Balancing a Forced Air System
Balancing a Forced Air System

About this Project

If some rooms in your home are consistently too hot or cold, don't blame the furnace. Instead, take on a simple "balancing" project.

To balance a forced-air system, you reduce the airflow to a room that is too warm. Warm air then reaches colder areas, typically those farthest from the furnace.

You already may have tried balancing by partially or totally closing registers in the hotter rooms. This cools the room off, but it doesn't redirect the air. Instead, look for dampers in the ductwork. Dampers are controlled by a handle or a locknut arrangement (see next page). You may find one at the point where each duct takes off from the furnace plenum. Not all duct systems have dampers, so consider installing some.

Identify which ducts serve each room, and label their dampers. Close them one at a time to determine which room isn't getting air.

Wait for a cold day to begin the balancing procedure shown on the next page. If only one or two rooms have airflow problems, you might be tempted to adjust only the dampers. Because balancing is a robbing-Peter-to-pay-Paul proposition, however, you'll get better results by tuning the entire system.

Time: 3 to 4 hours

Skills: Basic skills.


Getting Ready
1. Open all registers; open dampers by loosening locknuts and turning the damper handles parallel to the duct.

2. Synchronize several thermometers by laying them together for 30 minutes and noting differences.

3. Place a thermometer in each room, 3 feet above the floor but not above a supply register.

Making Adjustments
4. Begin to partially damper rooms, beginning with the one where your home's thermostat is located.

5. Check the thermometers in each room for changes. Continue to adjust dampers until all rooms are approximately the same temperature.

Note: If you have central cooling, you may need to rebalance the system in summer. Mark the seasonal position of each damper to make these changes easy.

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