Fans used to provide combustion air must be interlocked with which of the following?

Fans used to provide combustion air must be interlocked with which of the following?

• A. Gas valve, Operating control, Limit switch
• B. Thermostat, Barometer, Flame sensor
• C. Pressure switch, Draft inducer, Vent
• D. None of the above

Answer: (A)

Combustion air safety relies on tying the fuel supply to the actual operating and safety conditions. The gas valve must only open when the system’s control is calling for heat and the safety path is clear. Interlocking the gas valve with the operating control ensures fuel can’t flow unless a heat demand is present and the control is satisfied. Adding the limit switch provides a high‑temperature safety shutoff: if the furnace overheats or the venting is blocked, the limit switch trips and de-energizes the gas valve, stopping fuel flow. This trio—gas valve, operating control, and limit switch—works together to prevent fuel from being supplied under unsafe conditions, which is why it’s the correct interlock. The other options don’t provide that exact, integrated safety chain. A thermostat and flame sensor are part of control and flame monitoring but don’t guarantee interlock with the gas valve via a dedicated safety switch; a barometer isn’t used for combustion air interlocks in typical gas appliances. The combination of pressure switch, draft inducer, and vent relates to draft and venting monitoring, but it doesn’t directly couple the gas valve to the safe operating controls in the same interlocked way.

Combustion air safety relies on tying the fuel supply to the actual operating and safety conditions. The gas valve must only open when the system’s control is calling for heat and the safety path is clear. Interlocking the gas valve with the operating control ensures fuel can’t flow unless a heat demand is present and the control is satisfied. Adding the limit switch provides a high‑temperature safety shutoff: if the furnace overheats or the venting is blocked, the limit switch trips and de-energizes the gas valve, stopping fuel flow. This trio—gas valve, operating control, and limit switch—works together to prevent fuel from being supplied under unsafe conditions, which is why it’s the correct interlock.

The other options don’t provide that exact, integrated safety chain. A thermostat and flame sensor are part of control and flame monitoring but don’t guarantee interlock with the gas valve via a dedicated safety switch; a barometer isn’t used for combustion air interlocks in typical gas appliances. The combination of pressure switch, draft inducer, and vent relates to draft and venting monitoring, but it doesn’t directly couple the gas valve to the safe operating controls in the same interlocked way.