What size opening is required in the furnace room door for combustion air in a newly enclosed area with a furnace input of 100,000 Btuh and AWH input of 25,000 Btuh?

To determine the required size of the opening for combustion air in a furnace room, it is essential to calculate the total BTU input and apply the relevant standards for combustion air supply.

In this scenario, the total BTU input is the sum of the furnace input and the AWH (automatic water heater) input. Therefore, the total input is 100,000 Btuh (furnace) + 25,000 Btuh (AWH) = 125,000 Btuh.

The standard requirement for combustion air is that you need 1 square inch of combustion air opening for every 1,000 Btuh of total input for appliances operating in a confined space. By dividing the total BTU input (125,000 Btuh) by 1,000, you get the required combustion air opening in square inches:

125,000 Btuh ÷ 1,000 = 125 square inches.

Thus, the opening size required for combustion air to support the appliances in operation is indeed 125 square inches. This ensures that there is sufficient air for combustion, which is critical for the safe and efficient operation of the heating appliances in the space.