave and call 911 or Enbridge Gas. Otherwise wait at least 5 minutes before relighting.
Is it dangerous if a pilot light goes out? In nearly every case, no, because the appliance was designed on the assumption that one day it would. Furnaces, boilers, water heaters and fireplaces with a standing pilot all carry a flame-proving safety that shuts the gas when the pilot dies. The danger sits in the exceptions, and your nose finds most of them.
The thermocouple closes the gas safety valve when the pilot flame is lost
The tip of a thermocouple sits in the pilot flame. Heat makes it generate a tiny current, around 25 to 30 millivolts, which holds a small electromagnet open inside the gas valve. When the flame goes out the tip cools, the current fades and a spring snaps the safety valve shut, cutting gas to both the pilot and the main burner. Fireplaces and some boilers use a thermopile, a larger version of the same part that also powers the valve.
The tip takes a short while to cool, so a little gas escapes at the pilot orifice before the valve drops out. A pilot burns very little gas, and that brief release disperses. It is the reason every lighting plate tells you to wait: at least 5 minutes with the control knob at OFF before a flame goes anywhere near it.
When a pilot light that is out is dangerous
- You smell gas. A rotten-egg odour near an appliance with a dead pilot means the safety valve has not closed, or the leak is somewhere else. Do not relight and do not touch switches; leave and call Enbridge Gas at 1-866-763-5427 or 911 from outside. The full steps are in what to do if you smell gas.
- The appliance is very old. Some decades-old ranges, space heaters and floor furnaces were built with pilots that have no flame-proving safety, and on those an unlit pilot keeps releasing gas.
- The safety valve has failed. A pilot knob that does not need to be held in while lighting, or a main burner that will light with no pilot, means the valve is not doing its job and must be replaced before the appliance is used again.
- The fuel is propane. Propane is heavier than air and settles in basements, crawl spaces and floor drains instead of rising and dispersing the way natural gas does. On rural lots in Kilbride, Lowville, Campbellville, Carlisle and rural Caledon, give an extinguished propane pilot extra time and ventilate low areas before relighting.
- Exhaust blew it out. If a water heater or boiler pilot dies on windy days, or when the dryer and kitchen fan are running, the chimney may be back-drafting, and that brings carbon monoxide into the house. See how to tell if a water heater is back-drafting.
Relighting safely: wait at least 5 minutes first
Relighting a pilot is a homeowner task only when there is no gas smell and the lighting instructions are on the appliance. Follow the plate on the unit, not memory, because the sequence differs between valves.
- Turn the gas control knob to OFF and wait at least 5 minutes. With propane, wait longer and sniff at floor level.
- Turn the knob to PILOT, press and hold it down, and light the pilot with the built-in igniter or a long lighter.
- Keep the knob held for about a minute so the thermocouple heats through, then release it slowly.
- If the pilot stays lit, turn the knob to ON and set the thermostat. If it dies when you let go, turn to OFF, wait another 5 minutes and try once more.
- After two failed tries, stop. Each attempt puts more unburnt gas into the cabinet.
For a tank water heater, the model-by-model version is in how to relight a water heater pilot light.
A pilot that will not stay lit needs a licensed technician
A pilot that lights and dies the moment you release the knob almost always has a worn thermocouple, one that no longer makes enough current to hold the valve. A pilot that burns yellow, lazy or small has a dirty orifice, and its flame no longer wraps the thermocouple tip. One that goes out at random on an otherwise healthy appliance points to a draft problem: a spilling chimney, a missing draft-hood baffle, or a furnace room starved of combustion air after a renovation.
In Ontario, work on gas valves, pilots and burners belongs to a technician holding a TSSA gas licence. What it costs:
- Diagnostic visit to find why the pilot fails: $90–$150, folded into the repair if you proceed.
- Thermocouple replacement: $150–$300.
- Pilot orifice cleaning and flame adjustment: normally done during that first visit. A draft or venting fault is quoted in writing once the cause is found.
A furnace or boiler old enough to have a standing pilot is usually at least three decades old. The thermocouple is worth replacing, but ask the technician what else is worn, because that generation of equipment burns far more gas than the 95% AFUE minimum sold today.
ZK Mechanical repairs standing-pilot furnaces, boilers, water heaters and fireplaces across Burlington, Hamilton, Oakville and Milton, on natural gas and propane. The heating services page shows what a visit covers. Call (647) 801-1252 at any hour once the utility has made the house safe after a gas smell, or book a repair for a pilot that keeps dropping out.