Heating or cooling room thermostat diagnosis: how to troubleshoot a heating or cooling room thermostat that is not working at all or is not working properly. Here we suggest step by step procedures in diagnosing HVAC thermostat problems such as a thermostat that is not working at all or one that shows a blank thermostat display or a thermostat that does not turn heat on or off at the right temperatures.
Page top photo: a basic room thermostat showing the red and white wires being switched by the thermostat and a place where it's easy to check for voltage .
Green links show where you are. © Copyright 2015 InspectApedia.com, All Rights Reserved.
If your heat won't turn on at all, the problem may not be at the thermostat at all. Start at HEAT WON'T TURN ON. But if you think the problem is at the room thermostat for either cooling or heating, you can also start right here in the steps we give just below.
Question: Wall thermostat is not working - what do I do?
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. Here are the steps in figuring out what's wrong with a heating or cooling room thermostat:
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 referring 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.
Most room thermostats operate at 24VDC except line thermostats such as those used to switch some electric heat baseboards.
Useful Heating or Cooling Troubleshooting Articles
Following a list of articles useful in troubleshooting thermostats we include a series of reader questions & answers about troubleshooting room thermostat problems.
Because some controls are used in common on hot water heat, hot air heat, and steam boilers, readers should see these other articles:
Air conditioners & heat pumps: see A/C - HEAT PUMP CONTROLS & SWITCHES
For hot water or steam heat see BOILER CONTROLS & SWITCHES, and also
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
Readers needing to find and fix un-wanted air leaks, heat losses, or other energy wasters should
Readers should see ENERGY SAVINGS RETROFIT CASE STUDY and also
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.
Reader Question: How to Translate the error codes on digital thermostats
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: 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 thermostat, 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 control 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: 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
Take a look at HEAT WON'T TURN OFF.
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 the "More Reading" links at the bottom of this article 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: 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: 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: 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: 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 non-programmer digital thermostat 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 vacuum 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 thermostat 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 the "More Reading" links at the bottom of this article under THERMOSTATS see the article titled COOL OFF HEAT, Thermostat Switch
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 contactor or control board problem
Question: Heat/AC system cycles on and off too often - short cycling boiler or furnace
New, Heat/AC Honeywell Round Thermostat. (not digital)
(Dec 17, 2012) sean said:
The thermostat has been in the same place for 33years, and I replaced the thermostat and it still keeps cycling for heat but they are short cycles less than 3mins. I diconnected the thermostat wires from the board and the furnace still wants to run is it the board? how does a fan limit switch malfunctioning cause the burner to run I could see the fan running abnormally but not making the burner to run also. again the burner turns on and runs approx 2-3mins then the fan kicks on, directly the burner turns off, then the fan turns off after 1-2 mins then the burner comes on again, and the cycle continues and the temp in my home continues to rise and rise
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
Good detective work! Indeed if you disconnect the thermostat AT THE BOILER OR FURNACE and the system continues to run then there is a control problem at the heater, not a thermostat problem. But if you disconnected the thermostat AT THE THERMOSTAT then it's possible that the thermostat wires are shorted together somewhere enroute, thus "calling" for heat.
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 warrantee 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: 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: 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.
Continue reading at THERMOSTAT BACKUP BATTERIES or select a topic from the More Reading links shown below.
Suggested citation for this web page
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
No FAQs have been posted for this page. Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
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
HTML Comment Box is loading comments...
Technical Reviewers & References