Why does an inspector operate a system through its normal controls rather than at the equipment?

Operating a system through its normal controls tests the whole chain the occupant depends on: the thermostat or switch, the wiring between it and the equipment, any interlocks or safety devices in the path, and finally the equipment itself. Starting the equipment directly at the unit bypasses most of that chain, so a system that runs perfectly when jumped at the appliance may still be useless to the occupant because the thermostat is dead, a wire is broken, or a safety switch has opened. The failure is in the path, not the machine. Using normal controls also keeps the inspector within the operating conditions the equipment is designed for, and avoids the risk of forcing a unit to run with a safety device defeated. It is the same principle behind operating fixtures at the tap and switches at the wall rather than at the panel.

Operating through normal controls exercises the entire chain the occupant relies on — control, wiring, interlocks and equipment — so a fault anywhere in that path is revealed.

Inspection standards call for operating systems using normal operating controls. This exercises controls, wiring and interlocks along with the equipment, and avoids bypassing safety devices.

Test the Path, Not Just the Box. A furnace that runs when you jump it at the unit is still broken if the thermostat is dead.

Where a system fails to respond at its normal control, report the symptom rather than diagnosing the component. The cause may be anywhere along the chain.

• •Starting equipment at the unit and reporting the system as functional
• •Bypassing safety devices to force operation
• •Diagnosing the component instead of reporting the symptom