Furnace sizes are determined by the quantity of fuel that they consume in an hour. This
limits the choices for installation, and dictates what size furnace to install. If the total
heat loss for the house is 44,600 btu, and you wish to use an oil furnace, then a 56,000
btu furnace should be used. Even though your room totals are 44,600, you must distribute
all 56,000. To do this, the corresponding cfm of the equipment must be known.
If you know both the btu and cfm of your desired equipment, return to the calculator and
enter those output values. Otherwise, the charts below give approximate btu and cfm outputs
for oil furnaces, gas furnaces, hydro-air handlers, and cooling air handlers.
Always design the duct system for the greater cfm requirement if doing heating
and cooling with the same sytem.
If the heating requirement is 800 cfm, while the cooling requirement is 1000 cfm, then
the system must be designed to accommodate 1000 cfm.