What does the pressure switch on an induced-draft furnace prove before ignition is allowed?
- That the gas supply pressure at the valve inlet is high enough
- That the circulating blower is moving air across the heat exchanger
- That the air filter is clean enough for the furnace to run safely
- That the inducer is moving air through the heat exchanger and vent
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D. That the inducer is moving air through the heat exchanger and vent
The switch senses the negative pressure created by the draft inducer, confirming that combustion air is flowing and the vent is not blocked. It does not measure gas pressure, despite its name.
Reference: Appliance manufacturer installation and service instructions
About how many cubic feet of air are needed to completely burn one cubic foot of natural gas?
- 2 cubic feet
- 5 cubic feet
- 10 cubic feet
- 24 cubic feet
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C. 10 cubic feet
Natural gas needs about 10 cubic feet of air per cubic foot of gas for complete combustion. About 24 cubic feet is the figure for propane, which has more carbon and hydrogen in each cubic foot.
Reference: Combustion fundamentals; NFPA 54 Annex
What does the fixed maximum liquid level gauge (bleeder valve) tell the person filling a propane tank?
- The pressure inside the tank in psi
- The temperature of the liquid in the tank
- That the tank has no air or water in it
- That the liquid has reached the maximum fill level
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D. That the liquid has reached the maximum fill level
A dip tube ends at the maximum permitted liquid level. During filling the bleeder is opened; when it changes from clear vapor to a white liquid mist, the tank is full and filling stops.
Reference: NFPA 58 Sections 5.9 and 7.4 (fixed maximum liquid level gauge)