Quick answer
A boiler that is not turning on has one of three problems: no call for heat is reaching it, a safety is holding it off, or it tried to light and locked out. Check the emergency switch, the thermostat and the pressure gauge, which should read about 12 to 15 psi cold. Press reset once only; a second lockout needs a TSSA-licensed boiler technician.
If your boiler is not turning on, it is in one of three states, and telling them apart takes five minutes at the boiler. Either nothing is asking it to run, or a safety control is refusing permission, or it tried to light, failed and locked itself out. Each state leaves different evidence: a dark control, a gauge reading out of range, or a fault light and a reset button.
No call reaching the boiler: switch, thermostat or a zone-valve end switch
A boiler with no call for heat sits silent and looks dead even though nothing is broken. Start with power: the service switch on or beside the boiler, a second emergency switch that is often at the top of the basement stairs with a red cover plate, and the breaker. These get bumped by storage boxes and flipped by people who take them for light switches.
Next is the thermostat: fresh batteries, set to heat, set point above room temperature. A house with zone valves has one more link in the chain. The thermostat opens its zone valve, and only when the valve is fully open does a small end switch inside the valve head close and signal the boiler to fire. A valve with a tired motor or a failed end switch leaves the thermostat calling and the boiler hearing nothing, which is why one heating zone not working and a whole-house no-heat can share a cause.
Safeties holding the boiler off: pressure, low water, high limit, vent and condensate
A boiler that has power and a call but will not fire is usually being held off by a safety doing its job. Read the gauges before touching anything.
- Low water pressure: the gauge should sit at about 12 to 15 psi with the system cold. Many condensing boilers have a pressure sensor that blocks firing when the reading falls too far, and a needle near zero means water has left the system somewhere.
- Low-water cut-off: a probe in the boiler or the supply pipe that cuts the burner when it is not covered by water, usually with an indicator light. Never add cold water to a boiler that is hot and low on water; shut it off and let it cool.
- High-limit aquastat: if the boiler water is already at its limit, typically around 82 °C on a cast-iron boiler, the burner stays off until it cools. A hot boiler with cold radiators is a circulation fault, explained under why radiators are cold when the heating is on.
- Blocked vent or air intake on a condensing (mod-con) boiler: snow, ice, leaves or a nest at the plastic pipes outside stops the fan from proving airflow, so the boiler will not attempt ignition.
- Blocked condensate trap or frozen condensate line: a mod-con makes acidic water as it runs, and when the trap or drain plugs, that water backs up and the boiler shuts down to protect itself.
Ignition lockout: pilot out, failed igniter or flame sensor
A boiler in lockout tried to light, did not prove a flame and gave up. Older cast-iron boilers with a standing pilot simply have no pilot flame to light the burner. Newer boilers use a hot surface or spark igniter and a flame sensor; a cracked igniter gives no flame, and a fouled sensor cannot see the flame that is there, so the control shuts the gas after a few seconds.
The rule is one reset. Write down any code on the display first, because some controls clear it when the power is cycled, then press the reset button once and watch. If it locks out again, stop: each failed attempt can leave unburnt gas in the combustion chamber, and repeated resets are how a no-heat call becomes a hard, noisy ignition. In Ontario, work on the gas train, burner and ignition controls belongs to a TSSA-licensed gas technician.
What to check yourself, and when to call now
- Service switch, red emergency switch and breaker all on.
- Thermostat calling: heat mode, set point above room temperature, batteries fresh.
- Pressure gauge reading about 12 to 15 psi cold, and the temperature on the same gauge.
- Fuel: the gas valve handle in line with the pipe; on a rural lot in Kilbride, Carlisle or Caledon, propane or oil left in the tank.
- Outside: intake and exhaust pipes clear of snow and ice.
- One reset, then leave it alone.
Stop checking and call now in any of these cases.
- You smell gas: leave the house and call Enbridge Gas or 911 from outside.
- A carbon monoxide alarm has gone off, even once.
- The pressure gauge reads zero, or there is water on the floor under the boiler.
- The boiler locked out a second time after your one reset.
- The house is losing heat in freezing weather. A hydronic system holds water in every radiator and pipe, so read what to do if heating pipes or radiators freeze while you wait.
What a boiler no-start costs to diagnose and fix
The diagnostic visit is $90–$150 and is folded into the repair if you proceed. Boiler parts vary far more than furnace parts: a thermocouple on a 1980s cast-iron boiler and a control board on a wall-hung condensing unit are different orders of cost, and the brand decides whether the part is on the truck. For that reason the repair is quoted in writing once the fault is found; how much boiler repair costs explains what drives the number.
ZK Mechanical is based in Burlington and works on cast-iron, condensing and combi boilers, from pre-war brick houses in Hamilton and the west side of Toronto to newer homes with in-floor heat. For boiler repair, phone (647) 801-1252 on the 24/7 line, which carries no overtime or weekend surcharge, or send the code and gauge readings through the contact form.