“My thermostat says heat on but no heat. What’s the problem? How to get the heat back on?”
This is one of the most frustrating thermostat issues in the winter. You do everything right; set the heat “ON” on the thermostat but no heat comes from the furnace. We will look into all 14 common reasons why the thermostat says heating but no heat comes from the furnace or heat pump. Of course, we will also describe how to solve and troubleshoot ‘heat on but no heat’ issues for Nest and Honeywell thermostats.
Here are a few scenarios that you might be seeing when you set heat “ON” on the thermostat:
- Thermostat says heat on but the furnace not running. Basically, you set the heat on as you should, but the furnace does not respond; ie. furnace will not turn on with the thermostat. This classic ‘thermostat won’t turn on heat’ issue indicates either a thermostat-to-furnace communication problem (wrong thermostat wiring) or a faulty furnace.
- Furnace blowing but no heat despite the heat on the thermostat setting. You might see heat not blowing warm from your return vents. This might be caused by wrong thermostat wiring or a faulty furnace.
- Heat won’t turn on but AC will. If you have tried to figure out why the heat is not working in your house, you might have switched to AC to see if that works. Well, in many such cases, the air conditioning system turns on and works but, if you switch the thermostat back to heating, the furnace or heat pump just won’t heat.
- Thermostat not reaching set temperature. This is a related issue where we see some (but not enough) heating that we have covered in this article.
Even from the get-go, we see that the thermostat says heat on but we see no heating coming from the return vents or indoor air handler because of the main issues:
- Problems with thermostat wiring. If the wires provide inadequate signals to the heating system, you will see no heat coming from the furnace no matter what setting you set on the thermostat. Culprits include a wrongly wired thermostat (for new thermostats), bad thermostat wires or loose connections, a bad thermostat battery, the thermostat is set to too low temperature, faulty thermostat temperature sensor, and tripped circuit breaker (furnace is cut off from the power supply).
- Faulty furnace or heat pump. This is a situation where the furnace receives an adequate signal from the thermostat (heat “ON”) but just doesn’t turn on or turns on but we see no heat coming from the furnace. Causes include dirty furnace or heat pump filters, no fuel (propane, oil, or gas supply is off), gas furnace pilot not working (dirty pilot ignitor), faulty electronic ignitor, frozen outdoor heat pump coils and low refrigerant levels in heat pumps, 2nd stage in a two-stage furnace not turning on, undersized heating system or extremely low outdoor temperatures (air-source heat pump issues), faulty furnace blower, and blocked supply and/or return vents.
We will start with the thermostat wiring issues and proceed with the faulty furnace or heat pump issues. You can follow this thermostat not turning the heat on troubleshooting guide one by one (it’s quite an extensive set of potential culprits and it’s impossible to single out one reason that would cover 80%+ of these issues), and see what is the cause for your thermostat not turning on the heat and learn how to fit it:
1. Wrongly Wired Thermostat (Common For New Thermostats)
If you have a new Honeywell, Nest, or any other thermostat, chances are that it was wrongly wired. That’s also true if you have a new furnace or a heat pump that had to be wired via the thermostat. The issue here is quite straightforward: If your thermostat is not wired correctly, you can set the thermostat to “heat on”; the thermostat will say “heat on” but the thermostat itself will not power the right wires to actually turn on the furnace.
This might be something as simple as wrongly wiring the white wire terminals (W1 and/or W2) that are responsible for heating (gas furnace, heat pump). It might be that C wire, red R thermostat power wire, or dark blue B wire for the reverse valve to switch the heat pump from cooling to heating are off. It’s just impossible to know theoretically; you have to remove the thermostat control panel and check the wires one by one.
Thermostat wiring is a big topic; which color wires and the terminal does what and where they should go is explained in our thermostat wiring article here. For furnaces and heat pumps, we use 2-wire, 3-wire, 4-wire, or even 5-wire thermostats (Honeywell and Nest thermostats are the most commonly used ones).
Example: We had a case where the thermostat was wired ‘in reverse’. The higher the temperature you set, the lower temperature was actually acknowledged by the thermostat. When you set it to 80°F, the thermostat said heat on but there was no heat coming from the furnace. When you set it to 60°F, the furnace immediately starts running on 100% output and provides excess heat.
In any case, the first thing you should do when the thermostat says heat on but no heat comes from the vents or indoor air handler is to check if the thermostat wires are correctly connected to their corresponding thermostat terminals. If you see that not everything is in order, rewire the thermostat (or call the HVAC company or an electrician) that wired it in the first place.
If the thermostat is correctly wired, let’s move on (don’t screw in the thermostat control panel just yet):
2. Bad Thermostat Wires (Or Loose Connection)
Even if your thermostat is correctly wired (each wire goes to the right terminal), the wires themselves might not send the ‘heat on’ signal through. That will happen if the wires are bad (screwed too much, insulation missing) or if the connections to the terminals are loose.
In both cases, you will see that even if you put the heat on on the thermostat, there is no heat coming from the vents or indoor air handler.
Here’s the thing:
Thermostat wires are made out of copper. They can take a lot of twisting and turning and still work. But during the thermostat installation process, these wires can be stapled with a lot of pressure. This can cause the wires to ‘break’ or to be accidentally cut. If they are flexed too much to fit into the thermostat box, they can also fail to conduct the electric signals adequately.
Check the wires if they seem to be flexed excessively or if you spot a cut in the insulation around the wire (result of pressured stapling). Straighten the flexed thermostat wires out and/or replace the cut wires, if you find any. After you have done this, turn the heat on and see if the furnace or heat pump starts as it should.
Another thing are loose thermostat wire connections. As you know, the thermostat terminals have to grip the wire endings in order for adequate signal conductivity. Check if any of the wire connections are loose. You just take the wire and pull it a bit. If you see that the terminal doesn’t grip the wire as it should, re-staple the wire to the terminal.
If you had a faulty thermostat wire or a loose connection (and have now replaced the wire, re-stapled the terminal connection), you will see that the furnace or heat pump now starts and provides heat a few minutes after you have turned the heat on the thermostat.
3. Bad Thermostat Battery (For Battery-Powered Thermostats)
Some thermostats can use batteries as a primary or backup power source. Examples of battery-operated thermostats are Honeywell TH6110D1005/U FocusPRO 6000, Honeywell Home RTH7600D, ecobee SmartThermostat, and so on. Here is an example of a Nest thermostat that:
“… uses 2 AAA alkaline batteries as a backup, or to supplement if your system can’t deliver enough power.” (Google Nest Help)
It will be immediately obvious to you that a thermostat cannot function properly without a working power supply. Even if the thermostat says heat on, you may get no heat from your furnace or heat pump because the battery is not full enough to provide electricity for that specific electric signal.
Here the solution is pretty simple:
Just change the batteries. If the empty thermostat batteries are the reason, for example, that Honeywell thermostat says heat on but the heat is not turning on, changing the batteries is the quick and easy fix.
4. Thermostat Being Set To Too Low Temperature
This is a simple one. By default, when you set the thermostat to heat on, you will see “heat on” spelled out on most thermostats. However, don’t be misled that the heating system will actually always blow hot air through the return vents (furnaces) or via the indoor air handler (heat pumps).
If the thermostat set temperature is below, equal to, or just a degree or two higher than the indoor temperature, the furnace will often not provide heating.
Example 1: Let’s say the indoor temperature is already 74°F and you set the thermostat temperature to 72°F. Obviously, the furnace or heat pump will not provide heating because no heating it actually needed, since the indoor temperature is already above the thermostat set temperature. Despite that, we will see “heat on” written on the thermostat; this is the default setting and the source of confusion.
Example 2: Let’s say the indoor temperature is 71°F and the thermostat temperature is set to 72°F. With many thermostats (especially older thermostats), the heating will not come on since the deviation between the indoor temperature and the set temperature is a bit insignificant.
With heat pumps, the case may be that the indoor air handler is blowing only mildly warm air; this is because the heat pump doesn’t really need to pump out an enormous amount of heat to raise the temperature by a single degree. If it started blowing hot air, it might overshoot its 72°F degree target temperature.
In all of these cases (thermostat will, in most cases, say heat on), keep in mind that the indoor temperature might already be very close to the thermostat set temperature, or just increase the thermostat temperature to see the heat coming from the heating system.
5. Faulty Thermostat Temperature Sensor
How does a thermostat know what is the temperature indoors? It uses a sensor, of course. When you see the “heat on” on the thermostat, the furnace or heat pump will provide heating under only 1 condition:
If the indoor temperature is below the thermostat temperature, heat. That’s it.
Now, the problem you may describe as a thermostat says heat on but no heat will arise if the thermostat doesn’t accurately measure the indoor temperature. That happens when we have a faulty thermostat temperature sensor.
Example of how a faulty thermostat temperature sensor behaves: Let’s say the indoor temperature is 68°F and you set the thermostat temperature to 72°F. The thermostat will clearly state “heat on” but you will see no heating coming from the furnace or heat pump. That may be because the thermostat temperature sensor measured the indoor temperature of 74°F. If the sensor thinks the indoor temperature is already above the thermostat set temperature, it won’t provide the heating (despite “heat on” label).
Most often, you will sense something is wrong with the temperature sensor if the thermostat says 72°F but you feel like the temperature is way below that (65°F, for example). If you have a thermometer at home (not those digital body temperature ones, they don’t have a linear scale in the 60-80°F range), you can measure the indoor temperature with the thermometer and compare it with the temperature on the thermostat.
If there is a significant difference, you know you have a faulty temperature sensor (or your home thermometer is not adequately calibrated, that may be an option as well, theoretically).
Issues with the thermostat sensor are better left to the professionals. Here is an example of how complex these wirings can be:
You want to call your HVAC guy to come to check it out. If you don’t “know a guy”, we provide options for “get to know an HVAC guy” in your area at the end of this article.
6. Tripped Circuit Breaker (Furnace Is Cut Off From The Power Supply)
As you can see, we are slowly moving from the thermostat issues for heat on but no heat problems to the furnace or heat pump itself. In the middle, however, are the power supplies. No power supply to the heating system equals no heat.
In most cases, furnaces and heat pumps are cut from the power supply if the circuit breaker is tripped. With new installations, that may mean that the circuit breaker is undersized, and you will have to upgrade it.
With existing installations, the power draw (amps) increase is too much for the furnace or heat pump to handle, and the circuit breaker trips to protect our HVAC unit. This can happen for a number of reasons, including after a power outage, increased power draw due to dirty filters, low refrigerant levels in heat pumps, and so on.
The result of the tripped circuit breaker is always the same:
The thermostat will state heat on but no heat will actually be generated and distributed throughout your house. That’s because the furnace or heat pump is not turned on.
You can check if the circuit breaker is tripped yourself. Open the electric panel and see if the breaker connected to your furnace or heat pump is switched (and flip it back on).
Most often, we see the tripped circuit breaker with heat pumps since they run on electricity and can draw major amps (you would need two 30 amp or even three 3 amp, or two 60 amp breakers there).
Gas furnaces obviously don’t run on electricity; however, the modern ones are powered on via electronic ignitors (there are the hot surface ignitors, or HSI for short). To fire up the gas furnace, these ignitors require a big electric surge (to heat the ignitor). This surge can trip the breaker, and the furnace won’t come on. You will see a classic case of the furnace not turning on with the thermostat despite the “heat on” written on the thermostat.
These have been the thermostat heat on but no heat issues that stem from the thermostat doing something wrong. Now let’s have a look at what might be the problem at the other end – at the furnace or heat pump – that causes this misalignment between what the thermostat states (heat on) and what the heating system actually does (no heat):
7. Dirty Filters (Furnace And Heat Pump Filters)
As homeowners, we can sometimes roll our eyes when the HVAC guy says something is wrong because of the dirty filters. They are just filters, right? Well, yes, but it is pretty interesting to see how many potential HVAC problems can be avoided just by cleaning/replacing filters and letting our furnace or heat pump unobstructedly breathe as it should.
Here is what might happen if the filters are dirty or inadequate:
- Thermostat sends a signal for the furnace or heat pump to turn on, and it turns on. However, the very dirty filters obstruct so much that the furnace or heat pump turns off because it just cannot handle the pressure it has to work against (due to clogged filters).
- Thermostat sends a signal for the furnace or heat pump to turn on, and it turns on. If the filters are dirty (not very dirty, just a bit dirty), the airflow restriction might cause the furnace to provide heating very slowly. If you are standing at the return vent (furnace) or indoor air handler (heat pump), it might seem that the heating system is providing no heat (due to much lower airflow) despite the thermostat clearly stating “heat on”.
- Use of filters with too high a MERV rating. Furnaces, for example, mostly use MERV 6, MERV 8, and sometimes MERV 11 filters. These provide adequate airflow. If, however, we install higher MERV filters (MERV 13, MERV 14, or higher) to get cleaner air, they might be too neatly packed for the furnace to provide adequate airflow. We will see the reduction in airflow and may diagnose this as a furnace not providing heat.
The solution here is simple but very important:
Change furnace filters every 1-3 months. Check if the filters are dirty; if they are, this might be the culprit causing the decreased heating despite the thermostat stating “heat on”. If you have a higher MERV filter (MERV 13, for example), check the unit specs to see if these filters will work with your furnace or heat pump. If not, just replace them with MERV 8 filters (most common ones).
With clean and adequate filters, your furnace heat pump will be able to provide heat when you set the thermostat to heat on.
8. No Fuel For Furnaces (Gas Supply Is Off)
No fuel equals no heat. This is quite simple. With propane furnaces, the propane tank might be empty. The most common fuel supply issues are with natural gas furnaces. Without a steady supply of gas, the thermostat can say “heat on” all it wants, and send signals to the gas furnace to power on, but you get no heat because there is no gas that the furnace could burn to generate heat.
There are several reasons why the gas supply might be cut off, including:
- Gas network issues. Maybe they are working on the gas network, doing repairs, and have shut down the supply of gas for some hours (it happens).
- Unpaid bills. The gas supply might be cut off due to unpaid gas bills.
- Gas valve. To supply gas, the gas valve has to allow the flow of gas to your furnace. If the gas valve is closed, the gas won’t reach the furnace.
- Gas leak. This is a serious issue, as you know. Open windows, go out of the house and call the firefighters ASAP.
You can smell a gas leak and check the gas valve yourself. In most cases, however, it pays to call the gas company to check if they are doing something on the network (they should notify you beforehand) or if there are some unpaid bills you need to take care of.
When the gas supply is back on (or you have refueled the propane tank for propane furnaces), the furnace should start providing heat when the thermostat says heat on (if everything else is sound, of course).
9. Gas Furnace Pilot Not Working (Dirty Pilot Ignitor) Or Faulty Electronic Ignitor
Older furnaces have gas pilot ignitors. When you set the thermostat to heat on, this ignitor will start up the furnace, and the furnace will provide heat. If, however, something is wrong with the pilot, the thermostat might say that the heat is on but the furnace doesn’t provide heat because the pilot can’t turn it on.
The easiest way to tell if you have a faulty pilot is to check how old it is. Pilots have to be changed every 5 years or so. If you haven’t replaced the pilot in the last 5 years, chances are that the furnace doesn’t provide heating when the thermostat signals that heat is required because of a faulty gas pilot that needs to be replaced.
Since these older furnaces use gas pilots, they might also be dirty (residue of burning gas, it accumulates on the pilot). If your gas pilot is only a few years old, try to clean the pilot, and see if that fixes the problem.
Newer furnaces use electronic ignitors (HSI ignitors). There is a whole range of issues that can “break” these HSI ignitors; the most common one is improper voltage. These electronic ignitors are powered by electricity via the ICM ignition control module. Problems with the improper voltage provided by the ICM control module can trip the breaker; you can flip the breaker back in, but you should call an HVAC professional if the breaker keeps flipping.
10. Frozen Outdoor Heat Pump Coils (Low Refrigerant Levels, Dirty Filters, Etc.)
In some cases, the coils in the outdoor unit of a heat pump can freeze over. This will impede the heat exchange that is vital for the heat pump to provide heat. If the coils are frozen, the thermostat will say that the heat is on (it has done its job by signaling the heat pump to start heating), but the heat pump will be unable to perform the task.
You should check the coils in the outdoor unit. If you notice a distinct brownish or even white ice on the coils, you know that this is the reason why there is no heat in the house.
Outdoor coils can freeze over due to many different reasons, including:
- Low refrigerant levels.
- Dirty filters (airflow restrictions).
- Extremely low outdoor temperatures.
Frozen outdoor heat pump coils are a good reason to call your HVAC guy. If you run such a unit despite the ice, you can run into much bigger problems, even needing to replace the heat pump altogether.
11. 2nd Stage In A Two-Stage Furnace Not Turning On
Double the furnace stages, and double the potential issues. With two-stage furnaces, one stage (usually the 2nd one) might not come on. The result is that the thermostat says heat on, but you don’t get sufficient heat.
That’s because the second stage is responsible for extracting an extra 5-20% of heat out of the gas you are burning. If the second stage doesn’t come on, the furnace is automatically less powerful, and you might notice some heat coming from the return vents, but it might not be enough to adequately heat your home.
It is very hard to diagnose if the culprit for heat on but no heat from the furnace is the 2nd stage not coming on. If you suspect this might be an issue (have checked all other common culprits), the smart move is to call an HVAC expert to come take a look.
12. Undersized Heating System Or Extremely Low Outdoor Temperatures
The reason why the thermometer is set to heat on but doesn’t reach the set temperature can, during very cold weather, be either a too-small furnace or just a heat pump losing its efficiency.
In recent years, we have gotten quite a few strikes of extremely cold weather. Furnaces that were capable of adequately heating homes might be overwhelmed by such low temperatures. There is nothing you can do here as far as thermostats and furnaces are concerned; you can get some extra 1500W electric space heaters to supplement the furnaces that are not powerful enough for such cold weather.
With air-source heat pumps, the low outdoor temperatures is another issue altogether. As we have written in this outdoor temperature vs heat pump COP article, the efficiency (COP or Coefficient Of Performance) of the heat pump might fall from 3.5 COP or higher to below 2.0. That means the heating output is cut by half or even more in extremely cold weather.
You can think about installing a furnace that comes on when the outdoor temperatures are extremely cold or supplement the heating with 1500W electric space heaters.
Essentially, nothing is wrong with the thermostat or the heating unit; it’s just that the furnace or heat pump that is installed was not designed to withstand such low outdoor temperatures.
13. Faulty Furnace Blower
In limited cases, you can see that everything seems to be working finet. The thermostat is correctly wired, and the furnace turns on and burns gas, propane, diesel as it should. However, despite having heat on on the thermostat, there is no heat coming from the vents.
The problem sometimes lies in the ability of the furnace to distribute the heat throughout your home. This is a job for a furnace blower; this blower is basically a big fan that propels the heat generated by the furnace via air supply vents to your house.
If you have a faulty furnace blower, the heat will be generated, but it will not be distributed. You might be standing in front of a return vent, thermostat says heat on, the furnace is clearly running, but no heat comes through the vents.
In this case as well, the right move is to call an HVAC expert to check out the furnace blower and fix it.
14. Blocked Supply And/Or Return Vents
During the fall, we might move things around the house, and accidentally block the vents that are vital for the furnace to function. Essentially, the furnace requires supply vents to ‘inhale’ colder indoor air and return vents to ‘exhale’ warmer heated air.
Just check that nothing is put in front of these vents. In many cases, homeowners have already cleared the restrictions in front of return vents when checking why there is no heat from the vents when the thermostat says heat on.
The supply vents are just as essential, but we check them less frequently since we notice no heat (return vents) but rarely check no air supply (supply vents).
Conclusion And Who To Call When You Need Help
We hope that this overview of issues that can cause thermostat heat on but no heat gives you a good insight into what might be wrong. In many cases, we have also pointed out that it would be smart to call an HVAC expert.
If you have an HVAC expert already, you know who to call. If you don’t, you can fill out this form, note what problems you have with your furnace of heat pump, and you will get help from up to 4 HVAC experts in your area. We hope this helps you get the heating back on.
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