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Heating & Cooling Thermostats, how to install, set, troubleshoot & repair home page: this article series explains selection, installation, use, setting, and adjustment of heating or cooling system room thermostats or "wall thermostats". Here in the article links found at Related Topics and at the top of this article we provide a complete guide to buying, installing, using, adjusting, & repairing thermostats for heating or air conditioning or heat pump systems. This website answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
If your heating system is not working at all, start at HEAT WON'T TURN ON.
Because some controls are used in common on hot water heat, hot air heat, and steam boilers, readers should see these other articles:
For hot water or steam heat 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. Water heater thermostats are discussed separately at AQUASTAT CONTROL Functions and at Electric Water Heater Thermostats.
Readers needing to find and fix un-wanted air leaks, heat losses, or other energy wasters should see HEAT LOSS DETECTION TOOLS. Readers should see ENERGY SAVINGS RETROFIT CASE STUDY and also see HEAT LOSS DETECTION TOOLS and INSULATION INSPECTION & IMPROVEMENT for energy saving retrofit detailed guides.
Also see GAS PIPING, VALVES, CONTROLS for more details on how to inspect and test LP and natural gas piping, controls, valves, and tanks.
On this wall thermostat the red pointer shows the current room temperature.
The black pointer at the center of the scale at the bottom of the little window shows the temperature that the thermostat has been "set" to.
The "Set Temperature" is the temperature that the room occupant is requesting.
Moving the thermostat adjustment lever (located at the top, side, or bottom of the thermostat depending on the model) will change the set temperature up or down, causing the heating or air conditioning system to turn on or off as needed.
See Detailed Guide to Room Thermostats below.
The Nest™ Learning Thermostat - home automation features
Adapting data from the company's website:
Watch out: this smart thermostat is very appealing but currently in our OPINION it is a bit pricey at $250./ per thermostat plus $119 for concierge installation of one unit. You probably don't need to hire a concierge service to install the unit.
There is no question that the Nest thermostat is a beautiful product. We considered buying two Nests to test and review the product and to try controlling building temperature by cellphone from another country, but needing two thermostats for the test building we felt that the price was a stumbling block. In our OPINION the promise of a 20% reduction in heating costs is probably an "up-to" number that will only be reached if you are not already using a programmable room thermostat that permits night and weekend temperature setbacks.
The legal definition of "up to" in advertising law states roughly that 10% of any product or product installation (not just this one) using the term "up to" in advertising should meet the "up to" claim.
If however you want to use building temperature sensing or time of day to automagically control additional building components such as programmable LED lighting, the value of the Nest is increased accordingly. See LED BULB & LIGHTING TYPES for notes on programmable or controllable LED lights in buildings.
Bimetallic Element thermostats use a bimetallic spring consisting of strips of two different metals fused together, typically into a coil shape. As room air temperature causes the two metals to expand or contract at different rates, the effect is that the spring coils up or un-coils, moving a switch that turns heating or cooling or or off.
Bellows Element thermostats are filled with a volatile liquid that vaporizes at temperatures typically found indoors. As the liquid vaporizes pressure inside the bellows expands, translating temperature change into the movement of a contact to turn heating or cooling on or off in response to building temperature.
Thermistor-type thermostats use a tiny solid-state electronic component, a type of resistor whose electrical resistance changes in response to temperature. By combining the thermistor with appropriate additional circuits and controls the thermostat uses the change in resistance to control the switching of the thermostat's control of cooling or heating on or off in response to room temperature.
For details: at TEMPERATURE RESPONSE of Room Thermostats we give a detailed description of how room thermostats sense & respond to room temperature, and at HEAT ANTICIPATOR Adjustment and THERMOSTAT SWITCHES, INTERNAL we explain methods used to fine-tune thermostat operation in response to temperature swings, thus adjusting the burner on-cycle or fan operation if the heating system uses a fan.
Detailed photos of the major components of this thermostat and a description of how they work are provided below. If your heating system is not working at all, start at HEAT WON'T TURN ON.
Remote Control Thermostats & Remote Heat, Cooling, or Building Temperature Monitoring
A variety of remote control thermostats are offered by HVAC control manufacturers, permitting an owner to monitor, or even change building temperature control settings remotely by telephone or by web-enabled internet connection.
Proliphix provides a range of energy control thermostats that offer remote temperature control via web-enabled communication between an individual and the building's HVAC controls. Also see Honeywell Controls, and White Rodgers thermostat controls.
Heating thermostats: Here we use a traditional, and very simple dial-type round wall-mounted thermostat as an example. Notice that the thermostat face has two temperature scales and two pointers.
The top scale on the thermostat and the red arrow shows the "set" temperature that the thermostat is asking for.
The bottom scale shows the actual room temperature.
Rotating the thermostat dial to the right (clockwise) will increase the "set" temperature. Rotating the dial to the left will decrease the "set" temperature.
Heating thermostats are a simple switch to turn on a heating furnace, boiler, or other heat source. As we explain again below about cooling, for a heating thermostat to actually cause the heating system to turn on, the ambient temperature at the thermostat must be below the set point or temperature to which the thermostat has been set.
Make sure that your thermostat has been set to the proper position for the season: cooling or heating. The air conditioner will not run with the switch set to "heating" and conversely the heating system won't run if the thermostat has been set to "cooling."
An air conditioning system thermostat is a switch to turn on or off the A/C equipment as indoor air temperature varies around the thermostat's set point. Air conditioning thermostats are typically wall mounted in the living area. The thermostat, if it's a combination unit, may have both a temperature setting switch and a switch that can be moved from "cool" to "off", or "heat" positions.
For the thermostat to call for cooling it must be in the "cool" position and the temperature set to a level below the ambient air temperature at the thermostat location. Thermostats contain an internal temperature sensor which compares the air temperature at the thermostat to the temperature called-for by the user, turning the cooling (or heating) equipment on or off as appropriate.
Substantial heating or air conditioning cost savings can be achieved by manually setting back your thermostat when the building is unoccupied or when occupants are asleep. The thermostat is set several degrees (or more) cooler during heating season or warmer during cooling season.
The settings of thermostat "mode" switches into "heating" or "cooling" and the use of other thermostat switches to control an air conditioning blower fan or heating system blower fan are discussed in detail at Other Switches on a Room Thermostat.
Lots of websites offer calculators that will tell you how much you'll probably save in energy costs for every degree you set back your heating or cooling thermostat. Just don't set the thermostat down so low that you suffer costly damage from freezing pipes in winter or mold from high humidity in summer.
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.
Backup Batteries & Default Programs for Digital Room Thermostats
Some programmable digital room thermostats include an internal battery that will allow the device to remember the program that you have set even if there has been a power outage.
All programmable digital room thermostats are designed to return to a "safe" or "standard" temperature program to be sure that heat or air conditioning will continue to work in some reasonable range in case a power outage or dead internal battery causes the thermostat to forget how it was programmed by the user.
Frequently Asked Questions (FAQs) about heating & cooling room thermostats
Question: Why Does the Thermostat Display Jump Back to 83 After I set It to 68 Degrees?
In normal operating mode a digital thermostat whose indicator window shows only a single temperature number displays the current room temperature that it is sensing.
Watch out: if your heating system electrical power has been shut off, when power is restored the thermostat might have "forgotten" the program that you had set into it and you'll need to re-program the device by setting the correct time and then the desired room temperature set back time, and temperature.
Tony error codes are thermostat or heating/cooling device and model dependent. The meaning of HVAC unit error codes are usually printed on a sticker inside the unit cover or control or device cover. If yours has lost that information look for the installation and service manual for the equipment.
If that fails you'll need to call the manufacturer with your model number, serial number, and ask for the manual. That's because there is not a universal standard for digital thermostat error codes - the codes displayed and just what they mean vary among different heating & cooling devices. Some example thermostat and digistat error codes are listed below:
Our image above shows a Trane ComfortLink™ II digital thermostat that uses downloadable software that can be transferred into the device using an SD card or similar memory device. This is one of at least ten models offered by the company. Contact Trane Corporation at PO Box 9010, Tyler TX 75711, or via their website, www.trane.com or by calling Trane's Thermostat Customer Assistance Helpline at 1-877-880-3381 - don't expect to find detailed thermostat installation guides or error codes for Trane equipment online - the company does not provide that information.
Question: Control Voltage for Thermostats
I've read several pages of these troubleshooting tips and none of them refer to "control voltage" .I have found this to be the problem in more than one HVAC unit that I had to work on . FYI folks . - Charlie G 7/7/11
Question: We changed our A/C unit and Thermostat - now it Stopped Working
Hello, Mr. Friedman. I seem to be having a strange problem. We just had our A/C unit and thermostat changed in February, and it was working wonderfully up until today. We just got back from vacation yesterday and had left the thermostat at 80 degrees. We came home, and I switched it from 80 to 75 and then from 75 to 70. And it was working fine up until about three hours ago (it is now 7:15 pm).
The thermostat registers the inside temperature, but there is no side number indicating what temperature I wish to have it at. (Ex.: It's 85 degrees, and I would set it so that it would be 75 degrees, but that number isn't showing up). The fan will run if I switch it to "on", but not on auto. I have switched the batteries, called Honeywell, flipped all the switches I can find. I'm assuming that this would be the cause of the thermostat, as opposed to the actual unit itself?
We have been having problems for a while now, with strange things. (Last month, the carpet directly across from the unit was sopping wet, and took a week to get up! But there was no leakage directly at the AC unit.) It's been kind of a Murphy's Law joke. I'm not sure what I hope to hear, but any kind of advice would be gladly received.
I'm not sure about your model but many thermostats show the current room temperature and do not show the set temperature except while you are pressing the set buttons to change the temp up or down. If you have set the temperature well below the current room temp and the A/C won't start, then you will want to take a look at the air conditioning diagnosis suggestions found in the article linked to at Related Topics and titled LOST COOLING CAPACITY
About that wet carpeting, if your air conditioning condensate drain system is clogged or leaking that could be the problem; watch out for hidden mold or even insect damage or rot if such a leak is left un repaired.
Question: We changed our thermostat settings and now the A/C unit won't come back on
We had our thermostat set at 79 and it read 79. My wife had some friends over and they were hot so we turned it down to 75. The reading never changed from 79. I decided to turn it off and then on again to see if it would help. When I turned the switch back on to "cool" it would not turn back on. We usually hear a click and then it starts going. Nothing now. What do you suggest? - Mike 7/11/11
Mike on at least some air conditioning systems if you switch the compressor component off and then back on again quickly the compressor can have trouble re-starting against the head pressure in the system. Usually if you leave the system switched off for 30 minutes or longer you'll get past that problem, but it might be worth looking out for a hard-starting compressor.
Question: My new digital thermostat worked for just 6 hours then stopped
I replaced my thermostat with a digital one and it worked great for like six hours and then before it reached the desired temperature it just shut down. Now the ac nor the fan will come on. The fan will not even come on in manual mode. I flipped the breaker and nothing has worked. What is up with this A/C? Maybe I wired wrong but the unit was working perfectly for a hours. - Mike in Lubbock
Check for a wiring error, broken or loose connection, lost power.
Question: Our digital thermostat is blank - what causes the thermostat LCD screen to go blank and refuse to respond when I press thermostat keys or controls?
When we came home from the store the digital thermostat was blank. We tried to take off the face of the thermostat to look for a battery as we were doing that the thermostat kicked on. Problem was it started to blow hot air. We went back to the thermostat and when we fiddled with it it went off but then came back on this time it blew cold air. Everything fine right?
Reply: things to check if the LCD display on the room thermostat is blank or does not respond to touch
First make sure that your thermostat has power.
When the display on a digital thermostat goes blank that means that power has been lost to the unit. Power for a digital TT comes from one or at most two sources:
Next, check the thermostat setting mode and physical switches. Trane gives this advice if the display on their TCONT800 series Touch Screen programmable thermostat (illustrated at above left) is blank or not responding to touch:
If the display is not entirely blank, but does not respond to touch, check the thermostat's Advanced Settings to see if the device was put into locked mode.
Question: A/C system cools for just an hour, suspect thermostat or control board trouble
ok joe this is what i got. Ac system set to 75 get home flipped it on started to cool 1 hour later no more air. Flipped fan switch to on instead of auto on thermo, nothing. pulled the cover off fan unit and saw contactors go on to start fan motor than flipped off. Decided it must be a bad start /run capacitor, put a new one in and the same thing. I then pulled out the magnetic coil /contactor /control board and jumped the contactor that was for the fan motor. The motor started right up and ran fine. Either the thermostat went bad and is not sending the proper signal to the control board or the control board is bad. Got any good tests to figure what is bad? - Ray 7/19/11
Hey Ray, try this:
Well I got to your e mail a few hours too late . I replaced the contactor/relay to no avail, after scratching my head for about 20 minutes I decided to find your advice. I found an old thermo and viola!! the system came right on and cooled the house quickly. Hey i just wanted to say thank you for taking the time to help all of us save hundreds if not thousands of dollars with your expert advice. There are millions of people out there that are willing to tackle problems with a little guidance keep up the good work we all appreciate it even if you don't hear back from us. Ray D
Ray D - so glad you got things working, and that our website was helpful. We are dedicated to making our information as accurate, complete, useful, and unbiased as possible: we very much welcome critique, questions, or content suggestions for our web articles. And working together always gives us better information than struggling on alone.
Question: A/C blower making loud sound - doesn't respond to the thermostat properly
My ac blower was making a loud sound so it went bad i replace it with a new one and know the thermostat is not working properly the setting on the thermostat it works wrong when i set it on the auto and cool it does not turn on the blower when i set it on fan and cool it works but it does not shut off i change the thermostat and it does the same thing. Do you know what it could be? - Angelo 8/5/11
Angelo I'm not sure but most likely there was a wiring change needed, or perhaps someone shorted a control board or relay
Question: A/C freezes up after blower was shut off - compressor doesn't stop
Recently my A/C began to freeze up after the blower was shut off by thermostat at right temp, I noticed that the thermostat would shut off the blower but not the outside unit causing it to keep running and freeze up the inside coils, but when I turn thermostat to "ON" and just let it blow, it keep running and getting too cold. How do I find out what is making the thermostat only control the inside unit. - Kewl Jungles 8/8/11
Question: How do I Calibrate my Thermostat
How to calibrate the thermostat?My temp reads 4degrees lower than my set temp,in AC mode. - NC
Question: Inadequate cooling, tech says just keep running the system
We just had evaporator coils replaced on both a/c systems. One system has been cooling fine ever since. The other only seems to cool at night. The thermostat seems to be working OK and the air coming out of the vents is cool although perhaps not quite as cool as that coming out of the vents driven by the other system. The system that not working correctly gets to be six degrees warmer than the set temperature on the thermostat. The company that installed the new coils came back and just checked pressures at the condenser and said everything seems to be working OK and that it may take some time for the everything to cycle through the new equipment. Any suggestions of problems that may result from new coil installation or what else could be causing the problem? Mike
A/C equipment does not normally need a "break-in" period to begin working properly. There are a number of explanations for inadequate cooling, including in a new installation (where we figure we don't have blocked air filters etc) improper refrigerant charge, or a refrigerant metering device not working properly. Call the service company and ask for help from a more experienced technician
Question: A/C fan turns on but A/C won't turn off? Is it the thermostat
my fan will turn on and off when i set mode on thermistat, but ac will not turn off. Is it safe to say I need a new stat? - Roll 8/22/11
Roll, the thermostat is a simple "on-off" switch as far as your A/C equipment is concerned. I suspect the problem is elsewhere, in a relay or switch in the equipment itself.
fixed. cleaned contactor, now it does not lock on or off, works as designed. case closed.
Glad a simple cleaning step at the contactor fixed the thermostatic control. Just watch out: cleaning of electrical contact points needs to be as gentle and smooth as possible. I used to literally file off burned contactor points, but I learned that the contact point surfaces are plated and that once I too-aggressively filed them down they pitted and burned rather quickly. If the problem recurs, replace the entire thermostat or the contactor relay itself.
Dan: thanks for the advice. I used a small flat head with caution and then used a compressed can of dust off for a final cleaning. This is a 1991 Fedders that purrs like a kitten. I clean every year myself. It feeds to a 1991 Goodman Janitrol that has new fan motor and igniter. Runs great. Key is cleaning annually including electric board. I replace filters regularly.
Roll that sounds like a perfect wall thermostat clean-out job, and good advice for others whose thermostats seem to be acting strangely. Indeed on older thermostats I've found amazing clots of dust inside.
Question: hand held A/C remote control does not seem to be working
Have a hand held remote for TCL air-conditioner (split model TAC-30CHS/G). when we set temperature, conditioner blinks what you want it to be, then it goes to inside temperature (suppose to do this) but then it quickly jumps to 34 degrees & switches off. Guessing it is thermostat? Anne Webber 9/2/11
Anne, sounds odd to me too. Try
Question: My inside air handler blower fan won't stop running no matter what
My inside fan will not shut off even when the thermostat is in the off position and continually blows hot air. The outside unit is not coming on in either on or off positions. I've had to turn the breaker off on both sides. Would this be a problem with the thermostat, the inside fan circuit or the outside unit? - RG 9/6/11
Question: electric wall heater turned itself on when the thermostat was not calling for heat
My electric wall heater turned itself on last night. The thermostat was off or not set to a temp. During spring and summer I have my couch in front of it bc I don't use it. During the winter I arrange my apt accordingly to have proper clearance. But tonight I heard it running and I thought that was weird. I pulled my couch away and saw that the screen was literally red instead of white. I'm guessing it was too hot especially with my couch right up against it. So I turned it off from the breaker since at the point I couldn't turn the dial off anymore than it was. To turn it on I have to turn it above 50 degrees. Under 50 means its off. And it has been off since the end of winter. My place stays warmer than by itself so it should never cut on. Not sure why this happened. Just looking for any help. Bc had I not checked on it it could have caught fire.
If your electric wall heater came on while the thermostat was in the OFF position then your unit probably has either a bad internal switch, thermostat, or short circuit in the thermostat or control wiring. As you suggest, the unit is unsafe and should not be used before it is repaired or replaced. And indeed, a couch, curtains, or other combustibles should never be placed in front of an electric heater exactly because of the overheating and fire risk that you observed. Thank goodness you detected the condition and turned off power to the heater. Had no one been at home, or had people been asleep the result could have been a fatal fire.
Question: My A/C blows warm air for a while before cool air comes out
when I turn the air conditioner onto heat it blows out warm out for a few minutes and then cool air, would this be the thermostat or would it need gassing, it is just over five years old. - Marylin 3/14/12
Marylin, if the air produced by your system is adequately cool after the initial warm air delivery I suspect your building may have poorly insulated ductwork running through a hot attic of some length. The hot duct is warming the initial airflow.
Hi Dan, if it was the ductwork wouldn't it have played up by now i have had the unit for just over five years, when I go downstairs to the main unit and touch the pipe it is hot,but we just cant seem to get enough heat upstairs..
Yes Marylin I agree - that's part of the difficulty of guessing at a question with not much information. However depending on duct type, routing, condition, insulation can fall off, be torn off by animals, or open and leak - it's worth a careful inspection in any case.
Question: My thermostat only works on a call for heat; on call for cooling a fuse blows
when I call for heat the thermostat works, when I call for ac t blows the fuse what is going on - George 4/21/12
Question: How do we balance air temperatures in a triangular shaped room?
hi, i am at work in a triangular room, the shape is /_\, so at the bottom we have the regular wall, and the other 2 sides are formed of long big windows. at the top of the triangle it is always colder than at the bottom... the people in one side are always colder than the others.
We have 5 thermostats; the air vents are on the margins of the 2 glass walls; in the middle of the room we have another air vent; now we were told to keep all of them on Heat or Cold, and at the same temp.
Nevertheless the difference between temp in the room is quite big; for ex we set the temp at 73, and on heat, but the top of the triangle has temp 62 and the bottom 79.... Quite huge difference. could you help in advising what could be done or if it is ok nonetheless to set 2 thermostats on cold in the hotter part of the room and the other ones on heat in the cooler part?
Maria balancing temperature in a large but irregularly-shaped open space can be difficult, but the solution usually lies in not only where the conditioned air supply registers are located but also the location of the air returns. Also whenever anyone changes "just their local area" - like moving a suspended ceiling tile or changing their supply register outlet, they upset the balance of the system. You need an experienced HVAC designer/installer to take a look at the problems you describe.
Question: After installing a new thermostat the A/C worked fine, then started making noise
2 days ago I replaced my old honeywell digital thermostat with a new honeywell digital thermostat because the old one just stopped working - there would be nothing at all on the display and nothing would appear when I pressed any of the buttons. It was not a battery backed up type. After replacing the thermostat - the AC seemed to be working quite well and I was so pleased with myself. I left to run some errands and when I returned home I could hear the outside unit clicking loudly as if it were trying to come on but then shorting out and then trying again repeatedly. It kept doing this until I got inside and turned the thermostat to "off".
Several hours later I tried again by turning thermostat to "cool" and the ac came on and blew cold air fabulously for about 10 minutes then the outside unit started making that noise again (It's a terrible noise - you know something's wrong). The AC keeps blowing cold, even when the outside unit is making that noise until I turn it to the off position again. I didn't have the problem with the old thermostat and this only started happening when I replaced the thermostat. Did I buy the wrong Thermostat? What could be causing this?
I don't have enough information to make a good guess on this one, but it sounds as if you need a service tech to take a look at the compressor and outdoor fan. Clicking noises could be a bad relay, or even a loose fan blade or a stick that fell into the fan unit.
Question: our heat won't stay on
Thermostat has 2 readings, actual temp inside the home and the temp we set that we want inside the home, the heat comes on for 1 to 2 minutes then turns off, and will not warm up the home,we have to turn it completely off then on again for it to start again for another few minutes, what's wrong with the thermostat or is it that? - Sam
Sam, take a look at the room temperature and compare it to the "set" temperature of the thermostat. If the room temp is different from the set temperature then the thermostat is calling for heat (in your case). If the heater wont' keep running and the thermostat keeps calling for heat, the problem is most likely not the thermostat but a start or operating problem at the heater. It may be going off on safety, then automatically resetting - not a cycle that you should continue as it could be unsafe.
Question: thermostat can't be adjusted and the digital display just shows a blank space
My digital thermostat only shows a minimal temp (61) and can not adjust to desired temp, only a blank space. Nothing happens when I place unit on cool or heat and depress the related buttons. The furnace fan will run in the "on" position, but can not make any other adjustments. The thermostat will not respond. Any idea? - William Shassere, 5/29/12
William, it sounds as if the thermostat has lost a connection to the heating system or is defective. Some thermostats that incorporate a battery intended as backup to preserve the thermostat settings will show some data in their display when under battery power, even if their connection to the heating or cooling system has been lost or is "off".
Question: contactor in my AC condenser won't get the 24-volts it needs
I have a unit don't star the condensing unit the blower in the air handler work good but the contactor in the condensing unit does not get the 24V, I check the contactor with the tester an work good I check the transformer in the air handler and is good, thermostat is in cool mode and set up at 60F. room temp. is 87F Do you thinks is the thermostat? - Angel, 5/29/12
Angel, I'm not sure from just the info in your question, but I have this suggestion: it's easy to find out if the thermostat is the problem since that device is basically an "on-off" switch. Just take the thermostat out of the loop by disconnecting it and then jumping the two TT terminals.
Also see our article LOW VOLTAGE TRANSFORMER TEST
I have the thermostat in cool mode and on 55F.
Do you thing is the themostat? - Angel
No Angel, if the thermostat calls for cooling and the air handler blower operates, but the outside compressor/condenser unit does not start, the problem lies in the compressor/condenser unit and its controls and relays, not in the thermostat.
Question: thermostat won't turn off fan: our attic blower fan won't turn off
Our t-stat will not turn off the blower fan in the attic. The condenser unit ... - Anon 6/8/12
Our condenser unit stopped running so it's not cooling. I tried to turn the blower fan off from our simple nonprogramable digial termistat but it wouldn't turn off. I had to go up to the attic to turn the switch off. - Anon, June 2012
Check that the "FAN ON" switch is not set on your thermostat. Try setting that switch to AUTO or FAN OFF.
Then if that doesn't correct the problem, see Fan won't stop: What to do if the Air Conditioning or Heating System Blower Fan Runs Continuously and Won't Shut Off? or go to our blower fan diagnostic home page at FAN, AIR HANDLER BLOWER UNIT.
Question: thermostat won't turn off air conditioner
My Honeywell round thermostat is not shutting off my A/C. I have the position set to auto and the A/C works but won't shut the A/C off. Also, it appears not to be calibrated any longer in that I need to adjust the temperature to a much higher temp than actually desired. Is it a "dirty" stat? Help! Roy K, 6/19/12
My outside unit wont shut off unless I turn it off with the breaker. A friend told me it was the thermostat so I replaced it. While in Auto/Cooling the house cools and the fan shuts off, but the outside unit continues to run. Maybe a relay? - Tim 7/16/2012
AC will not turn off when thermostat is at set temperature it run all the time - Scott 8/24/12
this should be pretty easy to check since thermostat wiring is not complicated. I'm not sure of the age and type of your round wall thermostat - take a look at the photos in the article above - but if it's a bimetallic spring type and dust has clogged the unit it should be possible to simply blow or vaccum debris out of the thermostat. Be careful not to bend anything.
If the cooling system continues to run then the problem is not the thermostat itself.
- a short anywhere in the thermostat wiring that crosses the two wires together (this is equal to a thermostat "ON" call for A/C)
If the thermostat is not calling for heat, and PROVIDED that the thermostat wires are not accidentally shorted together somewhere (worth an ohmmeter check), then the AC equipment should not turn on. An indoor air handler blower may keep running if the fan is in ON or Manual rather than AUTO, but the outside unit would shut off. IF the outside unit does not shut off I suspect a relay or control board problem there.
- If the thermosat is not properly located it may not properly sense room temperature; for example locating a heating thermostat on a poorly insulated exterior wall or locating an air conditioner thermostat in hot sun can cause problems
Question: Air Conditioner sometimes won't turn on
BASICS: Apartment A/C unit with heat pump. Inside: Blower, motor, coils, clean drip pan. Outside: One-year-old compressor.
The trick is to turn the OFF-COOL-HEAT switch from COOL to OFF to COOL; then, the A/C comes on within a minute and resumes normal cycling (until the next random failure).
P.S. I just had them install a brand-new clock thermostat, to replace the old-fashioned manual one. Made no difference in the problem.
Question: Inside Blower Fan Runs in AUTO but not in ON - fan shuts off, coil ices up
why would the inside fan motor quit when in the auto position but yet run fine in the on position? this happens after working fine for 3 cycles in the auto position and then I have to switch to the on position because the fan shuts off and ices up the condenser. I have stat set @ 75 (stat is new) - Woody 7/21/12
Woody you've taken a good diagnostic step; we know the problem is in a control, control board, ran relay, or switch, or thermostat wiring, not the fan motor itself.
Let's get clear which fan we're discussing and where icing is occurring. If the indoor air handler fan won't run I'd expect the evaporator coil in the indoor air handler to ice-up. The condenser coil is in the outdoor compressor/condenser unit. You wouldn't expect to see icing there.
Question: Round Honeywell Digital Thermostat - exhaust purge fan keeps cycling
My mother's home has a round Honeywell digital thermostat (just like the one pictured for a previous question). When her a/c is running, the furnace exhaust purge fan comes on and cycles repeatedly (it will run for a short while, then shut off, then start right back up before it has a chance to completely stop). She was told it was a defective thermostat. Does that sound right? - DaveM 7/31/12
Question: Apartment heat is off for the summer and I don't want the Air Conditioning On but I'm Freezing Cold
The heat is turned OFF for the summer in my apartment building, and I do not want the air conditioning on. The Honeywell on the wall (rectangle), shows 72, but my portable thermostat shows 68 and I'm freezing. What should I do besides using a portable space heater? - Ellen 8/6/12
Ellen, in the links listed at Related Topics under THERMOSTATS see the article titled COOL OFF HEAT, Thermostat Switch
My wall-mounted thermostat is not working. - Ed 8/14/12
Reply: Things to check if a thermostat is just not working
Ed you're not giving us much to go on so my reply is a bit limited:
Question: what doers "TT" mean in thermostat and HVAC discussions?
I was reading your help forums on thermostat trouble shooting, and needed to know what you are refering to when you say TT? (The Thermostat?) (I have no power at my thermostat)
Scott, thanks for the question - it's an example of how people too familiar with a topic can speak in shorthand that is gobbledygook to everyone else.
TT is simply shorthand for "thermostat" in most discussions; for example in wiring up an aquastat the instructions might at times refer to the TT or "thermostat" terminals.
Question: why do we have to turn our heat up higher than previously just to get warm
Our heating and air unit is roughly 8 or 9 years old. When we first moved here (6 years ago) we kept the air set on 73-74 during the summer months. We were comfortable. Now, we have to keep it up to 79-80 and we still feel like the house is too cold. We had the unit checked out last year ( because we started having double and triple utility bills) and the guy said it was fine. Could this be thermostat problems? I don't really feel that the unit runs more than it should though. - Rachel 8/22/12
First, are we looking at the same outdoor temperatures as previously? If not, if it's colder now than previously, the problem could be with your home's rate of heat loss - bad insulation, leaky windows &c.
If weather conditions are the same, then we have some other questions to ask: you don't say if your heat is from hot water or forced warm air. In either of those systems, even if the equipment is running as before, you could have a simple maintenance problem like a clogged air filter or air bound radiator that's the problem. I'd need to know more.
Question: where to put batteries in our Hunter 44422 Thermostat - Hunter Fan Company Programmable Thermostat Instructions
Can't find where to put batteries on Hunter 44422 help - Jeanie 8/29/12
Reply: How to Change Batteries in the Hunter 44422 Programmable Thermostat
Jeanie, the Hunter 44422 you are discussing is a Hunter Fan Company programmable thermostat. You can get product information and instructions at the company's website www.hunterfan.com or contact the company directly at Hunter Fan 2500 Frisco Ave. Memphis, TN 38114 888-830-1326 www.hunterfan.com
Quoting the company's product literature for the Hunter 44422 thermostat,
This thermostat is designed to work on the following heating and cooling systems:
The batteries in this unit, or at least the one that I examined, are in the back of the hinged, removable thermostat assembly. You'll have to un-clip the thermostat from its top, swing it down and out on its hinge, and you'll see that using a plug connector the entire assembly is removable to make it easier to change the batteries without breaking something.
The company's instructions describe changing the batteries but omit a nice illustration of how to find them in the bottom-back of the assembly.
See http://www.hunterfan.com/uploadedFiles/Support/Owner_Manuals/44002.pdf for the manual for this thermostat.
Question: my A/C system won't turn on in response to the thermostat
My a/c contactor is not receiving the "call". If I push in the contactor button, it kicks on and everything seems to work. But, the thermostat won't kick it on. I have a brand new capacitor and contactor. If there was a problem with power getting to the unit, wouldn't it NOT come on if I pushed in the button on the contactor?? Any ideas? I looked for a switch on the blower motor, but none found. Help! - Art 8/30/12
- removing the thermostat from the equation by jumping the two on-wires (see our tables of thermostat wire identification) that would be connected if the thermostat calls for heat or cooling - whichever you are testing
- if that solves the problem then the issue is in the thermostat, else I suspect a contacator or control board problem
Question: Heat/AC system cycles on and off too often
New, Heat/AC Honeywell Round Thermostat. (not digital)
Yep; check for a bad heat anticipator, dust clogged thermostat, improper thermostat location in hot sun or too cold space, or in an area where air from the supply register blows right onto the thermostat
Question: rapid on-off cycling of outdoor compressor/condenser fan
I just bought a new house and I noticed that the fan on the outside unit is cycling on and off in 1-2 minute cycles with the thermostat set to off. This doesn't seem normal to me. I know that there was a freon servicing valve that was stuck and let all the freon escape. This was fixed prior to us closing and i'm not sure if that has any relation to what is going on now. I'll probably have to call in for warantee work unless you know why it's doing what it's doing. It seems like the wiring might be incorrectly installed, what do you think? - Joe 10/31/12
Joe I agree that rapid on-off equipment cycling is not proper but typically I suspect a bad control or sensor, not the wiring itself.
Question: Bonnair evaporative cooler went blank
My Bonnaire ducted evaporative air con controller went blank; there's power to the unit and no brakers were tripped. Any suggestions? - Mike 12/23/12
check for a broken or disconnected low voltage wire between the low voltage transformer and the thermostat. Also check the transformer itself. See
Question: Our digital thermostat keeps magically switching from heat to cool
Is this a bad thermostat or is it an indication of something else? I have a basic digital thermostat and the electric heat pump system is 5 years old. The outside temperature has ranged anywhere from 14 to 50 degrees outside. I have the temperature set at 68 on heat and can wake up and the house is in the low 60's. The thermostat will read that it is on cool. I run the settings on the thermostat back to the heating cycle and the house warms back up only to change back to cool maybe 8-24 hours later again. This has been going on for a while. - Janice 12/30/12
Janice, be sure that the manual switch on the thermostat is set to HEAT mode;
Question: Gemtech programmable thermostat troubles
I have a Gemtech GTP211D, a programmable thermostat. I set the temperature based on time pf day and day of the week. However, the thermostat program resets itself and loses all the settings. In addition, the heat and air will not be running according to my programmed temperature. Please advise. 12/30/12 - Eunice 12/31/12
Question: heating thermostat seems to show wrong temperature
My heater's thermostat is showing wrong temp. At off setting , it shows the room temp at about 80 when it feels like 60 degree in the house. I tried removing and putting batteries again but doesn't solve the problem. Do I need to replace it? - P. 1/2/2013
You might, but first check the thermostat's room temperature reading (not its SET temperature reading) against the actual room temperature - use a room thermometer; Then be sure the thermostat is set to HEAT mode and make sure that the SET temperature is above room temperature. Then heat should run until the sensed temperature at the thermosat turns it back off; See the next Q&A for more details.
Question: how do I fix a thermostat that I have to set lower than the level of heat I want?
My traditional honeywell simple dial-type thermostat on the wall has two pointers and two temp. scales. However I have to set it on 60 degrees to get heat temp to 70 degrees.
Reply: check these reasons for a room thermostat that keeps calling for heat
On most two-scale room thermostats, one scale reads the current or actual room temperature while the other scale is the set temperature - and is adjustable. When you move or adjust the thermostat to call for a different room temperature you will see the scale pointer move to your new setting on the "set" scale.
Remembrer that a thermostat is just an "on-off" switch that turns heating or cooling ON or OFF in response to the actual room temperature and the desired or "set" temperature you've specified. Make sure that your thermostat is propery set - to "Heat" mode with the fan switch set to "Auto".
You are describing having to set the thermostat lower (60F) than the desired temperature (70F) to get the proper level of heat in your home. In other words, you say that the room gets too hot - hotter than you asked-for. Here are some things that would cause that problem:
Keep us posted, what you learn will help other readers.
Question: can I replace my Honeywell round wall analog thermostat with a new programmable one on a steam heating system?
I have a gas-fired steam heat furnace (Burnham - approx 10 years old) with an analogue Honeywell round-dial type thermostat. Is it possible to replace the thermostat with a digital type with programmable heat settings or does the age and type of furnace preclude such luxury? - Anon 2/7/13
Replacing an old round analog room thermostat with a programmable unit should be no problem at all - the thermostat is basically just an on-off switch.
Question: what is the setting on this thermostat: precision of room wall thermostat settings, dials, readings
Questions & answers or comments about Room Thermostats for Air Conditioners & Heating Systems or heat pumps: thermostat operation, wiring, adjustment & repair procedures.
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