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AIR HANDLER / BLOWER UNITS
AIRBOUND HEAT SYSTEM REPAIRS
BACKDRAFTING HEATING EQUIPMENT
BACKUP HEAT for HEAT PUMPS
BLOWER FAN OPERATION & TESTING
BOOKSTORE - InspectAPedia
CIRCULATOR PUMPS & RELAYS
CONVECTOR HEATERS - HYDRONIC COILS
COOL OFF HEAT, Thermostat Switch
DEFINITION of Heating & Cooling Terms
DIAGNOSE & FIX AIR CONDITIONER / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DUCT SYSTEM & DUCT DEFECTS
ELECTRIC HEAT, DIAGNOSIS, REPAIR
ELECTRIC MOTOR OVERLOAD RESET SWITCH
ELECTRICAL POWER SWITCH FOR HEAT
FAN, AIR HANDLER BLOWER UNIT
FAN AUTO ON Thermostat Switch
FAN, COMPRESSOR/CONDENSER UNIT
FAN CONVECTOR HEATERS - HYDRONIC COILS
FAN LIMIT SWITCH
FLOODED HEATING EQUIPMENT REPAIR
HEATING LOSS DIAGNOSIS-BOILERS
HEATING LOSS DIAGNOSIS-FURNACES
HEATING SYSTEM INSPECT DIAGNOSE REPAIR
LOW VOLTAGE BUILDING WIRING
LOW VOLTAGE TRANSFORMER TEST
MANUALS & PARTS GUIDES - HVAC
NO HEAT - BOILER
NO HEAT - FURNACE
ODORS FROM HEATING SYSTEMS
OIL FILTERS on HEATING EQUIPMENT
PULSE COMBUSTION HEATERS
Reset Switch - Heater Primary Control
Reset Switch Broken - Quick Repair
RESET SWITCH - ELECTRIC MOTOR
Reset Switch - Stack Relays
STEAM HEATING SYSTEMS
THERMOSTATS, HEATING / COOLING
THERMOSTATS, WATER HEATER
THERMOSTATIC EXPANSION VALVES
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. This article also explains how to troubleshoot low-voltage transformers used in HVAC equipment.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
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 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, 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
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.
How to Use the "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.
How to Use the "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).
"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.
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.
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. 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 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.
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, 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.
For hot water heat or steam heat that won't stop, 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.
Question: How do you tell if a transformer is bad? How do you test a low voltage transformer?
How do you test a low voltage transformer? - Den 7/16/12
Reply: how to test a low voltage transformer for heating or air conditioning
Erwin if you disconnect the low voltage wires connected to the transformer terminals and use a VOM you should find voltage, usually 14V. If the device looks burned, hot, or smells, don't even bother testing. If it has been buzzing that's another clue.
Remove all of the the low voltage wires from the two (or more) screws on the top of the transformer. Usually you'll see a small gauge red and white wire simply connected under two screws on the upper surface of the transformer itself.
And thank you for the question - we have answered your question more extensively and have added details about low voltage transformers used on heaters and air conditioners and how the transformer is located, wired, and tested at LOW VOLTAGE TRANSFORMER TEST.
To test for a bad low voltage transformer you simply need a volt-ohm meter or a digital VOM. See USING DMMs VOMs SAFELY for advice on how to use a volt-ohm meter, and see Guide to Electrical Test Equipment for advice on how to electrical test equipment safely.
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. Because some controls are used in common on hot water heat, hot air heat, and steam boilers, readers should see these other articles: see BOILER CONTROLS & SWITCHES, and also see BOILER COMPONENTS & PARTS for a detailed list of heating boiler controls, other heating system components, parts such as circulator pumps & draft regulators. If your building uses warm air heat, see FURNACE CONTROLS & SWITCHES. If your building uses steam heat see STEAM HEATING SYSTEMS. Also see Heat Won't Turn Off - Stop Unwanted Heat.
Frequently Asked Questions (FAQs) About the COOL OFF ON, FAN OFF, FAN ON, AUTO, MAN switches on thermostats
Question: old Carrier Nova air conditioner that stopped working.
We have an old Carrier Nova air conditioner that stopped working.
Peter, I'd start with the diagnostic suggestions found at DIAGNOSE & FIX AIR CONDITIONER / HEAT PUMP
Sorry to read about such aggravation but at this point you need a service call from an experienced tech. Call someone else, talk about your concerns with the service manager, don't bad-mouth the prior tech or the new ones may figure you're hard to work for. Just explain that prior attempts to get things running didn't work and that tech gave up. Keep us posted.
For a single-speed fan, Peter, at the Thermostat turn the FAN ON switch to ON to see the fan run; Or on some air handlers that combine heating and cooling there should be a fan limit switch that may include a white knob that can be pulled our pushed to turn on the fan.
We have a new LG aircond 8month old it have runs only 5 minutes it becomes overload and compressor disconnected internally Jitu 8/11
Overloaded AC Compressor going off on reset? Don't keep trying to run it or you risk further damage or even a fire. You certainly need a service call from a professional.
Question: fan will not turn on and air conditioner won't turn on
I have central air and heat and the heat works fine and blows perfectly but the fan will not turn on and the air conditioner wont turn on. I recently replaced the thermostat. do you think it could be faulty? - Warren Wirth 7/12/11
Warren: first be sure that your new TT is in COOL mode and then try the fan in ON position to see if you can force it to run - if so move the switch to AUTO. Set the TT below room temp; If nothing is happening I'd double check the thermostat wiring and confirm that there is voltage on the TT wires too - if the wires were shorted during installation you could have blown the transformer.
Question: Blower fan comes on for "no reason"
I have a Goodman AC and gas furnace. Lately I noticed that the fan will come on without the temperature on the thermostat calling for AC
Al, while some systems include a defrost cycle that can run a fan without a thermostat calling for heat, I wouldn't expect that to happen every 15-20 minutes when the system is not in use. I suspect a wiring or control board problem.
Question: heat won't turn off
i set the lever to "off", but still the heating wont go off.. before the heating was on and at auto and it was not turning off after a while and then getting on again automatically like it should. - Sarah 2/9/2012
If your home has more than one heating zone, another thermostat could be calling for heat;
If your home has only a single heating zone, then I suspect thermostat wiring has shorted together or the thermostat is not working properly. For now, turn off power at the switch for your heater and call a service technician.
Question: Heat won't turn on
My heat won't turn on when its on AUTO mode. - John 10/30/11
I have a 2 zone oil fired boiler, central heat/air. The 1st floor stat operates heat/air, fan comes on, everything ok. 2nd fl, when turning stat to heat, fan comes on but boiler doesn't come on. - Walt, Tunkhannock, PA, 5/13/2012
Depending on your type of heat, see the diagnostic "no heat" procedure starting at
Your question mentions a fan and air but a boiler - boilers are hot water heating systems, so I'm not quite sure what you've got installed; are there two heaters, one a furnace and one a boiler? do you have a water-to-air heating system? What? We need to know what that second upstairs thermostat thinks it's controlling. Also see the two links I gave to John, just above.
Question: How do I set the thermostat to make the air coming out of registers cooler?
How do u change the Thermostat for it too blow cooler Air. - Christie 7/5/12
Christie, you've asked a helpful question - let's clear up some confusion about wall thermostats: the thermostat is not an accelerator - it doesn't make the air conditioner run faster or slower, nor warmer or cooler. Nope. The thermostat is basically an "ON-OFF" switch that turns the air conditioner ON or OFF in response to room temperature.
Question: test for a bad low voltage transformer
How do I test for a bad low voltage transformer - Den 7/2012
Question: the blower fan doesn't seem to work - it keeps trying to start
Carol, in the links listed at Related Topics check the article titled
I suspect you have a bad start/run capacitor on the fan or a bad control switch or board
Question: blower fan runs continuously - it won't turn off
If coils cleaned, good charge, proper air flow and no wires touching where it's not suppose to.... what will cause the blower to continuously run? - Jerry 8/16/12
Jerry, see the diagnostics at Fan won't stop: What to do if the Air Conditioning or Heating System Blower Fan Runs Continuously and Won't Shut Off?