Troubleshooting Broken Thermostats | HVAC Control

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

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.

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