Trane Rooftop Unit Not Heating in Mississauga — What We Found and How We Fixed It.
Last week, HVAC-GROUP received a service call for a Trane rooftop unit (RTU) not heating in
the Lakeshore and Dixie area of Mississauga. The building manager reported cold air blowing
through the supply vents and the unit repeatedly shutting down.
This type of issue is common during colder months in Mississauga, especially with older rooftop
units running nonstop. Below is a full breakdown of the diagnosis, the failed components, and
how the repair was completed.
Job Summary — What We Found on Arrival
When the technician inspected the Trane rooftop unit, he confirmed:
• The unit was not producing heat
• The blower was running but no warm air
• Control board was not receiving proper low-voltage power
• The ignition sequence failed before start-up
After opening the control cabinet and running a multimeter check, two components were found
failed:
1. Burned step-down transformer
2. Failed venter motor (inducer motor)
Both components are critical for the heating cycle.
We ordered replacements, installed them the same day, and fully tested the heating system.
What Does a Transformer Do in a Rooftop Unit?
The step-down transformer converts high-voltage 208/240V rooftop power into low-voltage 24V,
which the furnace control board, thermostat, and safety circuits use.
Why does a transformer fail?
Common reasons:
• Shorted wiring
• Failed gas valve or shorted low-voltage component
• Overheating due to dust or age
• Voltage surge
When the transformer burns out:
The entire heating control system stops working.
This was exactly the case at this Mississauga building.
What Does the Venter (Inducer) Motor Do?
The venter motor is responsible for safely moving combustion gases out of the heat exchanger
and up the flue.
Without it, the system cannot ignite—by design.
Signs of a bad venter motor
• Grinding or humming sounds
• Weak airflow through the exhaust
• Unit tries to start but shuts down
• Error codes indicating pressure switch failure
In this job, the venter motor had seized internally, causing the ignition cycle to fail.
Full Diagnostic Steps (What Our Technician Did)
1. Confirm the symptom
No heat, blower running, cold supply air.
2. Check low-voltage power
Found 0V at the control board. This immediately suggested a transformer issue.
3. Inspect transformer
Visually burned, no output voltage.
4. Test inducer motor circuit
Motor would not turn. Resistance reading showed the motor windings were failed.
5. Inspect safety switches & venting
Pressure switch did not close due to inducer failure.
6. Order & install new parts
Replaced:
• Step-down transformer
• Venter/inducer motor assembly
7. Test heating cycle
After replacement:
• Transformer powered control board
• Ignition started normally
• Flame established
• Heat delivered to building
Unit was restored to full operation.
Q&A — Common Questions Mississauga Building Owners Ask
Why does my rooftop unit stop heating suddenly?
Most common causes:
• Failed transformer
• Bad venter/inducer motor
• Igniter failure
• Dirty burners
• Cracked pressure tubing
• Safety switch lockout
Commercial rooftop units work hard in Ontario winters — component failure is common.
Can a burned transformer cause no heat?
Yes. Without 24V power, the gas valve, ignition system, and safety circuits won’t operate.
Does a bad venter motor stop the furnace from starting?
Absolutely. The unit will not ignite until the inducer motor creates proper draft.
How long does a rooftop unit repair take?
Most repairs take 1–3 hours, depending on part availability and access to the roof.
Should I repair or replace my rooftop unit?
Consider replacement if:
• Unit is over 15–18 years old
• Repairs are becoming frequent
• Efficiency is declining
• Heat exchanger or compressor is failing
For newer units, repair is usually the best option.
Final Result for This Mississauga Job
After replacing the failed transformer and venter motor:
• The Trane rooftop unit ignited correctly
• No more short cycling
• Proper gas flow and draft pressure
• Reliable heat throughout the building
The building manager at Lakeshore & Dixie confirmed temperatures stabilized within 45
minutes.
Need Rooftop Unit Repair in Mississauga?
If your Trane, Lennox, Carrier, or Goodman rooftop unit is:
• Not heating
• Blowing cold air
• Making noise
• Short cycling
• Showing error codes
We can help.
Local, same-day service across Mississauga.
HVAC-GROUP Mississauga — 416-807-2752
Reliable rooftop unit heating repair.
