Troubleshooting Broken Thermostats | HVAC Control

Troubleshooting Broken Thermostats

Troubleshooting Broken Thermostats

What you will learn from this article:
1) One of the most common trouble calls for HVAC
2) Basic checks of the thermostat to see if it is broken or?
3) The basics of the thermostat
4) Learn what a heating anticipator is and what type of thermostat uses them.
5) Lots of related useful links to help you learn your HVAC system

Troubleshooting Broken Thermostats – The main connection between the air conditioning and heating system and the end user is the thermostat. Beyond the air conditioning and heating thermostat many people know nothing about their systems that keep them comfortable year round. There comes a time when the thermostat has a problem or the end user wants to replace the thermostat because they think it is a broken thermostat. It is not uncommon for a person to call an HVAC contractor and tell the secretary that they think the thermostat is broken because their air conditioner or heating system does not work. In some instances this is the case while in others the thermostat is not broken but something is wrong with the air conditioning and heating equipment that needs to be repaired. In either case it is usually best to call an HVAC professional to look at the problem especially if you want to replace the thermostat. There are a few things you can check with your thermostat before you call an HVAC professional. A little thermostat troubleshooting is in order.

Related Link: How to Hook up a New Thermostat – A Closer Look

What To Check for Troubleshooting the Thermostat – Troubleshooting Broken Thermostats

  • An air conditioning and heating thermostat, even if it is a digital thermostat, is simply a temperature switch that turns the air conditioning and heating system on and off. As with any switch it needs electricity to function or cause a function with the air conditioning and heating equipment. Most residential and light commercial thermostats are powered by 24 volts A.C. The thermostat gets this 24 volts A.C. from the air conditioning and heating equipment. Usually this power comes from an air handler in an air conditioning and heating split system and from the main unit for air conditioning and heating package systems. Check the circuit breaker for the air handler to make sure it is not tripped. Also check the power switch which should be located near the air handling unit. It is not uncommon for people to mistake these switches for a light switch and turn them off. They unknowingly just turned the power off for their air conditioning and heating equipment including their thermostat. That is when they begin to think they have a broken thermostat. If you have a digital thermostat the power display and the thermostat power display light may not function if you have this problem. Some air conditioning and heating thermostats use batteries to power the back light which takes us to number two in air conditioning and heating thermostat troubleshooting. Check the power switch and breaker to ensure the power is on before assuming you have a broken thermostat
  • The back display light for the digital thermostat does not function but the air conditioning and heating system works fine. Check the batteries as some digital thermostats require AA or AAA batteries for the back light to function on the thermostat. There are two main types of digital thermostats; Digital thermostats that use batteries for the back light to function and digital thermostats that use the power from the air conditioning and heating equipment. If you don’t know which kind of air conditioning and heating thermostat you have then open the thermostat up and look inside. If you don’t see any batteries or a battery compartment inside the thermostat then your thermostat uses power stealing technology. It is probably an older model digital thermostat and if it is a programmable thermostat then every time you lose power you will lose the program inside which takes us to the next step in air conditioning and heating thermostat troubleshooting.
  • The programmable thermostat is always losing its program. This is most likely one of the older thermostats that use the power from the air conditioning and heating equipment and every time you lose power you lose the program inside the thermostat. Programming an air conditioning and heating programmable thermostat can be a chore for many people to figure out so a problem like this can be frustrating especially if you live in an area where there are constant power failures. The only solution to this problem is to replace the programmable thermostat with one that uses power stealing technology (it uses power from your air conditioning and heating system to hold the programs) and has a battery back-up just in case you lose power. This can save you lots of trouble and frustration in the long run keeping you from reprogramming the thermostat over and over again when power failures occur.

Related Link: How a Programmable Thermostat Works – A Closer Look

If the problem is persistent with the thermostat it is important get back to thermostat basics.

We have many other related articles. Please use the search feature to the right to help you find other related articles.

Troubleshooting Broken Thermostats – Thermostat Troubleshooting Basics

  1. Is the thermostat installed in the proper location. This is very important as a thermostat installed on a wall that has high heat gain or heat loss will never offer you comfort as it will react mostly to the temperature of the wall and not the air in the living space. Additionally, thermostats installed where direct sunlight can hit the thermostat is going to be a problem also. A thermostat located near an outside door or window will be affected every time the door or window is opened or closed. A thermostat should be located close to the return (where the filter is installed) so that it sense and reacts to the air returning to the air handler for conditioning. Additionally, thermostats located near heat sources like hot or cold water pipes, radiant heaters, fireplaces, electrical devices which produce heat, etc… will never offer accurate temperatures and conditioning for the space.
  2. Is there a big hole behind the thermostat that will feed cool or warm drafts to the back of the thermostat? All thermostats have a hole behind them where the wires come into the thermostat from the air handling unit. Check this and if you find a big hole behind the thermostat stuff some insulation in this hole and cover it with a piece of tape. This will prevent drafts from affecting the thermostat.
  3. Has the heating anticipator been properly set by a qualified air conditioning and heating technician? The heat anticipator is on mechanical non-digital thermostats and needs to be set according to the amp draw on the heating control circuit. The heat anticipator offers a small amount of energy savings and prevents thermostat overshooting for you as it shuts off the main burners because the fan will continue to run and dissipate the heat which remains in the furnace or heat. Digital and programmable thermostats have built in heating and cooling anticipators which automatically set themselves with no manual adjustments. The mechanical thermostat needs a manual adjustment and you need a tool called an amp meter to determine the proper setting. The cooling anticipator in the mechanical thermostat requires no manual adjustment.
  4. If you have a mechanical thermostat with a mercury bulb switching mechanism inside it this thermostat needs to be level. If the thermostat is not level you never get an accurate temperature in the residence or business.

Related Link: Learn more about thermostat types – A Closer Look

Other problems that can occur with your air conditioning and heating thermostat need to be discussed with an HVAC professional as these problems can be technical and require special tools to fix. It is always a wise choice to call in an HVAC professional when you have problems with your air conditioning and heating equipment including your air conditioning and heating thermostat.

For more on Troubleshooting Broken Thermostats and learning about thermostats click here.

Troubleshooting Broken Thermostats

High Performance HVAC

Troubleshooting Broken Thermostats


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  1. how do you check to see if the thermometer itself is faulty? I have a digital thermometer that now always reads 74 degrees. When it’s hot outside the unit runs constantly an eventually shuts off and blows hot air. When it’s cool the temp remainws 74.

    1. If it always says 74 then something is wrong. Check the temperature by using a temperature sensor you know to be reliable to check the temperature near the thermostat. If its bad replace. And it is possible you could have more than one problem there. Call a professional to have it checked out.

  2. This information is helpful and I did learn some. However, I need further trouble shooting. My thermostat is digital with batteries powering the backlight, it has options for heat, cool and fan. Until recently, I was able to take the cover off and “reset” the thermostat so the A/C would kick back on. Now only the fan works, even after several resets and battery changes. Is it my thermostat or an HVAC issue?

    1. From what you are describing Kayla I would say it sounds like an HVAC issue. Best thing to do at this point is to call in a pro to check it out. Good Luck!!

  3. I have a strange question. We moved into a new house June 6 and it has geothermal heating and cooling. I don’t know anything about these systems. for 2 1/2 weeks it was on 73 and stayed 73 worked great. we did not touch anything because we did not want to mess it up. From out of nowhere it started getting hotter in the house the temp is 80 at times almost unbearable. We had a guy who works on them come out said the air going in fine and temp out fine. He does not no what the problem might be asked me turn down to 70 and leave it. He said keep eye on light on side when I see any lights above green one let him know. I wonder if the thermostat might not be working correctly. I am so hot last night it was colder outside than in here I opened windows in my room. It was fine before I don’t understand what changed. Please give me advice! Should I call a different person to look at it. By the way lights not changed. I swear think warm air comes out sometimes it is too hot in here.

    1. It could be anything and yes I recommend you call someone who specializes in geothermal systems. I am guessing your system has a refrigerant leak and it is freezing up sometimes. When it is not cooling you will experience reduced airflow from the vents. Call someone who specializes in this type of system and they will set you straight. And I am only guessing your system has a leak from the symptoms you describe. To be absolutely sure call a geothermal specialist.

  4. We have a digital thermostat that uses power stealing, no battery backup. The thermostat constantly needs to have the time reprogrammed on it (5+ times daily) and sometimes the display goes blank. Heater kicks on/off according to programming just fine. Have verified the circuit breaker is not tripped and the power switch is on. Has our thermostat gone bad? What else can we verify or should we call an hvac technician to have it replaced?

    Yes………….it is time to be replaced and preferably with one that has a battery back-up. I have had this type of thermostat before and it is a pain to reprogram every time there is a glitch in the power system.

  5. Honeywell RTH110B thermostat. The digital display is on…fan set to auto. Last night it did not come on at set temp (72)..display shows it is 79. Checked air handler cb …ok. Do I have a thermostat concern or unit in attic.
    Thanks in advance.

    Not enough information provided to make an solid assessment. The best thing to do at this point is to call a professional to check it out.

  6. The temperature display on the thermostat shows only one digit. How do we fix it?

  7. My thermostat will not show anything on the display what can be wrong . And the Air Conditioner is not working !

    1. I can only guess but you most likely have a problem with power somewhere. Check the breakers and switches for the inside unit. Follow the instructions here before calling an HVAC contractor to repair the problem.

  8. We just bought a brand new a/c unit because our old one needed to be replaced as it would constantly only put out cool air. The new one was working for a week before it started to put out cool air again. It is set at 73 degrees as the display shows, but the display says it is 81 degrees in here. I cannot figure out if the thermostat is the issue.

    1. call the installing company. It should be under warranty.

  9. My thermostat got hit and the digital screen is broke, that happen a few days ago now the air ant working just the fan in the house I guess, the outside fan ant working… Anyone know what I should do.

  10. My temperature control does nothing. Can’t turn it off, set temperature, nothing you touch will respond. This is a Honeywellcontrol for my Rheem 3 ton unit.

    1. Power problem or the thermostat is bad are the likely causes.

  11. I sprayed disinfectant spray onto the a/c screen that displays the degree number temperature. But now the display is mingle with numbers and words. Will the screen of the thermostat return to normal after it drys? I’m too afraid to tell my parent. 😟 ):

    1. As with many electronics a thermostat is susceptible to getting wet inside. If you are lucky it will dry out and return to normal operation but it is also likely it is damaged and needs to be replaced.

  12. I have to keep turning thermostat on and off for it to work. Is it the thermostat? Evert once in a while you hear it click and it comes on.

    1. Just guessing as you did not provide much information but it is likely something wrong with the HVAC system.

  13. I have a gas ac package unit the ac has never had a problem , but now the heat will run fine 85% of the time and then randomly not come on after the thermostat clicks. If i reset the breaker it will come right back on. Also it seems to work fine on program, mainly does it when i use the hold button.

    1. Sounds like you need to have a technician look at the unit before you press the button to reset it. Only guessing but it likely an ignition problem and that needs the attention of a trained and qualified technician.

  14. Dear Sir I have a Totaline 8 button programable thermostat. The LCD is reading -HP and bounces back and forth to the normal screen displaying tempature and mode. Unit is currently not working.

    1. The HP with Totaline thermostats typically means it is in Heat Pump mode. If the unit is not running then likely there is a problem with the equipment and not the thermostat.

  15. We have a heat pump and on the display inside the house we have a reading for the outside temp. That number says that it is 103 degrees outside and in reality it is 24 degrees outside. This isn’t the thermostat for the house, this is just the reading for the outside temp. What can I do about this? Is it something that I need to be worried about?

    1. Sounds like the temperature sensor needs to be replaced. Technically it is called a thermistor. It is important that this device be installed properly. The thermistor or outside air temperature sensor needs to be installed where no other heat source can cause it to read incorrectly. eg……a dryer vent, a flue, or even in the direct sunlight can cause it to read incorrectly. Additionally, the correct type of wire needs to be used in running the the wire from the thermostat to where the thermistor is located. The wire should 16 or 18 gauge shielded wire as other wiring can have cause incorrect readings. All these things need to be considered including the thermistor can go bad over time for various reasons. Additionally, if the programming in the thermostat ties into the controlling the outside unit when the temperature drops then you will have a problem if the system is an air source heat pump and the thermostat is programmed to turn off the outside unit when the temperature drops below the programmed set point and use the inside back up heat source. Call your local HVAC contractor and have them check it out.

  16. My programmable thermostat is not showing the word aux. the pump does turn on and airhandler. But the 2nd stage heatt does not kick in to help.

    1. The second stage or auxiliary heating (commonly referred to as back-up heating for a heat pump) should only kick in when there is an offset in temperatures. For example, if it is extremely cold outside the air source heat pump will not get enough heat from the air outside to keep up with demand (actually the homes or structures heat loss). The thermostat temperature reading will slowly continue to drop. Typically, depending on the thermostat set up, when the temperature falls 3 degrees Fahrenheit below the current set point only then will the auxiliary heating kick on (via a thermostat command). On some thermostats the offset is adjustable so that can be narrowed to say 2 degrees or expanded to to say 4 degrees. There could also be another problem that needs the attention of a technician to check out. Turn the thermostat setting up by three degrees to see if the auxiliary heating comes on. If it does there is not a problem.

  17. Hello, I recently had my thermostat replaced. Shortly afterwards I had to have my furnace repaired. The thermostat worked fine up until then. Now it seems to be having issues. The thermostat no longer displays heat on when the furnace is running and the furnace seems to run longer now. Any suggestions? Thanks in advance.

    1. Sounds like an anticipator problem but it could be something else. Is it an older mechanical type of thermostat? If so then there is a manual setting for the heating anticipator inside the thermostat. This would cause the furnace to run longer or shorter depending on the anticipator setting. An HVAC technician can set it for you. The setting is based on the amperage draw of the heating control circuit and requires a special meter to read that amperage draw. Newer digital thermostats are set automatically.

  18. I just replaced an older digital honeywell thermostat that kept shutting off due to low battery. Each time I would replace the battery in that previous unit, it would sometimes work fine for months, other times only for a few hours and then the low battery indicator would start again. Where we live, sub zero temps outside right now mean that a few hours without heat drops the temp inside pretty drastically. We woke up with the temp inside at 59 because the thermostat had shut off sometime during the night. I assumed that it was going bad so I replaced it with a new Honeywell digital thermostat with brand new batteries. It had been working for about 4 hours when all of a sudden the low battery indicator showed up. Could it be bad wiring going from the thermostat to the furnace? Any suggestions?

    1. Not sure whats going on there. Most thermostats are powered by the low voltage transformer that should be located inside the HVAC equipment. I couldn’t tell you exactly what it is but it is probably a very good idea to call in an HVAC tech to properly troubleshoot the problem.

  19. I have a Carrier HVAC unit with digital thermostat. Purchased unit in June 2009. Currently on 5th compressor, replaced coil at 6 mos. Thank God I Purchased 10 yr parts & labor warranty. But now having problem w/thermostat. Set on 65. Furnace just keeps running, have to adjust temp down to get unit to cut off. And while it was cold here couple days temperature fluctuates on thermostat from 64 to 65 causing auxiliary heat to come on. Opening the cover to reset temp can also trigger temp displayed to show lower temp when cover opened. My heat ran nonstop last night & today while at work. Have never replaced thermostat with all other problems. What is “life” of thermostat typically? Does this sound like thermostat problem? Sorry to be long winded. PS I hate this HVAC unit. Thank you for your help

    1. I’ve seen antique thermostats work just fine along with newer thermostats. Many times, people think something is wrong with their thermostat when it is really the equipment. The temperature has been extremely cold recently so, from what you are describing, it is possible there is nothing wrong with the system all. It is simply trying to keep up with the heavy demand placed on it from the cold weather. Most HVAC systems, when installed properly, take into account an average degree day for the area where you reside. When temperatures go to the extreme then your system will work overtime to keep up. Without being there to check it out and only going by the information provide I can only make assumptions here but I have been on trouble calls (in extreme temperatures) where I found nothing wrong with the system. I can only recommend to the people to add more insulation or consider getting their house wrapped and have old doors and windows replaced. Weather-proofing your home can go a long ways in saving the energy your HVAC system produces. The average degree day is one factor mentioned above for properly sizing your system……………some other factors involved in properly sizing the system are materials and insulation in the home. These are used for heat loss and heat gain calculations. I am only surmising your system is working properly. If you are uncomfortable call your HVAC contractor to check it out.

  20. Hello Richard,

    I just had a sensor replaced that had burned out on the furnace. Ever since, the Honeywell Digital non-programmable thermostat (no model number) keeps resetting itself to 75 degrees every few hours. Have tried new batteries, shutting it off and on, etc., but nothing will make it hold a setting for more than a short time. Do you know of any way to make it stop resetting, or do we need a new thermostat? Thank you!

    1. The power for most thermostats comes from the furnace or air handler. That, in most furnaces and air handlers, is where the transformer is that provides power for the low voltage circuits in your HVAC system. I would check to make sure that the thermostat has not been programmed to some obscure setting. After that I would cycle the power off and on to the furnace or air handler and see if the thermostat held its setting. If not then, barring no other checks for your particular system, I would replace the thermostat.

  21. My thermostat is acting crazy its flashing all kinds of things. The unit is running but blowing cold air!?!? Please help

    1. Please call an HVAC contractor to fix this. Don’t be cold. As cold as the weather has been recently the cold can become dangerous and even cause severe damage to your home by freezing the pipes and such.

  22. My Lennox thermostat has suddenly started changing time on the program to 12pm or so after I reset program and it runs awhile. Reverts to 12 pm setting. What is solution

    1. It could possibly be a power issue but it could also be that the thermostat is going bad. Best to call your local HVAC contractor and have them check it out. There are switches near the air handler or furnace that look like wall switches and some people mistake these for light switches. If you turn that switch off it will kill power to the air handler and the thermostat. The switches are not always clearly marked so make sure no one is turning that switch on and off again. While the thermostat should have a small back up battery inside it these batteries need to be changed from time to time. I think most Lennox thermostat have a small round battery inside them kind of like you find in a watch or a computer. Try changing that battery……it could be the problem.

  23. I have a question about my digital thermostat. I like to keep the temperature set @ 72°, but for some reason the temperature goes up to 84°, ( only when my daughter come home ) can the set temperature of 72° go up by itself. Or someone is messing with the control panel.

    1. I would suspect someone wants the temperature to be higher. It could also be a programmable thermostat and it is set to change at a specific time. So before you point any fingers figure the model and manufacturer and download the instructions for the thermostat unless, of course, you have the instructions for the thermostat somewhere. There are newer programmable thermostats that can be programmed and locked with a PIN # so only people with the PIN # can change anything on the thermostat.

  24. My Bryant thermostat keeps blinking HP don’t know what the problem is outside compressor will not kick on

    1. The HP is an error code but I am not 100% familiar with Bryant error codes so I am only guessing. If the outdoor unit is not kicking it is possible the unit kicked off because of “head pressure” which could be the problem with your unit but I am not 100% certain because it could be something depending on Bryant’s error codes. A head pressure problem is a problem with the condensing unit outside and it means you have a problem with the refrigeration system. A couple of issues involving this could be condenser fan motor failure or the coils are blocked from passing air through them. There are some other more technical issues that it could but it is probably best that you call an HVAC contractor to solve the problem for you as I never recommend a homeowner opening up the cabinet to their condenser as there is high voltage in there and you really need to know what you are doing when you are inside the unit.

      It is a protection feature in the diagnostics of the air conditioner or heat pump to protect the system from excessive pressures. Sometimes this issue is caused by lack of maintenance or a part failure of some kind.

  25. Have a trane tstat and the display will go blank for a little while and then comes back on. It will do this sporadically and when the display goes off the air handler comes on but the outside unit does not kick on and as soon as the display comes back on the air handler shuts off. Any ideas could it possibly be the tstat it is an older unit approximately 10 years old.

    1. I can’t say without looking at that problem.

  26. Hi, My thermostat works correctly except for the fan on/auto button. On auto it works as expected. If I switch to ON to have the fan run continuously – nothing happens. Is this a thermostat problem? thanks

    1. It is possible it is a wiring problem but it could also be something with controls in the air handler. It is probably best to call an HVAC contractor to solve this issue.

  27. Thank you for the help..we are experiencing an issue with our a/c unit. It will run and cool the house but if we flip it off at the thermostat the fan will cycle on and off by itself..we have checked the contactor, it’s fine! our thermostat is a digital battery operated (Honeywell) Does this sound like a possible thermostat issue?

    1. Not sure about that one. Does your thermostat or system have a ventilation mode (although some manufacturers can call it something else)? If you do not know the answer to that then you have to read the user manuals for both the thermostat and the HVAC unit. This feature is usually only found on high end equipment for residential systems.

      1. Yesterday I attempted to install a Nest. When I turned the power back on, the unit was not getting power via through the low voltage wires. I then reattached the old thermostat thinking I had done something wrong on the Nest install and now it is not getting power as well (it had worked fine before turning off/on the power). I put batteries in the old thermostat and the AC is now not responding to being tuned on and the temp turned down. Is there an additional breaker near the unit or something else that “trips” when you turn off the power? Next to the compressor there is box with a large fuse in it that has two finger holes and when you pull it out there is are slots for two small copper rectangles. Does this need replaced? Thank you very much for your help.

        1. Sounds like you blew the transformer which provides low voltage control power for the system. It is best to call a professional to fix this for you. This usually happens when you do not kill the main power to the air handler when you change the thermostat. Then when you are handling the wires you accidentally create a dead short and it blows the transformer. Hopefully the transformer was fused and you don’t have to replace it. You simply have to replace the blown fuse. Again, there is high voltage in the air handler and I always recommend a professional do this.

          1. Thank you so much. Will call the pros in tomorrow. Have a great week.

  28. For future reference, do I need to flip the “light switch” that is above the handler under the house, or pull the 120 V fuse that is near the compressor on the wall outside before attempting this? Or do I just need to not touch the low voltage wires that go into the thermostat? Thanks again.

    1. I explain this in other articles that have to do with replacing or installing a new thermostat. You can do what you describe or you can simply go to the circuit breaker panel and kill the circuits for both the condenser and the air handler. The control transformer is most often found in the air handler but I have seen some in the condenser so it is best to kill both to avoid any problems. The air handler and the condenser should be on separate circuits in the breaker panel.

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