How to set the room thermostat switches: this article explains the use, setting, and adjustment of the FAN On-Off and MAN-AUTO switches on room thermostats.
We explain why you might want to leave the fan ON continuously, and we review use of the MODE switch to change between COOL, OFF or HEAT. We also explain what to do if your air handler blower fan won't turn off and keeps running continuously.
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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.
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.
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).
"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.
Watch out: 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 keep switching your air conditioner, heat pump, or heating system on and off rapidly.
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.
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. Also
see Heat Won't Turn Off - Stop Unwanted Heat explanation of why heat may continue to come out of radiators or baseboards even though you have turned down the thermostat. Stop wasted heat by making sure you are not delivering heat when it is not needed or not wanted.
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 °F and the room temperature drops below 68 °F the heat will come on. If temperature rises above 68 °F 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.
Details about this switch are
at FAN ON AUTO MAN Thermostat Switch
Here we explain 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.
There are reasons to leave the fan in the ON position on a heating or cooling system, such as for more even indoor temperature distribution or improved indoor air filtration. But we do not recommend that you do this without first asking for advice from your heating and service technician. Not all blower fan motors are designed for continuous operation.
Details about continuous operation of heating or air conditioning blower fans are
at CONTINUOUS BLOWER FAN OPERATION .
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 on the Room Thermostat 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. Details about this switch are
at FAN ON AUTO MAN Thermostat Switch
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 at the Limit Switch in the Air Handler Unit is set to MAN or ON, that switch will cause the air handler unit's blower to operate continuously. More details about the fan control on the limit switch in the air handler are |
at FAN WONT STOP - LIMIT SWITCH.
At CONTINUOUS BLOWER FAN OPERATION we explain how at either the wall thermostat or at the switch in the air handler switches can be set to keep the blower fan on. Sometimes we want to do this for improved air filtration or more even temperature control in a building.
If after checking the switches above 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, or if you've checked the switches we describe above and you want to diagnose the fan yourself, go to our blower fan diagnostic home page
at FAN, AIR HANDLER BLOWER UNIT.
Other snafus such as a shorted thermostat wire can also cause the A/C or warm air furnace fan to run continuously. Those problems are explained
at THERMOSTAT WON'T TURN OFF.
For hot water heat or steam heat that won't stop,
see HEAT WON'T TURN OFF for an explanation of why heat may continue to come out of radiators or baseboards even though you have turned down the thermostat. Stop wasted heat by making sure you are not delivering heat when it is not needed or not wanted.
A friend's a/c unit has the blower fan that will only work in auto, it will not work on manual. On the other hand, even when you turn the unit from cool to off, the blower still remains on.
The only way the blower will turn off, is to turn the fan to manual.
He changed the thermostat, and the unit will cool to the desired temp and kick off, but you can't turn the fan off on your own unless you turn it to manual. Anyone ran into this before? - John 8/17/11
The wiring to the unit depends on if you have a heat pump, or straight a/c unit. It also depends on how the installer has wired the thermostat. With the heat pump, the red is power, the yellow is for cooling, the white is for heat, the green is for the fan, the orange is for the reversing valve, and the blue is usually always common.
Again, you will have to check to see how the installer has ran the wires from the thermostat. If you have a a/c only unit outside, you will only have a red wire, and a white wire coming form the inside unit, to the outside unit. These will go on the contactor to send 24 volts to the coil to pull the contactor in. These are the traditional wiring schematics, again, check the thermostat to see if this is the case with yours.
Bryan thanks for that succinct summary of usual thermostat wiring details and color codes. As it will help other readers I'll add it at "INSTALL & WIRE Thermostats" - if you want to be credited along with contact information let me know by using our website CONTACT link that displays our email addresses. Best, Dan. Also, readers with thermostat wiring questions should
see THERMOSTAT WIRE CONNECTIONS - Ed.
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