Why Does my Thermostat say Heat On but No Heat?

It’s very bewildering and distressing that according to thermostat say heat on but no heat is coming from vents. Issues related to furnace ruin your day or even ruin many days if you leave the issue unchecked or until the technician visit your house to solve your issue related to furnace.

thermostat say heat on but no heat

Service calls are also not so much cheap. However, it’s also feasible that you can get by without an HVAC expert. Before you spend money on the furnace inspection, try to solve this issue of thermostat say heat on but no heat by yourself.

Many house owners easily found several issue which stops the heating of furnace. Sometimes it’s as easy as you are flipping the switch. Before you find any help starts some troubleshooting by yourself and ty to solve the thermostat say heat on but no heat issue by yourself.

Why does my Thermostat say Heat On but No Heat?

There are many issues through which your furnace is unable to provide heat instead of just Thermostat setting. To understand that why your furnace is not kicking on, you have to examine furnace issues by yourself or with the help of any professional. But many peoples who have very little experience in HVAC systems are also able to address many issues like thermostat say heat on but no heat and many other issues are mentioned below.

I also suggest you that if you think that you are not able to solve or diagnose such issues then feel free to contact your HVAC professional.

You are having dirty air filter and full with debris:

Well, I have to tell you that this is one of the major cause which stops your furnace from providing heat. Prior to anything else, you must comprehend how air filters work? Air filters stops all the dust particles which is present in air and keep your furnace maintained and improve its performance and quality.

air filters full of dirt and debris

If you leave the filters dirty then it is accumulated with the dust particles and debris which finally clog the air filters. So, in this situation it stops the flow of air which make your system overheated and the system will shut down. Through this your thermostat say heat on but no heat.

Now replace your previous filters with the brand new one. Find the position of air filters and take out your filters, here I insist you that kindly note the size and model number of previous filters before purchasing the new filters. You know why I insist you to check this because air filters come in different types, sizes and it’s having different MRV RATINGS.

Note the arrow before placing the new filters in the system that arrow position must be towards your system. We meet with many house owners who even do not know that what is the right time to replace your air filters? because if you don’t change your air filters according to schedule then your thermostat say heat on but no heat, because the air filters full with dust and debris and its stops the flow of warm air.

 But don’t worry you choose tempindoors we will tell you each and everything about HVAC SYSTEMS. So, the answer is, you have to clean your furnace filters after 30 to 90 days, if its in bad condition means full with debris and dust particles then replace your old air filters with the brand new one.

Also Read: Why is my air conditioner leaking water?

Supply of gas must be turned On:

Well, you know very well that gas is necessary for the gas furnaces to work. So, make sure that the gas valves are open. if the valves of gas is not opened then your obviously your thermostat say heat on but no heat because gas valves are closed. If you don’t know where the valve is then follow gas pipe which is coming out from your furnace in order to find the gas valves. Gas valves are present in 6 feet of your furnace. It contains red or might be yellow nobs. Different houses have different designs of valve.

Your valve of gas is open if it’s in the position parallel position to gas pipe but if it’s in opposite or crosswise position then your gas valve is in the closed position or I say closed. If it’s in the closed position, then turn the handle in parallel position of the pipe in order to open the gas valve. If there’s any leakage of gas, then close the gas valve immediately and call your HVAC professional.

supply of gas issue

In case, if your house supply of gas is on but there’s no gas then contact with your gas company to verify that your bills are paid or unpaid and also check that If there’s any working going on, on the maintenance of gas line.

Wrong setting in thermostat:

Basically, thermostat works like a mind of the furnace and that’s why you have to make sure that your furnace thermostat having incorrect settings or correct settings. Because If the furnace thermostat has incorrect settings then your thermostat say heat on but no heat.

Let’s now talk about how to solve this issue. First of all, increase your thermostat temperature about 5⁰ degrees. Because increasing temperature induce to turn your system on. Also ensure, after furnace providing heating decrease its temperature.

If still, thermostat say heat on but no heat means your furnace is not able to provide heat then make sure that your thermostat contains such settings:

  • In setting heat is “ON”.
  • Set your fan setting to “Auto”.
  • Set your temperature above your rooms temperature.
  • Do not set your temperature to high which is extremely different from the outside temperature.
  • Already “programmed schedule” is “running”.

If its set to already programmed schedule or the settings is totally change then you will see such issue that thermostat say heat on but no heat.

Heater might be cut off from power source:

In all types or furnaces even the gas furnace also required electricity to run. Tripped Circuit Breakers or if the switch is closed stops electricity flow in furnace. You must inspect your furnace’s energy switch first. The power switch is exactly similar to your simple light switch.

Switch is very close your furnace, but here what matters is the position of switch. If switch is in Up position, then it’s in “OFF” position then you have to change its position to down which means your switch is “ON”. After that wait for your furnace to start. But if it still shows that there’s no heat means your thermostat say heat on but no heat.

So, check your circuit breaker if its closed turn it on, even after that if your furnace is unable to provide heat to the house. Check your circuit breakers panel which is normally located in laundry room, basement and in utility rooms etc. Check switch here which is labeled with “furnace” “FAU” which means (Forced Air Unit). Or it is labeled with “Air Handler“.

If the furnace switch is on then check all other appliances are having power or not. If other appliances are not having power, then it might be possible that power is out.

If circuit-breaker is tripped, then you will see that switch is in center position. If you change its position to “ON” again. Then this switch will trip again. So, to solve this problem turn off the switch and then on this switch again to reset all the settings. After all the setting if your circuit breaker tripped again then call your HVAC PROFESSIONAL to diagnose this problem. That is one of the major problem through which it shows thermostat say heat on but no heat.

Dirty Flame sensors:

Dirty flame sensor is also important reason through which your thermostat say heat on but no heat. The main function of flame sensor is that they detect the burners are ignited or not when the valve of gas is open. If the burners do not ignite then immediately flame sensor turn off the gas supply just to prevent gas leakage. Flame sensors are extended in the burners flame and its coated with the soot or carbon deposits. Dirty flame sensors are not able to sense any flame that’s why it’s necessary to clean the flame sensors.

If your furnace is running, then turn off the switch of furnace with the help of circuit breakers and let your furnace cool for 30 minutes. After that inspect your furnace flame sensors.

Now, find the flame sensor with the help of furnace manual. Remove the access panel by using screwdriver. Flame sensor look alike a thin metal rod placed near burners. This sensor is mounted with quarter inch hex screw attached with the wires.

flame sensors full with dirt

Subsequently, unscrew the screw and carefully unplug the sensor. Cut the cables connecting the flame sensors. If your metal rod is fully blackened or encrusted, then you have to clean the flame sensor of furnace. after all of this check that your thermostat say heat on but no heat issue still there or not.

The average lifespan of flame sensor is 5 years. If you didn’t change flame sensor from many years or you see cracks on the sensor, then you have to replace it with brand new one.

Malfunctioning in thermostat:

If there’s any problem in thermostat, then it won’t allow your furnace to turn on or off smoothly and not working efficiently. Here are some reasons that why your thermostat say heat on but no heat are mentioned below:

Weak batteries:

Basically weak batteries are not able to provide enough energy to thermostat. So, that whys its unable to perform its all functions. Some thermostats show the icon of low battery or have audible warning signals. Well, I suggest you to change the thermostat batteries before heating or cooling seasons starts.

Broken sensors:

Broken sensor is one of the major reason that why thermostat say heat on but no heat. If the temperature sensor is broken then its unable to measure the room temperature accurately and in this situation you will not get satisfactory results. The average life span of temperature sensor is 5 to 10 years.

Dirty thermostats:

Well, the dust or dirt is accumulated in the thermostat (expect smart thermostat) which causes the thermostat to perform its function in a proper way. Dust or dirt is the second major reason why thermostat say heat on but no heat. In thermostat cleaning. First of all, you have to remove its cover. If there’s any screws remove such screws and then clean all the dust with the help of soft brushes. Also inspect that if there’s any loose connection by yourself. If you are unable to complete this task, hire a qualified electrician to perform it for you.

Incorrect installations:

If your thermostat is installed recently then there might be incorrect installation. Make a call to an electrician for checking and rewiring purpose.


calibration comes under the reason in which your thermostat say heat on but no heat. If the temperature of the room is not matched with the thermostat temperature then old thermostat can be calibrated. You will need one mercury thermometer with tape. Tape such thermometer inch away from the furnace thermostat to the wall.

Then wait for 13 minutes and after that check the readings which is shown on the thermometer and on the thermostat. If there’s a difference of 1⁰ or 2⁰ degrees in temperature, then you do not need thermostat calibration. But if the readings have a major difference then you have to clean the thermostat before calibrating. So, in result that it won’t show this issue of thermostat say heat on but no heat.

There is anticipator adjustment levers in the thermostat. If the furnace is not able to on then move such levers away from longer settings (one mark at a time). Again wait for 13 minutes and after that check again the readings of thermostat and thermometer and judge that you need extra adjustments or not. so, these are all the major reasons through which your thermostat say heat on but no heat.


These are some major reasons that you will face issue like thermostat say heat on but no heat.

  • If you are furnace filters full with dirt or dust, then change it.
  • Turn off or on the gas valve or make a call to gas company.
  • Incorrect setting of thermostat. Review all the settings and make changes if necessary.
  • No power supply, make sure that circuit breaker or switch are on.
  • If you are having dirty flame sensors, clean it or replace it.
  • Malfunctioning in thermostat. Change thermostat batteries, inspect loose wires, call electrician if required.

If you still face issue like thermostat say heat on but no heat, then call your HVAC professional to diagnose each and every problem.

if you want to learn more about HVAC systems. I hope these articles helps you a lot in making your day better. Here are some articles mentioned below:

Frequently Asked Questions

If thermostat show words like “heat on” and blinks repeatedly on the screen of thermostat, then lockout mode is on. Lockout mode run for 5 minutes because it prevents your furnace from overheating and short cycling. Lockout mode is on due to power shortage.

Cut your furnace power supply, if you want to stop without thermostat. Find the power switch which is exactly similar to your other house switches. Turn off the switch in order to turn off the heat.

Yes, you have to connect the wires with the circuit board directly. But professional or knowledgeable person should do this act because its very risky and dangerous.

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