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Guide to Air Conditioning & Heating Blower Fan AUTO ON & COOL OFF HEAT Switches on Thermostats
HeatAPedia ^©

• What are the Other Little Switches For on a Room Thermostat: Heat, Air Conditioning like COOL OFF HEAT and FAN ON AUTO ?
• How does the MODE switch work on a digital thermostat to choose among OFF HEAT COOL AUTO ?
• What to do if the air conditioner blower fan runs continuously and won't turn off
• How & why to force an air conditioning or heating fan to run continuously

This article explains the use, setting, and adjustment of the FAN On-Off and MAN-AUTO switches on room thermostats. We provide A Guide to Finding, Using, and Adjusting Thermostats for Heating & Air Conditioning Furnaces & Boilers, Heat Pumps or Electric Furnaces or Boilers.

This website answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect residential heating systems to inform home owners, buyers, and home inspectors of common heating system defects.

The articles at this website describe the basic components of a home heating system, how to find the rated heating capacity of an heating system by examining various data tags and components, how to recognize common heating system operating or safety defects, and how to save money on home heating costs. We include product safety recall and other heating system hazards.

Also see GAS PIPING, VALVES, CONTROLS for more details on how to inspect and test LP and natural gas piping, controls, valves, and tanks.

Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

FAN SWITCH: Guide to Air Conditioning, Heat Pump, or Heating System Blower FAN ON-AUTO or FAN ON-MAN Settings on Wall Thermostats

How does the FAN switch work on a heating or cooling thermostat and how should I use the FAN switch? Details about this switch are at FAN ON AUTO Thermostat Switch where we discuss how to to use it and how to determine how the switch has been set on a digital thermostat. The basics are just below.

The air conditioning or heating wall-mounted thermostat may have a fan or blower control switch with (usually) two set positions "ON" and "AUTO" or sometimes "MAN" or "AUTO"

On these switches located usually on the top, side, or bottom of air conditioning or heating thermostats, the "ON" position is not what you might think. But it's simple, as you'll see.

"ON" on the fan switch will cause the heating or air conditioning fan or blower to run continuously. "MAN" or "manual" is the same as "ON" in this situation. There are advantages of running an air conditioning or heating blower fan continuously - which we discuss at CONTINUOUS BLOWER FAN OPERATION.

"AUTO" on the fan switch on your thermostat is the normal switch position for the fan control. Setting the fan control to "AUTO" will allow the fan to turn on when the air conditioning system (or heating system) are ready to blow cool (or warm) air into the building, and to turn off automatically when cooling or heating are not needed.

Why Run an Air Conditioner or Heating Blower Fan Continuously?

There are reasons to leave the fan in the ON position on a heating or cooling system, but we do not recommend that you do this without first asking for advice from your heating and service technician. Details about continuous operation of heating or air conditioning blower fans are at CONTINUOUS BLOWER FAN OPERATION .

MODE SWITCH: Guide to Air Conditioning, Heat Pump, or Heating System COOL-OFF-HEAT Settings on Room Thermostat Switches

How does the COOL-OFF-HEAT Switch work on a heating or cooling thermostat? Details are just below.

The thing that confuses some building owners is that a thermostat that is used to control air conditioning as well as heating may have a "mode" switch that allows the occupant to change the thermostat between "Cooling" or "Air" or "Air Conditioning" mode and "Heat" or "Heating" mode.

There may be a third position labeled "Off" which instructs the heating or air conditioning system not to run at all.

"COOL" thermostat switch setting position: During the cooling season when you need air conditioning, set this switch to the "COOL" position. This tells the thermostat what to do as temperature rises above the thermostat's setting (turn on the air conditioning system) or when room temperature falls below the thermostat's set temperature (turn off the air conditioner).

It's a good idea not to turn your air conditioning system on in cooling mode when it has been cold outside in the last 24hours. Unless your system is a heat pump designed to switch back and forth in all seasons, or unless a special heater has been installed on your air conditioning compressor, suddenly turning the air conditioning system (and the compressor component of it) on when it has been cold can damage the compressor.

"OFF" switch on the thermostat or on the fan switch, if there is one, will tell the thermostat to keep the equipment it is controlling turned off regardless of whether we're in heating or cooling season. On thermostats usually the "OFF" position is in the middle between "COOL" and "HEAT" positions. Why do you think this might be? It's not smart to switch a heat pump or a combination heating and cooling system right from "COOL" over quickly to "HEAT" or vice-versa.

Imagine driving down the highway with the automatic transmission on your car set to the "D" for "drive" - would you switch the transmission into "R" for "Reverse" at 60 MPH? (No "R" does not stand for "Race"). Well you would only do it once - after which your transmission would perhaps be in the road behind the car.

DON'T DO THIS.

Taking photos for these articles I kept switching my digital thermostat between OFF HEAT COOL AUTO - which led to having no heat at all. I screwed up the control. The "fix": I turned everything off at the service switch at my heater for five minutes to reset the system, turned things back on, and re-set the thermostat to MODE=AUTO and FAN=AUTO.

"HEAT" thermostat switch setting position: During the heating season, when you will need warmth, set this switch to the "HEAT" position. This tells the thermostat what to do when temperatures fall (turn on the heating system) or when temperatures rise to the thermostat setting (turn off the heating system).

MODE BUTTON: Guide to Air Conditioning, Heat Pump, or Heating System COOL-OFF-HEAT-AUTO Settings on Room Thermostat Switches that use a MODE Button

Unlike the round or square wall thermostat shown above, your room thermostat might look like this Carrier wall thermostat shown below (photo at left). Notice the buttons in the closeup (below right)?

The MODE button is at the upper right on this Carrier programmable digital thermostat. Pressing the MODE button repeatedly will cycle the thermostat among these overall thermostat control settings:

OFF - turn everything off - no heating, no cooling, no fan, no nothing

HEAT - put the thermostat in HEATING mode

COOL - put the thermostat in COOLING mode

AUTO - put the thermostat into AUTOMATIC mode - the thermostat will call for the heat to turn on or the air conditioning to come on depending on the temperature set on the thermostat. For example if your thermostat is set to 68 degF and the room temperature drops below 68 degF the heat will come on. If temperature rises above 68 degF the cooling system will come on - presuming your building HVAC system includes both heating and cooling.

In our lab we leave this thermostat MODE set to AUTO.

WARNING: as we explained just above, don't keep cycling among these modes - you might make your thermostat get nauseous.

HEATING and COOLING Air Filtering Suggestions

See these articles about how to use air filters, where to buy them, and how to turn on and off HVAC blower fans

• AIR FILTERS for HVAC SYSTEMS - about air filters
• OPTIMUM INDOOR AIR FILTERS - optimum filter designs for best filtration
• CONTINUOUS BLOWER FAN OPERATION - advantages of continuous air filtration in improving indoor air quality
• AIR FILTER EFFECTIVENESS - how air filter effectiveness is measured
• FIBERGLASS & AIR FILTERS - questions and opinions about fiberglass shedding hazards from air filters
• SOURCES FOR AIR FILTERS - where to buy air filters
• OTHER AIR CLEANERS
• Other Switches on a Room Thermostat - how to set the HVAC blower fan to continuous operation, and what to do if the blower fan won't turn on or won't turn off

What to do if the Air Conditioning or Heating System Blower Fan Runs Continuously and Won't Shut Off?

Some readers have written to say that their air conditioning blower was running continuously and they didn't know why.

If the blower or fan switch is set to MAN or ON, It could be that the FAN switch had simply been set to force the fan to run all of the time. Change the setting to AUTO and see what happens.

Other causes for an air conditioning blower fan that does not turn off could be a problem with the control circuit board for the air conditioning system, or there could be other operating system problems that are preventing the system from cooling air to the desired temperature.

If the blower or FAN switch is set to AUTO and the fan never turns off, call your air conditioning or heating repair company for diagnostic help.

...

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THERMOSTATS
  Types of Building & Room Thermostats
  How Thermostats Work
  Detailed Guide to Room Thermostats
  How to Set the Thermostat
  COOL OFF HEAT Thermostat Switch
  FAN ON AUTO Thermostat Switch
  HEAT ANTICIPATOR Adjustment
  HEAT ANTICIPATOR Mini Ammeter to Check
  HEAT PUMP Thermostats - Outdoors
  INSTALL & WIRE Thermostats
  TEMPERATURE RESPONSE of Room Thermostats
  SWITCH FUNCTIONS on a Room Thermostat

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