Many home heating and cooling systems move air throughout the home using a forced-air distribution system. A gas furnace, oil furnace, and electric furnace all do this, as does a central air conditioner.  Rather than repeat this information in each of the system guides, let's take a look at how air--no matter how it's made hot or cool--can be distributed efficiently.

How Does It Work?

This forced-air system filter is located on a ledge above the blower. Others are accessed through a narrow door in the blower chamber.

A forced-air distribution system simply picks up air from the heating or cooling source, typically a large furnace or air conditioner, and moves it through the house in large pipes called heating ducts. The return system brings cooler air to the heating device. An encased electric fan, called a blower, moves the air through the system as needed.

What Can Go Wrong?

Thermostats and other controls can malfunction. Filters become dirty and blocked. Blower belts get old and need replacing. Motors burn out.

How Can I Identify the Problem?

If the air isn't being distributed, first check the comfort controls to make sure they are working properly. Also make sure that distribution isn't stopped by a blocked filter. Clean or replace filters as needed. Also clean the blower blades.

If the blower is running, but not distributing air, check the blower belt or direct drive (see below); replace a faulty belt (see below).

If the blower motor does not work, test the motor and replace it if it proves faulty (see below).

If the blower is working, but air distribution isn't even throughout the house, balance the system (see below).

Fix-It Tip

A forced-air distribution system can be damaged by pests that build nests in it. Check for such debris, clean it out, and reseal the duct with duct tape (yes, that’s where the name for the tape came from) against future damage.

What Parts, Materials, and Tools Do I Need?

Replacement parts for forced-air systems are available through larger hardware stores, heating system supply houses, and the blower manufacturer. Leaks in ducts are relatively easy to seal using duct tape. Tools and parts you may need include these:

• Screwdrivers
• Wrenches
• Lubricant
• Replacement filter
• Multimeter

What Are the Steps to Fixing It?

Replace a filter:

1. Turn off power to the furnace at the electrical service panel .
2. Find the filter. It is typically near or above the blower motor.
3. Wearing breathing protection, carefully remove the filter. Clean or replace it as needed. Some filters can be washed while others require replacement. Check the owner's manual for specific information and model number.
4. Reinstall the access panel and restore power to the furnace.

Adjust blower-belt tension:

1. Turn off power to the furnace at the electrical service panel .
2. Remove the access panel to the blower and inspect the belt for any cracks or signs of wear.
3. Check for slack in the blower belt by depressing it midway between the pulleys. If there is more than about 1 inch slack, tighten the belt adjustment; if less, loosen the adjustment. Typically, there is an adjustment bolt on or near the pulleys.
4. Reinstall the access panel and restore power to the furnace.

Test and replace a direct-drive blower motor:

1. The blower motor has numerous electrical leads to and from it. Look for an electrical diagram near the blower for connection information or check the owner's manual.

   Turn off power at the electrical service panel or the unit-disconnect switch.
2. Remove access panels to the blower.
3. Remove the motor wire leads, marking them for later reinstallation.
4. Carefully discharge and disconnect the capacitor following the blower manufacturer's instructions. Remember that a capacitor acts like a high-voltage battery that can shock you even when it is disconnected.
5. Remove retaining bolts on the mounting bracket.
6. Test the motor following instructions in the Motor guide. Replace the motor with one of the exact size, model, and rating if needed.

Balance the distribution system:

1. Open all registers throughout the house.
2. Find and open all dampers within the forced-air ducting system. They may be under the floor (in a basement or crawl space) or in a ceiling (in an attic or access area).
3. Place identical thermometers 3 feet from major ducts for 30 to 60 minutes and note temperature differences. To verify, rotate thermometer locations and retest.
4. Adjust the dampers as needed to produce even heat through each room of the house.
5. Once the system is balanced, mark the position of each damper for future reference. If you also have an air conditioning system that uses the forced-air distribution system, turn the furnace off and the AC on to rerun the test. Adjust and mark the dampers for balance during the cooling season.