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 switch 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 (Shown below). The typical settings for the stops are 70 to 90 "Off", 130 to 150 "On", and the "Limit" to the bottom of the slot (lowest setting) as long as you don't get nuisance shutting off of the fire. As you might guess they also control the gas valve or oil burner if the furnace gets too hot. If you have problems with the blower not shutting off, set the "Off" setting to the bottom then after the burner cycles wait about 3 to 4 minutes and hold the dial and raise the stop until it shuts off. Run the burner several more cycles to make sure it works reliably (use your best judgment). 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.

Conventional fan/limit made by Honeywell. This one has an assist heater.
Image of Conventional Fan/Limit made by Honeywell (Robertshaw looks similar). This one has an assist heater as evident by the red and yellow wires in the center. A normal control will not have these wires!

    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 switch from "auto" to "on" will override the low speed and cause the blower to run on high. Most newer furnaces have a central control board to change speeds.

    Nuisance problems: The following are problems that are more annoyances than functionality problems and should be addressed as such: As a general rule it is ok for the fan to run if it doesn't need to. It is not ok for the fan not to run if it should be on.
  1. Blower won't shut off. There are several reasons why the fan won't shut off but rule #1: is unless you are sure what is wrong with it, don't F--- with it until Monday morning when the supply houses are open and you can get parts. Rule #2 if you are in airconditioning mode see rule #1.
  2. Blower comes on in high speed right away on your gas or oil forced air furnace. This is caused by the "G" terminal on the thermostat being activated most likely because it is an electric heat thermostat or a dual mode thermostat set in electric heat mode. (see rule #1 above).
  3. Blower comes on one more time after a heat call then shuts off. Solution: raise the "ON" temperature on the control shown above. If the control is being difficult to move see rule #1 above!
  4. You have had some work done to your gas or oil furnace and the air coming out is not as hot as it used to be. This is caused by the fan having been moved to a higher speed for the heat mode. While the air isn't as hot as it was before there is no down side to this other than the mind-set that hotter is better. In fact the furnace will be more efficient. But if you feel the need for hotter air move the fan to a lower speed. See rule #1 above.
  5. The blower shuts off at some time in the heat cycle but comes back on again reliably. Lower the "Off" setting on the fan control. See Above if it is a conventional control. Replace your module if it is an electronic control (this is a problem that needs correcting).
  6. You have a heat pump that uses a heat sequencer for the fan control and it doesn't always come on. Replace the heat sequencer if it is a week day, install a relay (if you have one) if it is a week end. (this is a problem that needs correcting)
  7. You have a Trane heat pump and the (honeywell) delay module goes bad (Fan won't run). Rewire it so the Common and Green wires activate the fan relay then decide what you will do with it once you get a price for that OEM. module. (This is a problem that needs correcting). If fan won't shut off read and heed rule #1 again.


    I recommend the nationwide chains Grainger or Johnstone as a source of parts. Written by Scott Meenen n3sjh.

furnace side of thermo-disc fan blower control setting side if thermodisk thermostat for furnace fan
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 and high limits are almost identical.
Description of this control
Camstat furnace fan blower thermostat with heat assist Long view of camstat blower fan 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. Camstat makes a version that has no assist heater.
The settings on the dial is the temperature that the control will shut off the fan. 


    Definitions:
  1. 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.
  2. Fan: a paddle type air moving device used where noise is not a major consideration. Will use more energy with more back pressure.
  3. Combustion blower: A blower used on high efficiency gas furnaces or oil burners to move combustion air. usually 1/20 to 1/6  horsepower.
  4. OEM: Original equipment manufacturer.
  5. Horsepower: 746 watts
  6. RPM: revolutions per minute.
  7. Service factor: the extent to which a motor can be safely overloaded beyond its name plate ratting without over heating.
  8. Air over horsepower: The rating of a motor assuming air flow through the windings usually as a result of the air moving device.
  9. SAE: Society of Automotive Engineers.
  10. High efficiency furnace: Furnace that uses over 85% of the energy in the gas.
  11. 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.
Related Pages:
    Wiring colors.
    Fan Motor Replacement.
    Problem solving.
    FAQ.
    More F.A.Q.
    More definitions.
    Gas burner system page.
    Oil heat page.
    Heat pump repair page.
    Heat pump defrost cycle.
    What is a combustion blower motor system?

    This page will be updated soon
 Good Luck Scott

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Written By:  Scott Meenen N3SJH of:
G&S MECHANICAL SERVICES.
Specializing in Mechanical, Controls and Electrical Modifications Of
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                 This text written by: Scott Meenen * G & S Mechanical

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