| Room | Heat Load | Cool Load | Heating CFM | Cooling CFM | Design CFM |
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Room CFM: the bridge between load calculation and duct design
A room-by-room load calculation tells you how many BTU/hr each room needs. The CFM calculation converts that into airflow — the quantity of conditioned air the duct system must deliver to each room. These CFM values drive every duct branch size, every register selection, and every balancing damper setting. Getting CFM right at this step makes the rest of the duct design straightforward.
The CFM formula
For heating: CFM = Heating load (BTU/hr) / (1.1 x (Supply temp - Room temp)). For cooling: CFM = Sensible cooling load (BTU/hr) / (1.1 x (Room temp - Supply temp)). The factor 1.1 combines air density and specific heat: 0.075 lb/ft3 x 0.24 BTU/lb.F x 60 min/hr = 1.08, rounded to 1.1. See the sensible heat calculator for more on this formula.
Design CFM: heating vs. cooling
In a heating-dominated Canadian climate, the heating CFM for each room is usually larger than the cooling CFM. The design CFM for each branch duct is the larger of the two values — typically the heating CFM for most Canadian rooms. The total system CFM should match the air handler's rated airflow within 10%. If the sum of room CFMs is too high or too low, check your supply air temperatures and room load calculations. Use the room load calculator for room-by-room load inputs.
Frequently Asked Questions
CFM = Room load (BTU/hr) / (1.1 x Supply air DT). For heating: supply air DT = supply air temperature minus room setpoint. For a room with 4,000 BTU/hr heating load, supply air at 120°F, and a 70°F setpoint: CFM = 4000 / (1.1 x 50) = 73 CFM. For cooling: supply air DT = room setpoint minus supply air temperature. For a room with 3,000 BTU/hr sensible cooling load and 55°F supply air: CFM = 3000 / (1.1 x 15) = 182 CFM. Use the larger of heating and cooling CFM as the design value for the branch duct.
Heat pumps deliver supply air at lower temperatures than gas furnaces. At outdoor temperatures above 0°C, a heat pump typically delivers 90-105°F supply air. At outdoor design conditions (below -15°C in cold climates), supply air temperature may drop to 80-90°F. This lower supply temperature means more CFM is needed for the same heating load. At 90°F supply air with a 70°F room setpoint, the DT is only 20°F versus 50°F for a furnace, requiring 2.5 times more CFM for the same load. This is why heat pump duct systems need larger ducts or supplemental heat to avoid excessive airflow and noise.