Quick answer
A radiant tube heater that won't ignite is usually failing its airflow check or its spark. A blocked combustion-air inlet or exhaust keeps the pressure switch from closing, so the control never tries to light. If it sparks without lighting, suspect the electrode, flame sensing or gas pressure. After repeated failed tries the control locks out. Overhead gas work needs a licensed technician.
When a radiant tube heater won't ignite, the point where it stops in its start-up sequence tells you most of what you need to know. These low-intensity infrared heaters, the long steel tubes with a reflector hung under the ceiling of a shop, warehouse bay or large garage, all light the same way. A fan proves airflow, a spark lights the burner, and a sensor confirms flame. Watch one call for heat from the floor and note which step never happens.
The ignition sequence, and what each stopping point means
- Thermostat calls and nothing runs: no power at the heater, a tripped breaker, a failed thermostat or a dead transformer.
- Fan runs but there is no spark: the pressure (air-proving) switch has not closed. The control will not spark until it has proof of airflow through the tube.
- Spark clicks but no flame appears: no gas at the burner, low gas pressure, or a spark too weak or too far out of position to light the mixture.
- Lights, then drops out within seconds: the flame is not being sensed, so the control closes the gas valve and starts over.
- Nothing after several attempts: the control has locked out. Most reset when power or the thermostat call is interrupted, but a lockout that returns needs a diagnosis, not more resets.
Blocked air inlet or exhaust is the usual cause in a working shop
A tube heater pulls combustion air in at the burner box and pushes exhaust out the far end of the tube. Shops are hard on both ends. Sawdust, grinding dust, overspray and cobwebs load the inlet screen and the burner, birds and wasps nest in wall and roof terminations over the summer, and a vent cap can ice over in a January wind.
Any restriction lowers the pressure the fan creates, the air-proving switch stays open, and the heater sits with the fan running and no spark. The switch also fails by itself: its diaphragm stiffens with age, and the small sensing hose cracks or fills with condensation. A technician reads the pressure with a manometer and compares it to the switch's rated set point, which separates a bad switch from a real blockage in minutes.
Never jumper the switch to get heat for the afternoon. It is the only thing stopping the burner from firing into a blocked exhaust, and that is how a nuisance fault becomes carbon monoxide in an occupied building.
Spark igniter gap, cracked electrode and weak flame sensing
If the heater sparks and will not light, or lights and drops out, the fault is at the burner. The ceramic insulator on the spark electrode cracks from heat cycling, and the spark then jumps to ground inside the burner box where it cannot reach the gas. The gap also widens as the electrode tip erodes, until the spark is too weak to light a cold burner.
Flame sensing works by passing a tiny current through the flame to ground. A sensor rod coated in oxide, a loose ground wire or a burner that has shifted on its mount can each drop that signal below what the control module accepts. The module is the last suspect, not the first: it is the costliest of these parts and is usually working correctly, reporting a fault that lives somewhere else.
Gas pressure and line sizing on long shop runs
A tube heater at the back of a building is often the last appliance on a long gas line, and line sizing depends on length as much as on load. Piping that was adequate for one unit heater can starve a tube heater added later, especially when a rooftop unit or a second heater fires at the same moment. The telltale is a heater that lights on a mild afternoon and fails on the coldest morning, when everything in the building is calling at once.
The test is a manometer on the heater's inlet with every gas appliance running. If the pressure sags below the minimum on the rating plate, the repair is a gas line upgrade or a regulator correction, sized from the tables in CSA B149.1, the gas code Ontario adopts. On propane, common on rural properties in Caledon and Flamborough, a low tank in deep cold produces the same symptom.
What you can check from the floor, and why the rest is not a ladder job
- The thermostat is set above room temperature and has power or fresh batteries.
- The breaker is on and the heater's cord or disconnect has not been knocked loose.
- The outside intake and exhaust terminations are clear of nests, snow and ice, viewed from the ground.
- The status light on the burner box, if you can see it: note the flash pattern for the technician.
- Whether other gas appliances in the building are running normally, which points toward or away from a supply problem.
Everything past that list is overhead gas work. These heaters hang well above head height, the tube and reflector stay hot long after shutdown, and the burner box holds line voltage, spark voltage and a gas valve in one enclosure. In Ontario the gas side must be handled by a TSSA-licensed gas technician, and in a workplace the lift, fall protection and lockout are part of the job. An extension ladder and a screwdriver are how people get hurt on these.
There is no flat price for a tube heater repair, because it depends on the part, the mounting height and whether a lift has to be brought in. You get a written price after the diagnosis and before any work starts.
ZK Mechanical installs and repairs infrared tube heaters and unit heaters for shops and warehouses from Burlington through Hamilton, Milton and Mississauga. See infrared tube heater installation and repair for commercial buildings, garage heater installation for home shops and detached garages, or send us the model and the symptom. A smaller fan-forced unit heater that will not light is covered in why a gas garage heater won't ignite.