How Furnace Fans and Blowers Work

    I get many inquiries about Furnace Blowers and fans and how they work.
For definitions see below. For the purposes of this text I will use the word fan and blower interchangeably.
     Most older gas furnaces that have standing pilot systems and most oil hot air furnaces use a thermostat located in the hot air plenum that detects when the furnace is hot and turns on the blower. These thermostats (more commonly called the Fan/Limit control) are usually made by honeywell or white-rodgers. The proper setting for the stops are 90 off, 130 to 150 on, and the limit to the bottom of the slot (lowest setting). As you might guess they also control the gas valve or oil burner if the furnace gets too hot. If you have to replace one of these you will have to measure the length of the probe that extends into the plenum. There are controls made that are just a flush disk that extends into a hole about 1 inch in diameter. These are necessary when the heat exchanger makes it difficult to use a conventional limit.
    Another fan/limit unit is made by Camstat and can have the added feature of an electric heater to cause the fan to come on after a certain period of time even if the furnace hasn't gotten hot enough. This feature is useful on furnaces that have high airflow and would cause the control to cycle after the plenum has reached proper temperature. Honeywell and White-Rodgers also makes one like this too.
    Newer furnaces will have an electronic control board to turn on the blower. This control can be part of the furnace control board or a stand alone box made by one of the popular control manufactures Honeywell, White-Rodgers, Fenwall, Johnson Controls, Robertshaw, Steveco (White-Rodgers), Mars (General Electric), Penn-Baso (Johnson Controls).
    Most all electronic fan controls have adjustments to adjust how long the fan runs and how long it waits to turn on the fan. This is usually a jumper wire that can be moved to stops marked 90, 120 320 ect. seconds of on time after the burner shuts off. The on time is usually not adjustable. When one of these devices fail the only repair is to replace it.
    If you have central air conditioning then you will have a relay added to turn on the blower. Most furnaces use the lower speeds for heating and high speed for cooling. Older furnaces will still use the limit control to turn the fan on low but switching the setting on the thermostat from "auto" to "on" will override the low speed and cause the blower to run on high. Newer furnaces have a central control board to change speeds.
    I recommend the nationwide chains Grainger or Johnstone as a source of parts. Written by Scott Meenen n3sjh.

Images of a thermodisk fan control (limit controls look like this too) used on some furnaces where there is not enough room for a probe type control.
Most older furnaces use a probe type control which is shown below.
Note: Electric furnace sequencers look somewhat like this
and clothes dryer thermostats are almost identical.

Description of this control

Image of Camstat (brand name) probe type fan control.
This control has an assist heater (top terminals) to make the fan come on after a predetermined period of time with the gas valve or oil burner.
This type of control is especially useful in a situation where the furnace has plenty of air flow and would shut the control off soon after the fan starts.
The settings on the dial is the temperature that the control will shut off.

Definitions:

Blower: usually a squirrel cage centrifugal air moving device. Will move large volumes of air relatively quiet. Will use less energy with more back pressure.
Fan: a paddle type air moving device used where noise is not a major consideration. Will use more energy with more back pressure.
Combustion blower: A blower used on high efficiency gas furnaces or oil burners to move combustion air. usually 1/20 to 1/6 horsepower.
OEM: Original equipment manufacturer.
Horsepower: 746 watts
RPM: revolutions per minute.
Service factor: the extent to which a motor can be safely overloaded beyond its name plate ratting without over heating.
Air over horsepower: The rating of a motor assuming air flow through the windings usually as a result of the air moving device.
SAE: Society of Automotive Engineers.
High efficiency furnace: Furnace that uses over 85% of the energy in the gas.
Condensing furnace: Gas furnace that uses over 92% of the energy in the gas and condenses the gas into liquid condensate and hot air. If your furnace has PVC pipe venting it then you have a condensing furnace.

This page will be updated soon
Good Luck Scott

Written By: Scott Meenen N3SJH of:
G&S MECHANICAL SERVICES.
Specializing in Mechanical, Controls and Electrical Modifications Of
Heating, Air-conditioning, Refrigeration, Cold storage,
Ice Production and Food preservation. Anything having to do with Heat and Energy.
Serving MD, DC, and Northern VA.
Contact us by pager: 1-877-467-2914 Pager Id: 32505
Page us by e-mail 4103560613.3732505@pagenet.net 240Chrs max.

Email us at: jsmeenen@toad.net

This text written by: Scott Meenen * G & S Mechanical

Go to or return to the G&S Mechanical home page.
For a list of all files on this site go to the site map.