If replacing an old Natural Gas Fired Furnace that was firing at 140 MBTUH, what size furnace should be installed if the heat loss calculations allow for 45 BTUH per square foot for a 1200 square foot house?

To determine the appropriate size of the furnace to install, one must first calculate the total heat loss for the house based on the given rate and area. The heat loss calculation indicates that 45 BTUH is required for each square foot of the house. For a 1200 square foot house, the total heat loss can be calculated as follows:

Heat loss = 45 BTUH/sq ft × 1200 sq ft = 54,000 BTUH

Next, to convert this value into MBTUH (million BTUs per hour), it can be divided by 1,000,000:

54,000 BTUH ÷ 1,000,000 = 0.054 MBTUH

However, since MBTUH is often expressed in thousands (or M), it's more practical to express 54,000 BTUH as 54 MBTUH, which doesn't fit the provided options. Consequently, the closest standard size furnace option should be chosen, and the sizing often factors in a safety margin for fluctuating conditions and system efficiencies.

In the choices given, 75 MBTUH is the correct answer because it provides sufficient heating capacity while also allowing for some additional capacity to ensure comfort and efficiency in various weather conditions.