Pre-purge of the system (by starting the blower for a specified time to provide a minimum air volume change) before ignition of main burner is not required by ANSI code, but is frequently specified by the user. This should be provided whenever a building contains flammable dusts or vapors that could potentially reach and collect within a direct gas-fired unit while it is idle.
To assure safe operation of a direct gas-fired heating system, a number of Safety Interlocks are commonly provided:
A. Damper Limit Switch – Dampers must be fully open before unit will operate.
B. Air Proving Switch – Located near outlet of unit to prove air flow before igniting burner.
C. Flame Supervision – Discussed above. Shuts down unit on flame failure.
D. High Temperature Limit – Signals unit to reduce gas supply when maximum allowable temperature is reached. If air temperature exceeds limit, the unit is shut down.
E. High Gas Pressure Switch – Shuts down unit to protect regulator against spikes in inlet gas pressure.
F. Low Gas Pressure Switch – Shuts down unit when low inlet pressure detected. Redundant safety device, since low flame would also shut down unit.
G. Safety Shut-off Valve – Responds to various inputs listed above – closes off gas supply.