A rooftop unit that blows cold air in January is more than a comfort problem. Staff get cold, customers leave, pipes near exterior walls are at risk, and in a restaurant the kitchen can start pulling frozen air through the doors. Most no-heat calls come down to a handful of causes, and a few simple observations make the repair faster.
First: is it really the rooftop unit?
Before anyone goes on the roof, confirm the thermostat is calling for heat. Check that it is set to heat, the setpoint is above the room temperature, any schedule or occupancy override is correct, and the display has power. A dead thermostat, a schedule stuck in unoccupied mode or a building automation setting can look exactly like a failed unit.
1. Ignition and flame problems
On gas-fired units, the most common no-heat faults are on the ignition side: a hot surface igniter that has cracked, a dirty or weak flame sensor, or a gas valve that is not opening. The usual symptom is a unit that tries to light, fails, retries a few times and then locks out. The fan may still run, so the space gets air but no heat.
2. Airflow and the high-limit switch
If air cannot move through the unit, the heat exchanger overheats and the high-limit switch shuts the burners off to protect it. Blocked filters, a broken or slipping blower belt, a failed blower motor or closed dampers all cause this. A unit that heats for a few minutes and then stops, over and over, often has an airflow problem rather than a burner problem.
3. Pressure switches and venting
The inducer fan has to prove draft before the burners light. A failed inducer motor, a blocked or iced vent, a cracked pressure-switch hose or condensate problems can all stop the sequence. Calgary winters make this more common: snow drifts and ice around rooftop vents are a frequent cause of cold-weather lockouts.
4. Controls, boards and power
A tripped breaker or disconnect, a blown control fuse, a failed transformer or a control board fault can leave the unit completely dead. Economizer and damper controls can also cause trouble: an economizer stuck open on a cold day can pull in enough outdoor air that a working burner still cannot keep up.
5. Gas supply
A closed gas cock, a supply problem or a regulator issue can affect one unit or the whole building. If several gas appliances stopped at the same time, mention that when you call.
What you can safely check
- Thermostat: heat mode, setpoint, schedule and power.
- Breakers: look for a tripped breaker for the rooftop unit, but do not keep resetting one that trips again.
- Filters: if you can reach them safely from inside, note whether they are visibly clogged.
- Pattern: does it never light, light and drop out, or run but blow cool air?
Leave rooftop access, gas components and electrical panels inside the unit to a technician. Roofs are slippery in winter and rooftop units carry line voltage and gas.
What to tell your technician
- Which unit or area is affected, if the building has several.
- What the thermostat shows and when the problem started.
- Whether you hear the unit start, click or retry.
- Any error codes or flashing lights you can see safely.
- Whether it has happened before, or after a cold snap or storm.
Preventing the next no-heat call
Most of these faults show warning signs before they cause a full outage: a weak flame signal, a worn belt, a dirty filter or a slow inducer. A fall heating check catches most of them before the first deep freeze, and a commercial HVAC maintenance plan keeps it on a schedule.
YYC Mechanical provides rooftop unit repair and commercial heating repair across Calgary, including Engineered Air, Carrier, York and Lennox units.