What does a complete duct design include?

A complete duct design includes: room-by-room CFM from room-by-room heating and cooling loads and supply air temperatures, available static pressure from fan ESP and component losses, total effective length of the longest duct run, design friction rate, branch duct sizes for each room, trunk duct sizes for each section, register and grille sizes, and supply/return balance verification. The design ensures every room receives its design CFM simultaneously when the system operates at design conditions. This tool generates a simplified duct schedule covering all these elements.

Do I need room-by-room loads before doing duct design?

Yes. Duct design uses room CFM values derived from room-by-room heating and cooling loads. Without accurate room-by-room loads, you cannot determine how much airflow each room needs. Use the room load calculator or heat load calculator to get room-by-room BTU/hr values, then convert to CFM using the CFM calculator before using this duct system designer. The total system CFM from all rooms must match the air handler's rated airflow within 10% for the design to be valid.

🌸 Complete Duct Design

Duct System Designer

The complete duct design tool. Enter your system parameters and room loads to generate a full duct schedule — branch sizes, CFM per room, velocity checks, and supply/return balance. Start with room loads from the room load calculator and fan data from the available static pressure calculator.

⚙ System Parameters

Fan Rated ESP *

IWG

Total Component Losses i

IWG

Longest Run TEL *

Room Setpoint

°F

Heating Supply Temp

°F

Cooling Supply Temp

°F

🏠 Room Loads

Room

Heat Load (BTU/hr)

Cool Load (BTU/hr)

Duct Type

Max TEL (ft)

🌸 Duct System Schedule

🌸 Branch Duct Schedule

Room	Heat Load	Cool Load	Heat CFM	Cool CFM	Design CFM	Round Duct	Velocity	Status

Export:

Duct design: the workflow every HVAC contractor needs

Most residential HVAC systems in Canada are installed without a formal duct design. The contractor runs ducts based on experience and available space, sizes branches by rule of thumb, and calls it done. The result is a system that works adequately at the thermostat but leaves bedrooms cold in winter, rooms uncomfortable in summer, and generates noise complaints from over-velocity branch ducts. A proper duct design takes 2-3 hours for a typical house and produces a duct schedule that specifies every duct size, every CFM, and verifies that the system will balance correctly on day one.

The five-step duct design process

Step 1: Complete room-by-room heating and cooling loads (use the room load calculator). Step 2: Calculate room CFM from loads and supply air temperatures (the CFM calculator). Step 3: Calculate available static pressure and design friction rate (the available static pressure calculator). Step 4: Calculate total effective length for the longest duct run including fittings (the equivalent length calculator). Step 5: Size every duct at the design friction rate — which is exactly what this tool does. The output is a duct schedule you can hand to a sheet metal fabricator.

Frequently Asked Questions

How accurate is this duct system designer for permit applications?+

This tool generates a duct schedule based on the inputs you provide, following industry-standard friction rate methodology. The accuracy depends entirely on the quality of your room-by-room loads, your fan data, and your TEL measurement. If you have accurate loads from the heat load calculator and accurate TEL from the equivalent length calculator, the output duct sizes will be correct per standard design principles. For permit submission in Canadian jurisdictions that require formal duct design documentation, export the schedule and include your load calculation summary, fan data sheet, and TEL calculation. Some jurisdictions require a licensed engineer's stamp — check with your local AHJ before submitting.

What if my total room CFM doesn't match my air handler rating?+

If the sum of room CFM differs from your air handler rating by more than 10%, you need to reconcile the discrepancy before proceeding. If room CFM sum is higher than the air handler: your loads may be overestimated, or you need a larger air handler. If room CFM sum is lower: your loads may be underestimated, or you can select a smaller, more efficient air handler. Do not simply scale CFM values to match the air handler — this will result in rooms with incorrect airflow. Use the heat load calculator to verify your loads and select equipment with the furnace sizing calculator before finalizing the duct design.