Furnace Air Flow Requirements

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.

Furnace air flow requirements.

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. 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. 10 times 130 cfm equals 1300 cfm. Although we cannot control the loss of air we can properly diagnose.

This airflow rule is applied to determine furnace airflow for three types of furnaces. Single stage models are like a bicycle with one gear. 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. An insufficient return system or one too large will reduce the efficiency of the heating and increase energy bills.

L q c p ρ t h t r 1 where. Required air volume in an air heating system. Fresh air requirements and furnace operation. Think of heating stages as gears on a bicycle or a car.

Multiply that figure by the factor form the table below depending on the type of furnace to find required system airflow. 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. It s great on a flat stretch of road but takes a lot more energy to go up a steep hill. Required air flow rate in an air heating system can be calculated as.

Divide the rated btu by 10 000. Condensing furnaces require 150 cfm per 10 000 btu input. Today s forced air furnaces are available in single stage two stage or multi stage models. 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.

Gas fired furnaces start with the rated btu input of the furnace. 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. Supply air from furnaces and air conditioners should be on outside walls. Specify a sealed combustion furnace or boiler which will bring outside air directly into the burner and exhaust flue gases combustion products directly to the outside without the need for a draft.

Jim johnson december 22. A furnace return system must collect all the air being put out by that system. We will also learn that a gas furnace requires a substantial amount of fresh air to operate properly. To find required airflow divide the rated furnace input by 10 000 btu.

If the supply ducts are in the floor then the return air should be located up high.