Which of the following appliances is most affected by altitude regarding gas input ratings?

The gas furnace is most affected by altitude regarding gas input ratings primarily due to the changes in air density and atmospheric pressure that occur at higher elevations. As altitude increases, the air becomes less dense, which means there is less oxygen available for the combustion process.

Gas appliances, like furnaces, are designed to burn a specific mixture of gas and air to maintain efficient operation and safety. At higher altitudes, if the gas input is not adjusted, the furnace may burn too rich (excess gas relative to air) which can result in inadequate combustion, increased carbon monoxide production, and lower efficiency.

To compensate for these changes, manufacturers typically recommend adjustments to the gas input ratings for furnaces installed at high altitudes. This may include changing the orifices or modifying the gas pressure settings to ensure the appliance operates safely and efficiently despite the reduced air density.

Other appliances listed, such as electric stoves, heat pumps, and solar water heaters, do not rely on gas combustion for operation, making them less affected by altitude in terms of gas input ratings. Heat pumps operate on electricity and are designed to work effectively regardless of elevation, while electric stoves and solar water heaters are not impacted by air density in a similar manner to combustion-based systems.