If your garage exhaust fans never stop, you are paying for electricity you do not need, and it is usually a symptom of a carbon monoxide system that is no longer working properly. Here is the short list of causes, in the order we find them.

1. A drifted sensor is calling for ventilation that is not needed

Garage CO sensors are electrochemical: a cell reacts to carbon monoxide and produces a small current. That cell ages with time, heat and humidity, and the reading drifts. When it drifts high, the sensor reports CO that is not there, the panel calls the fans, and the fans stay on. Nothing looks broken. The panel shows a number, the number is simply wrong.

2. A faulted sensor forces the fans to a safe state

Most systems are designed to fail safe: if a sensor fails, expires, or loses communication, the panel drives ventilation on and leaves it on. That is correct behaviour, and it is also expensive. Left unaddressed for a year, a single dead sensor can run an entire fan bank continuously.

3. Someone bypassed the controls

This one is common in buildings that have changed hands or changed vendors. The CO system started nuisance-alarming, nobody could get it serviced, and an electrician wired the fans to run permanently to make the problem go away. The fans run, the garage is ventilated, and the CO detection system is effectively decorative. The building is now paying maximum energy cost and has no working gas detection.

What the code actually allows

Enclosed parking garages require mechanical ventilation (IBC §406.4.2, with the mechanical code provisions). That ventilation must either run continuously or be operated automatically by carbon monoxide detection. Demand-based control is not a nice-to-have that you bolt on for savings, it is the alternative to running fans around the clock. Separately, where a gas detection system is installed, California Fire Code §916.11 requires it to be inspected and tested at least annually, with sensors calibrated on the manufacturer’s schedule.

What it is costing you

Exhaust fans in a parking structure are large motors, often several of them, and continuous operation means 8,760 hours a year. When the CO system controls them properly, they run only when vehicle activity actually produces exhaust, which in most garages is a small share of the day.

Working from fan horsepower, local energy cost and realistic run hours, moving a garage from continuous operation to demand-based CO control delivers up to 85% savings on garage ventilation energy. The actual figure for any given building depends on its fans, its occupancy pattern and its utility rate, and adding variable frequency drives changes the arithmetic again. But the direction is consistent: continuous fan operation is usually the single largest controllable electrical load in a parking structure, and it shows up on every monthly bill until someone fixes the cause.

How this gets fixed

• Calibrate every sensor with certified span gas and identify the ones that have drifted or expired
• Replace end-of-life sensors rather than re-zeroing them
• Verify the ventilation response: the fans should start at setpoint and stop when the reading clears
• Undo any bypass and return the fans to the panel’s control
• Where fans are on variable frequency drives, confirm the drives are actually modulating rather than pinned

ACES Monitoring works on gas detection and the fan control tied to it, in enclosed garages nationwide. If your fans have been running non-stop, that is generally the cheapest problem on this list to prove, because a calibration visit tells you immediately which sensors are lying.

Sources: IBC §406.4.2 and IMC ventilation provisions · California Fire Code §916.11.