Generator Exhaust Finds the Open Window

A standby generator is usually positioned for the reasons that matter on installation day: where the cable run is shortest, where the fuel tanker can reach, where the slab already exists. Exhaust discharge is decided late and locally, and during commissioning the machine runs for twenty minutes while nobody is in the building.

Then an outage lasts nine hours, the building is occupied, and the exhaust runs beside a window that is open because the air conditioning is off.

Carbon monoxide does not announce itself

It has no smell, no colour and no irritancy. The early symptoms — headache, nausea, tiredness, poor concentration — are the symptoms of a long day in a hot building during a power cut. People attribute them to the situation and stay where they are, which is exactly what makes it dangerous indoors.

What to check, physically

• Discharge point against intakes. Walk the distance between the exhaust outlet and every fresh-air intake, operable window and door on that elevation. Include intakes on the roof if the exhaust terminates at low level.
• Height and direction. The outlet should discharge upward and clear of the building envelope, not sideways at head height into a courtyard where plume recirculates.
• Prevailing wind. A discharge that is safe on a still commissioning day may track straight back along the facade on a normal afternoon.
• Joints and silencer condition. Exhaust systems leak at flanges and flexible connections, and a leak inside a plant room puts the gas into a space people enter during the run.
• Enclosure ventilation. Containerised sets need their own airflow, and a blocked louvre causes both overheating and a build-up of products of combustion.

Detection belongs where people are

Carbon monoxide detection in the plant room protects the technician. Detection in the occupied spaces nearest the discharge protects everyone else, and that is where it is usually missing. Detectors also have a service life — the sensing element degrades whether or not it has ever alarmed — and a detector past its replacement date is a decoration.

The test worth running

Run the set on load for the duration of a realistic outage, with the building occupied as it normally would be, and measure at the intakes and in the nearest rooms. A twenty-minute monthly exercise proves the generator starts. It proves nothing at all about what the building breathes during the ninth hour.