Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
AIR HANDLER / BLOWER UNITS
AIRBOUND HEAT SYSTEM REPAIR by WATER FEED VALVE
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
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DUCT SYSTEM & DUCT DEFECTS
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
ZONE VALVES, HEATING
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 2014 InspectApedia.com, All Rights Reserved.
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 °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.
FAN SWITCH: Guide to Air Conditioning, Heat Pump, or Heating System Blower FAN ON-AUTO or FAN ON-MAN Settings on Wall Thermostats
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 .
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 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.
Is it Time to Call the HVAC Repairman to fix a Fan Stuck On problem?
We have an old Carrier Nova air conditioner that stopped working.
An aircond mechanic fiddled about for ages and ended up changing the control switch to
an LE75 Temperature Control.
It now works but only heats or cools by about plus or minus 2 degrees Celsius.
As well there is only one fan speed (low) The others won't pull in but I can hear them trying.
The mechanic won't come back and others throw their hands in the air as they don't know where to start.
Any suggestions about how to tackle this problem?
Sydney, Australia - 6/10/11
Peter, I'd start with the diagnostic suggestions found at DIAGNOSTIC GUIDES A/C / 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.
Thanks for your reply,
To refine my question. ..where is the best place to start to look to get the
fan operating at full speed?
Cheers and thanks.
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.
On a system that has a dual speed fan, I find that if the thermostat SET temperature is significantly below the current room temperature the control board in the HVAC air handler will step up the fan to its faster speed.
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.
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.
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
The fan will come on for about 2mins and shut off.Then about 15-20mins later do the same thing. No AC just fan. And it will do the same without the thermostat being connected
Any clues as to what might be the issue?
Thanks for your help - Al at firstname.lastname@example.org, 8/27/12
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.
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.
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
or see HEAT WON'T TURN ON
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.
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.
First make sure your thermostat is the one that controls your air conditioner. In some homes different or separate thermostats control heating and cooling. If you are at the air conditioner thermostat be sure it is set in COOL mode; typically the fan switch is set to AUTO. Then set the desired temperature to a number lower than room temperature and your A/C should operate. If at that point equipment does not turn on there is a problem to troubleshoot - but first let's summarize:
So if you SET the thermostat to call for a temperature lower than the actual room temperature, the A/C will run until the room temperature drops to the SET number.
Now if the A/C runs forever and never cools the room down to the SET temperature then either the SET number is lower than the capability of the air conditioning system (say setting the temp to 60 °F when it's 110 outside at a home that has a lot of heat gain)
The air conditioner needs repair.
Before calling the repairman check for
- a dirty air filter or closed air supply registers - both of which reduce the cool air flow
And if you click on "Air Conditioning" in the links across the top of this page, then on the left side of that web page you'll see an article titled "LOST COOLING CAPACITY" that offers some homeowner diagnostic steps that can help figure out why the air conditioning system is not cooling as well as you'd like.
How do I test for a bad low voltage transformer - Den 7/2012
To test for a bad low voltage transformer you simply need a volt-ohm meter or a digital VOM.
Details of exactly how to test the transformer are at LOW VOLTAGE TRANSFORMER TEST - separate article.
The compressor outside of the house is running and is cooling.
The blower start but after less than a minute the blower stop and the compressor still run outside of the house.
The blower always try to start but stop all the time.
After a while the pipe is frozen.
Why does the blower stops after a few seconds - Carol 8/10/12
Carol, in the links listed at Related Topics check the article titled
BLOWER FAN OPERATION & TESTING
I suspect you have a bad start/run capacitor on the fan or a bad control switch or board
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?
Questions & answers or comments about finding, using, diagnosing, repairing, or replacing water pressure gauges on private well systems.
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Search the InspectApedia website