Shop Unit Heater Not Heating or Tripping

A shop heater tends to fail on the first properly cold morning of the season. It sat all summer, and now it’s being asked to run hard. You walk in and it’s either blowing cold air, doing nothing, or tripping the breaker every time the thermostat calls for heat.

This guide is about electric unit heaters — the ceiling- or wall-hung fan-forced heaters common in shops, garages and small industrial bays. Gas-fired unit heaters are covered briefly at the end.

How an electric unit heater works

Knowing the parts makes the symptoms easier to read:

• Heating elements — one or more, depending on size.
• Fan motor — pushes air across the elements.
• Contactor — switches power to the elements when the thermostat calls for heat.
• High-limit (thermal cutout) — shuts the elements off if the heater overheats. Some reset on their own when they cool. Others need a manual reset.
• Thermostat — built into the heater, or mounted on the wall.
• Control transformer and fuses — on many larger heaters, especially 208 V and 600 V three-phase units, the controls run at a lower voltage through a small transformer.
• Supply — a breaker in the panel and usually a local disconnect.

Fan runs, but the air is cold

This is the most common complaint. The heater is getting power and the fan works, but the elements aren’t heating.

Likely causes:

• The contactor isn’t closing, or its contacts have burned. Contactors on shop heaters cycle thousands of times over a winter, and worn contacts eventually stop passing current.
• The high-limit has tripped and hasn’t reset, or has failed open.
• An element has burned open.
• The thermostat isn’t calling for heat — so the fan is running on a fan-only setting or a separate circuit.

If the heater gives some heat but less than it used to, one element or one element circuit is usually out. On a three-phase heater, a lost phase or one burned contactor pole does the same thing.

Nothing happens at all

No fan, no heat, nothing:

• The breaker is tripped, or the local disconnect is off.
• A control transformer or control fuse has failed, so the contactor can’t be energized.
• The thermostat has failed, or its wiring has been damaged.
• A manual-reset high-limit has tripped and shut the whole unit down, on models that work that way.

Check the breaker and disconnect. Beyond that, finding which part has failed means testing inside the heater with the covers off. That’s electrical work.

It trips the breaker

A heater that trips its breaker when it comes on, or after running a few minutes, usually has:

• A grounded or shorted element. Element sheaths crack with age and heat cycles. A cracked element can trip the breaker immediately, or only once it heats up.
• A loose or burned connection at the element terminals, the contactor or the disconnect. Heaters draw a heavy, steady load, so a poor connection gets hot, gets worse and eventually trips or burns open.
• Damaged wiring in or near the heater — heat-damaged insulation near elements, or wiring chafed where it enters the heater.
• A failing breaker that has tripped many times. Worth proving before replacing.

Don’t keep resetting it, and never replace the breaker with a larger one. The breaker is sized to protect the wiring feeding the heater.

It heats, then shuts off, then comes back

A heater that cycles off on its own, blows cool for a while and then heats again is often tripping its auto-reset high-limit. The high-limit is doing its job. The question is why the heater is overheating:

• The fan is slow or failing. Worn bearings or a failing motor mean less air across the elements, so they overheat.
• Airflow is blocked. Shops are dusty. Dust, lint, sawdust or overspray build up on the fan and elements over years. A wood shop heater can be clogged enough to overheat within one season.
• The fan blade is loose on the motor shaft.

A clogged heater is also a fire risk. Dust on hot elements can smoulder. Cleaning is worth doing every fall with the power locked off.

It runs all the time and can’t keep up

If everything works but the shop won’t come up to temperature on cold days, check:

• The heater may be on the wrong voltage. A heater rated for 240 V connected to a 208 V supply puts out about three-quarters of its rated heat. This comes up when a heater is moved from a house or acreage shop into a commercial building, or when used equipment is bought. The voltage on the nameplate should match the supply.
• Part of the heater isn’t working — one element or one phase out, as above.
• The heater is undersized for the shop, or the shop has had doors, insulation or ventilation changes. That’s a heating-load question, not a fault.
• The thermostat is in a bad spot — in the heater’s own airstream, beside an overhead door, or on a cold outside wall.

Gas-fired unit heaters

Natural gas and propane unit heaters also have electrical parts: the fan motor, the control board, the igniter, the thermostat and the power supply. I can troubleshoot the electrical side — supply, thermostat wiring, fan motors and controls. Anything involving gas valves, burners, venting or combustion belongs to a licensed gas fitter or heating contractor.

What you can check yourself

• The breaker and the heater’s disconnect.
• The thermostat setting and mode, and whether it’s calling for heat.
• Whether the fan runs, and whether the heater gives any heat.
• The nameplate voltage against what the panel says the circuit is.
• Whether the heater and its fan are visibly clogged with dust. Only clean it with the power off and locked out.

Stop and call when:

• The breaker trips again after one reset.
• There’s a burning smell, scorching or discolouration.
• The fix means opening the heater’s control compartment, element terminals or disconnect.

Getting it fixed

I service electric unit heaters and their controls in shops and garages across Red Deer, Blackfalds, Penhold and Red Deer County. That covers elements, contactors, high-limits, thermostats, control transformers, supply circuits and disconnects. It’s part of shop and garage electrical work. Thermostat and contactor wiring falls under electrical controls. If you’re adding a heater and not sure the panel can take it, that’s covered under panels and equipment power.

Questions I get asked

Can I replace the element or contactor myself? Those parts are inside the heater on the line-voltage side — 240 V, 208 V or 600 V. Replacing them is electrical work, and a wrong connection can cause the next failure. Checking the breaker, thermostat and cleanliness is fine.

Is it worth repairing an old unit heater? Often, yes. Elements, contactors, high-limits and fan motors are usually available. If the heater is badly corroded or the parts are obsolete, replacing it can make more sense. That’s decided after it’s diagnosed.

Why does my heater smell when it first comes on in fall? Usually dust burning off the elements after a summer of sitting. It should fade quickly. A smell that persists, or smells like burning plastic or wiring, isn’t normal — shut it off and have it checked.

Can a heater be moved from one shop to another? Yes, if the supply matches the nameplate voltage and phase and the circuit is sized for it. Check before moving it, not after.

Heater down?

Call 403-872-8698, or send a photo of the heater’s nameplate and the panel through the service request form. The nameplate tells me the voltage, phase and wattage before I head out.

Todd Grasse, Red Seal Journeyman Electrician. Grasse Electric is a licensed electrical contractor in Alberta, insured and WCB covered.

Written by Todd Grasse, Red Seal Journeyman Electrician, Grasse Electric — Red Deer, Alberta. Published September 25, 2026.