Rooftop Not Heating — B2B Diagnostic Guide

When a rooftop unit stops heating a commercial or institutional building, the impact quickly goes beyond comfort: reduced productivity, occupant complaints, risk to water systems in cold weather and, on combustion appliances, a safety concern. The symptom is easy to observe — supply air stays cold — but its origins are varied and rarely confined to a single component.

Where does the problem usually sit?

On a rooftop unit, a lack of heat generally traces back to four families of causes. First the demand: setpoint, mode, schedule, remote lockout or a BMS control point that never calls for heat. Then the heating section itself — ignition sequence, flame control, gas supply or electric stage depending on the equipment. Next venting and combustion air: an induced-draft fan, a pressure switch or an obstructed flue interrupts the sequence before ignition even happens. Finally airflow on the supply side: clogged filters, mispositioned dampers or an economizer stuck on outdoor air can trip a temperature limit control. Assigning the cause remotely is risky — each family calls for its own measurements.

What can be checked safely from inside the building?

Start by confirming that a real heating demand exists: setpoint, seasonal mode, occupancy schedule and no lockout in the management system. Note whether the blower runs, whether one or several zones are affected, whether the unit locks out after a few attempts and at what time of day. Record visible alarms at the interface and the condition of accessible filters. On the other hand, do not open electrical panels or the combustion section, and do not reset repeatedly: voltage, hot components and flue gases are real hazards. If you detect a gas smell, apply your gas safety procedure immediately before anything else.

Why leave the diagnosis to a professional?

Telling a control fault apart from an ignition, venting or airflow problem requires instruments, safe roof access and a structured verification method. A gas heating and combustion intervention validates the full sequence rather than replacing parts at random, and a preventive maintenance program ahead of the cold season avoids the breakdown at the worst possible moment. Montréal Combustion diagnoses and restores rooftop units for commercial buildings across the Greater Montréal area, North Shore and South Shore. 450-473-0909.

Frequently Asked Questions

The blower runs but the air stays cold — is the heating section always at fault?

Not necessarily. A missing or mistransmitted demand, the wrong mode or schedule, an economizer stuck on outdoor air or a tripped safety device can produce the same symptom. Only an on-site diagnosis can tell them apart.

Can we reset a unit ourselves when it locks out in heating?

A one-time reset may be provided for by the manufacturer, but repeating it on a combustion appliance hides a fault and increases risk. When a lockout keeps coming back, call a qualified technician instead of restarting the unit over and over.

Is a loss of heating a risk beyond comfort?

Yes. In cold weather, losing heat can compromise freeze protection of water systems and business continuity. And on a combustion appliance, a venting or ignition fault raises a safety issue that requires professional verification.

Can a clogged filter prevent heating?

Insufficient airflow raises the temperature in the heat exchanger and can trip a limit control, interrupting heating or causing short cycles. It is a frequent lead, but it does not remove the need to check the other causes.