How to Troubleshoot a Forced-Air Distribution System
How to Troubleshoot a Forced-Air Distribution System
by Walter Curtis

Forced-Air Distribution System Overview

Fueled by gas, electricity, or oil, a forced-air distribution system is just what the name implies. Air is forced from the furnace through ducts to registers in various rooms. Besides warming the air, the blower system that distributes the warmed air also returns the cold air to the furnace so it can be rewarmed and distributed to the rooms again.

A forced-air distribution system uses a blower to distribute warmed air and to
return cold air to the furnace so it can be rewarmed and distributed again.

A forced-air system is also efficient for distributing cool air from a central air conditioner with the same ducts, registers, and blower. There is little that can go wrong with a forced-air system. The big problems typically include noise and blockage of airflow, usually caused by dirt or by furniture or draperies blocking the registers. Forced-air systems should be cleaned and maintained regularly.

Floor registers are slip-fit into ducts or are held by retaining screws on the frame of the register. Wall and ceiling registers are also held in place by retaining screws on the frame of the register. Duct joints are usually slip-fit and held with sheet-metal screws or duct tape. The ducts are supported by wire or metal strap hangers nailed or screwed to wooden framing members such as studs and rafters. All of these parts are easy to disassemble. Lay them out in order as you work so you'll be able to reassemble them properly.

A problem can arise with this type of system in which the temperature of different rooms varies widely. Find out how to solve this problem in the next section.
Balancing a Forced-Air Distribution System

Forced-air systems often go out of balance, causing some of the rooms in a home to be too hot or too cold. The furnace is usually not to blame; the problem is caused by ducts and registers that are not properly set. You should balance the system while the furnace is turned on.
To balance a forced-air system, open all the ducts and registers in the system. There may be dampers in various ducts that need to be turned to open the ducts. The damper is open when it's turned parallel with the top and bottom of the ducting.

Next, gather six or seven thermometers and get them all to have about the same temperature reading. You can do this by laying out the thermometers together for about 30 minutes and then noting any discrepancies. Tape the thermometers on the walls of each room so each thermometer is about 36 inches up from the floor, away from the hot air register or cold air return. Wait one hour, then take a thermometer reading in each room when the heat is on. If one room shows a higher temperature than an adjoining room, close the damper or register slightly in the hotter room.

Follow this procedure for each room in your home, opening and closing dampers and registers until the same temperature is maintained in each room or the temperature balance you want is reached. The thermostat to the furnace should be kept at the same reading while you balance the system.

It's also important to know how to improve the flow of warm air in your house, so check out the tips mentioned in the next section.

Adjusting the Blower Speed

An increase in blower speed can sometimes improve the flow of warm air through your home. A decrease can make the system quieter. You can increase or decrease the blower speed by slightly adjusting the pulley on the blower drive motor.

To adjust the blower speed, loosen the setscrew that holds the pulley
to the drive shaft, and turn the pulley on the shaft.

To increase blower speed, slightly loosen the setscrew that holds the pulley to the driveshaft. Move or turn the pulley clockwise on the shaft one turn, then tighten setscrew. If more speed is desired, turn the pulley clockwise two turns.

For decreasing blower speed, loosen the setscrew that holds the pulley to the driveshaft. Move or turn the pulley counterclockwise on the driveshaft one turn, then tighten the setscrew. If less speed is desired, turn the pulley counterclockwise two turns.

To adjust the blower speed, loosen the setscrew that holds the pulley
to the drive shaft, and turn the pulley on the shaft.

The motor and blower pulley may also get out of alignment. This causes the blower to be noisy and cuts down on the efficiency of your distribution system. To check alignment, place a carpenters' square against the outside of the motor and blower pulleys. The pulleys should be in a straight line and at right angles to the motor shaft.

If the pulleys are not lined up at right angles to the motor housing, loosen the setscrew holding the motor pulley and move the pulley backward or forward as needed to align it properly. If the setscrew is jammed or rusted and won't loosen or if the pulleys are out of alignment, loosen the mounting bolts on the motor and slide the motor backward or forward until the pulleys are properly aligned.

Noise and heat loss from ducts are common problems with forced-air distribution systems. In the next section, we'll discuss how to solve these issues.

Noise Problems and Heat Loss

Forced-air distribution systems sometimes are noisy or have heat loss problems. Solve these common problems using the tips listed below.

Noise Problems

Air forced through the ducts of a forced-air system can cause vibration and noise if the ducts are not firmly connected. The best way to stop this noise is to add duct hangers to the ducting system. The hangers are usually wrapped around or across the ducting and nailed or screwed to the stud or rafter framing.

At the elbows of the ducts, where air moving through the ducts changes direction, the duct sections can become loose or separated. Push loose sections back together, and tape the joints firmly with duct tape to stop vibrations.

Noise can also be caused by inadequate lubrication, worn or damaged belts, or too high blower speed.

System Care Guide

The problems that can cause your forced-air distribution system to malfunction can get complicated.
Heat Loss From Ducts

If ducts run through cold basements or exterior crawl spaces, wrap the ducts with fiberglass insulation. Spiral insulation around the ducting and secure it with duct tape, wire, or heavy cord. Or, wrap the ducts with aluminum-face insulating tape, sold in wide rolls and available at heating supply stores.

When a forced-air system malfunctions, it can seem like a daunting challenge. But the tips mentioned in this article will help you solve the crisis yourself.

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