In a space of 8' x 20' x 20', what is the required air volume for a 100,000 btu/hr furnace and an 80,000 btu/hr water heater?

To determine the required air volume for a furnace and a water heater, it's essential to understand the relationship between the BTU output of these appliances and the volume of air needed for combustion. This ensures that there is enough oxygen for efficient fuel combustion and to prevent the buildup of harmful gases.

In cases like this, one common rule of thumb for appliances using gas is to allow for 1 square inch of ventilation per every 1,000 BTUs of output in a confined space. To calculate the total BTUs:

• The furnace outputs 100,000 BTUs.

• The water heater outputs 80,000 BTUs.

Adding these together gives a total output of 180,000 BTUs. Following the guideline, the needed air volume in square inches would be calculated as follows:

180,000 BTUs / 1,000 BTUs per square inch = 180 square inches.

This calculation directly supports the choice identified, as it aligns with the ventilation requirements based on the total BTU input. Thus, the choice indicating 180 square inches is indeed the correct answer, ensuring adequate air for proper combustion and safety.