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Forced Warm-Air Heat

Caring for the Forced Warm-Air Heating System
Balancing the Heat
Setting the Fan Control


Low installation cost, fast heat delivery, and reliability make forced warm-air systems a popular heating choice. The system is versatile and lends itself to the addition of central air conditioning. In this system, a blower pulls air from the rooms into the cold-air return and return duct, through a filter, and into the furnace. There the air is heated. It then flows back to the rooms through the warm-air ducts and registers.

Caring For the Forced Warm-Air Heating System

To ensure trouble-free operation, service the system as follows:

  • Clean or replace the filter monthly during the heating season.
  • Brush and vacuum heat exchanger surfaces annually (see owner's manual for instructions).
  • Clean the blower blades at the start of each heating season. Add a few drops of motor oil to each oil cup if your blower is equipped with them.
  • Check and adjust the belt alignment and tension if your furnace has a belt-driven blower. To replace a worn belt, loosen the motor adjustment bolt, remove the old belt, and attach a new one.
  • Examine the ducts annually for leaks; seal any leaks with duct tape.

Balancing the Heat

  • If some rooms are too hot or cold, try adjusting the dampers in the registers and, if your system has them, the dampers in the warm-air ducts.
  • Leaving the thermostat at one setting, let the system run for 3 hours to stabilize the temperatures. Open the dampers wide in the coldest rooms. Then adjust the dampers room by room until temperatures are balanced. Wait half an hour after each adjustment before rechecking or readjusting.
  • Speeding up the blower may help heat chronically cold rooms. Adjust the motor pulley of a belt-driven blower or, for a direct-drive blower, change the electrical connections (see owner's manual).

Setting the Fan Control

If you're chilled by a blast of cool air when the blower turns on, try adjusting the fan control.

  • CAUTION: If your furnace has a combination fan and limit control, do not touch the pointer on the limit control side. This pointer turns off the furnace if the maximum allowable air temperature is exceeded.
  • As the blower turns on, hold your hand in front of the warm-air register farthest from the furnace.
  • Ideally, your hand should feel neither cooler nor warmer.
  • If it feels cooler, uncover the control and move the fan control's ON pointer a few degrees lower.
  • To adjust the temperature of the air coming out of the registers, move the ON pointer to set the temperature at which the blower turns on, the OFF pointer to set the temperature at which it turns off.
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