Balancing forced-air registers
Applies to: Assumes a ducted warm-air system with adjustable supply registers and at least one return path per floor.
Last worked on 2014-09-24
Forced-air systems distribute by resistance, exactly as water systems do. The rooms that are wrong are usually the ones furthest from the plant or the ones with the shortest returns.
Supply is only half of it
Every volume of air pushed into a room has to leave it. If the return path is a door undercut and the door is shut, the room pressurises and the supply flow falls off, regardless of how far the register is opened.
This is why a bedroom that is fine with the door open and cold with it shut is a return problem, not a supply problem, and why opening the register further makes almost no difference.
Balance from the extremes inward
Start with every register fully open and identify the two rooms at the ends of the range: the one that overheats and the one that never arrives. Restrict the overheating one first, because that is what makes air available to the rest of the system.
Work in small steps and give the system time between adjustments. Each change alters the resistance seen by every other outlet, so a large correction at one register produces a set of new problems elsewhere.
Seasonal difference is normal
Warm air rises, so a balance set in heating season is usually wrong for cooling and the other way around. Systems with both often have two positions marked at each register, and the seasonal change is part of operating the system rather than a fault in it.
What to check before adjusting anything
- Filter
- A loaded filter reduces flow everywhere and makes balancing meaningless. Replace it and let the system settle first.
- Return path
- Confirm each room can exhaust air with its door closed, by undercut, transfer grille or dedicated return.
- Duct leakage
- Ducts running through unheated space lose both air and heat. Sealing joints changes the balance more than any register adjustment.
- Register obstruction
- Furniture, rugs and curtains over an outlet remove most of its throw. Move them before concluding the outlet is starved.
Balancing is slow and it is finished when the rooms are acceptable, not when the readings match. The measurable target is comfort across the whole floor, not equal flow at every outlet.