Why is one room always cold in winter even though the furnace works fine?

A room that stays cold while the rest of the house is comfortable almost always has an airflow problem, not a heat loss problem. Either the supply duct to that room is undersized, has too many fittings creating high pressure drop, has a closed or partially closed damper, has a disconnected or leaking duct section, or the register is blocked or too small. Use a register airflow hood or anemometer to measure actual CFM at the register, then compare to the design CFM from your load calculation. If measured CFM is less than 60% of design, you have a duct problem to fix, not an insulation problem.

How do I balance airflow between rooms in my house?

True air balancing requires measuring airflow at every register with an airflow hood or anemometer, then adjusting volume dampers in each branch duct to hit the design CFM for each room. Start by measuring all registers at current settings. Calculate design CFM for each room from its heating and cooling load. Compare measured to design. Throttle dampers in over-supplied rooms (reducing their flow forces more air to under-supplied rooms). Never fully close a damper — minimum 10-15% open to prevent coil freeze and excessive static pressure. Re-measure after each adjustment. This process takes 2-4 hours for a typical house.

⚖ Air Balancing

Supply Air Balancing Calculator

Enter design CFM and measured CFM for each room to identify which rooms are over or under-supplied. Shows exactly how much each register needs to be adjusted and flags rooms with serious airflow deficits causing comfort complaints.

Unit System:

⚙ System Parameters

Total Design System CFM *

CFM

Measured Total CFM (optional)

CFM

Acceptable Tolerance

ASHRAE recommends ±15% of design CFM

🏠 Room-by-Room Airflow

Room Name

Design CFM

Measured CFM

Status

⚖ Air Balance Results

Room	Design CFM	Measured CFM	Variance	Status	Action

Export:

Air balancing: the last step every HVAC installation skips

In Canada, fewer than 5% of residential HVAC installations include any form of air balancing after commissioning. The system is turned on, it heats and cools the house to setpoint, and the contractor leaves. But "heats to setpoint" in the thermostat location is not the same as comfortable temperatures in every room. Master bedrooms on the far end of the house, bonus rooms above garages, and rooms with long duct runs are almost always under-supplied — and the homeowner lives with it for years, adding space heaters, closing doors, and complaining about the system.

What ASHRAE says about air balancing

ASHRAE Standard 111 (Measurement, Testing, Adjusting, and Balancing of Building HVAC Systems) requires that all supply outlets be adjusted to within 10% of design CFM for commercial systems and 15% for residential. In practice, residential installers in Canada rarely measure airflow at registers at all. A simple anemometer or flow hood test on commissioning day, combined with this calculator, takes 45 minutes and identifies every room that will generate a comfort complaint before the homeowner moves in.

When damper adjustment isn't enough

If a room is receiving less than 50% of design CFM and all dampers are fully open, the problem is the duct itself — undersized, kinked flex, blocked with debris, disconnected, or poorly designed. Damper balancing only redistributes the available pressure; it can't fix a fundamentally undersized branch. Use the air velocity calculator and friction loss calculator to diagnose whether the duct can physically deliver design flow.

Frequently Asked Questions

How do I measure airflow at a register without professional equipment?+

A basic anemometer (digital wind speed meter) costs $30-60 and is accurate enough for balancing work. Hold it centered at the register face, take readings at 4-5 spots across the face, and average them. Multiply the average velocity (FPM) by the register free area (in ft²). Free area is typically 60-70% of the face area for residential registers. A 10x4 inch register with 65% free area has 0.028 ft² free area. At 500 FPM average velocity: CFM = 500 x 0.028 = 139 CFM. A proper flow hood is more accurate and costs $400-800, but the anemometer method is within 10-15% accuracy for balancing purposes.

Can I balance my system in summer and have it stay balanced in winter?+

Yes, with one caveat. If your system uses the same ductwork for both heating and cooling (standard forced air), damper settings that work in summer will generally work in winter too, because airflow distribution is determined by duct geometry and damper positions — not by whether you are heating or cooling. The only adjustment needed seasonally is for rooms with very different heating vs. cooling loads (like a sunny south-facing room that needs lots of cooling CFM but relatively little heating CFM). Some contractors install separate summer/winter balancing damper positions for these rooms, but this is rarely done in residential practice.