A service call on a flat roof that reveals a complex failure
At AirGreen , we often work on complex commercial and residential HVAC systems. Today's service call took us to Sainte-Thérèse , on Montreal's North Shore , for an in-depth diagnosis of a central Lennox system installed on the roof of a modern apartment building.
The client reported a lack of air conditioning in several units of the building, despite the system appearing to be functioning. The unit in question—a dual-compressor Lennox rooftop —was responsible for servicing all of the block's air ducts.
Initial diagnosis: compressor failure and electrical problem
Upon opening the control panel, our technicians immediately noticed several telltale signs:
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Loose wires and slightly blackened insulation in the control compartment.
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Internal circuit breakers tripped , indicating a previous electrical overload.
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The main compressor appeared to be inactive despite the thermostat request.
Using a professional multimeter , we checked the voltage on the compressor and relay terminals. The result: the main contactor was no longer engaging properly, preventing the compressor from starting.
Step 1 – Inspect the electrical circuit and starting components
The problem was caused by a faulty power relay and a weak start capacitor . Both of these are essential for the compressor to function properly.
We have proceeded to:
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Replacing the power relay with a genuine Lennox part.
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Installation of a new dual capacitor , calibrated for the compressor and the fan.
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A complete check of the wiring to ensure that no oxidation or poor contact persists.
Step 2 – Checking the refrigeration circuit
After power was restored, the compressor started, but the suction pressure indicated an abnormally low refrigerant level . A visual inspection revealed traces of oil on a brazed connection in the circuit—a typical sign of a microleak .
So we have:
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Performed a nitrogen pressure test to locate the leak.
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Repaired the leaking solder joint .
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Pulled a full vacuum to -500 microns to remove all moisture.
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Recharged the system with R-410A according to the manufacturer's specifications.
Step 3 – Performance optimization and system calibration
Once the charge was restored, we carried out a series of tests:
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Monitoring of pressures and temperatures using a pressure gauge and thermocouple.
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Measurement of subcooling and superheat to ensure optimal performance.
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Checking the air flow at the duct outlet.
The results were perfect:
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Stable suction pressure at 118 psi.
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Air blown at 14°C colder than ambient air.
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Electricity consumption returned to normal.
Final result: comfort restored and prevention assured
After the repair, air conditioning was fully restored throughout the building . The property manager immediately noticed the difference.
Before leaving the premises, our team:
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Cleaned the electrical compartment.
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Tighten all connections.
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Recommended a seasonal preventive inspection to avoid the return of a breakdown related to humidity or vibration.
Why choose AirGreen for your HVAC systems on the North Shore
At AirGreen, we know that rooftop units require advanced technical expertise . Our technicians are trained to:
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Quickly diagnose electrical and refrigeration faults.
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Safely handle R-410A and R-32 refrigerant gases .
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Repair or replace original manufacturer parts .
Thanks to our rapid interventions in Sainte-Thérèse, Laval, Montreal and on the South Shore , we provide reliable and long-lasting HVAC service , supported by our certified know-how.
AirGreen – Air Conditioning and Heating , your trusted partner for all your Lennox central HVAC system repair needs, whether you are on the North Shore , the South Shore or in the Montreal metropolitan area .