Furnace Air Flow Requirements

Natural draft furnaces 100 cfm per 10 000 btu of rated btu input induced draft furnaces 130 cfm per 10 000 btu of rated btu input.

Furnace air flow requirements.

10 times 130 cfm equals 1300 cfm. When this outside air is drawn into the structure while the furnace is operating in the heating mode the air that s coming is a going to be cold. This is called ventilation air. Multiply that figure by the factor form the table below depending on the type of furnace to find required system airflow.

L q c p ρ t h t r 1 where. 100 000 divided by 10 000 equals a factor of 10. The return air vent openings need to be on the opposite side of the room so the conditioned air is pulled across the room. To find required airflow divide the rated furnace input by 10 000 btu.

Fresh air requirements and furnace operation. Single stage models are like a bicycle with one gear. This air is used in combustion and as a re placement for the air that a modern home loses through outward flow due to mechanical ventilation. Required air flow rate in an air heating system can be calculated as.

And that means that we need to account for that additional load on the furnace when it s sized so that it will be able. We will also learn that a gas furnace requires a substantial amount of fresh air to operate properly. In milder climates with lower annual heating costs the extra investment required to go from 80 to 90 to 95 efficiency may be hard to justify. L air flow rate m 3 s q heat loss covered by the air heating system kw c p specific heat air 1 005 kj kg o c ρ density of air 1 2 kg m 3 t h heating air temperature.

Required air volume in an air heating system. Think of heating stages as gears on a bicycle or a car. It s great on a flat stretch of road but takes a lot more energy to go up a steep hill. If the supply ducts are in the floor then the return air should be located up high.

Supply air from furnaces and air conditioners should be on outside walls. Condensing furnaces require 150 cfm per 10 000 btu input. Gas fired furnaces start with the rated btu input of the furnace. Jim johnson december 22.

A furnace return system must collect all the air being put out by that system. Today s forced air furnaces are available in single stage two stage or multi stage models. An insufficient return system or one too large will reduce the efficiency of the heating and increase energy bills. This airflow rule is applied to determine furnace airflow for three types of furnaces.

If the furnace is putting out 1 000 cubic feet per minute of air flow the return system must collect 1 000 cubic feet and take it back to the unit.