Only Getting 15 Volts At Thermostat: 5 Reasons & Solutions!

In most household thermostats, you should be getting around 24 volts. If you get less voltage than this, there must be a problem with your thermostat that requires an immediate fix. 

So, why are you only getting 15 volts at thermostat? 

The reason behind getting only 15 volts from your thermostat can be a loose or open wire connection or a blown fuse. Also, if the thermostat’s components are dirty, the transformer is not grounded properly or the battery is low, you can get low voltage. To fix this, check the wiring connection, clean the contacts, and replace the fuse if required. Also, use fresh batteries and ground the transformer properly. 

To understand these reasons in detail with the proper solution, keep reading. 

Only Getting 15 Volts At Thermostat: Reasons & Solutions

There are a bunch of reasons why your thermostat is only receiving 15 volts. Here are those reasons with detailed solutions.

Reason Solution 
Loose Or Open ConnectionCheck and properly connect the wiring 
Blown FuseReplace the fuse
Dirty ComponentClean the thermostat and its component
Low BatteryReplace the battery
Transformer is Not GroundedGround the transformer properly 

Reason 1: Loose Or Open Connection

The wiring connection of a 24-v thermostat is already very complicated as they have too many wires. If any wires get loose, damaged, or open, you will get low voltage from your thermostat.

If you have a zone controller and you are getting 15V from only a particular zone, the wiring to that zone can be damaged or loose. 

This type of situation might happen where you are getting no voltage between R and C, or only C wire low voltage. These all symptoms are the result of a loose wire connection. 


Check the wiring of your thermostat to other components including the 24-V transformer, AC, heat pump, and fan. Verify that all of the cables are properly connected. If not, tighten the connection. Visibly check the wiring for any damage and wear. If the wires are not in good condition, replace them.

Sometimes, the cable is broken internally and it’s not possible to identify it visibly. In this case, you have to take the cable of the zone you are getting low voltage and try it in another zone. If the later zone also starts getting 15 volts, that means the cable was damaged internally and you have to replace it.

Reason 2: Blown Fuse

Another common reason for getting only 15 volts from your thermostat can be a blown fuse. Technically, you should not get any power when the fuse is blown. But still, some people claim to face this issue where they get 15-16 volts from the thermostat before they discover a blown fuse.


Here’s how to replace a fuse in the thermostat

Step 1: Make sure to shut off the power supply to the thermostat by switching off the circuit breaker that controls it.

Step 2: Remove the cover to reach the internal components of the thermostat. Step 3: Look for the fuse inside the thermostat. It’s usually a small, cylindrical component with metal caps on both ends. It has a thin filament inside. Visibly check the filament. If it is cracked or separated, the fuse has blown, and you must replace it.

how to replace a fuse in the thermostat

Step 4: Gently pull the old fuse out of its holder. You can use a fuse puller to get a grip on it.

Step 5: Insert a new fuse of the same type and rating into the holder.

Step 6: Reassemble the thermostat cover once the new fuse is securely in place.

Step 7: Switch back the circuit breaker on to power the thermostat again. 

Now the voltage problem should be solved. If not, go to the next reason. 

Reason 3: Dirty Component

If the wiring and components of the thermostat are dirty it can cause a low voltage problem. Also, the thermostat itself can accumulate dirt, especially if you keep this in the kitchen. This can also cause your thermostat to lose power intermittently. 


You need to clean all the components of the thermostat. Here’s how to clean the thermostat.

Step 1: Turn off the power of the thermostat by switching off the circuit breaker or removing the batteries, depending on the type of thermostat you have. 

Step 2: Pull the thermostat cover off gently to remove it. Some thermostats have covers that snap off, while others may have screws that need to be removed.

Step 3: Use a soft-bristled brush to gently remove any dirt, or debris from the inside of the thermostat, especially the contact point. Be careful not to touch any of the internal components or wiring. You can also use compressed air if you think the contact points are difficult to reach with a brush. Here are some air can blowers you can choose from. 

Compressed AirFeaturePrice 
Falcon Dust Compressed GasThe small container and thin head pipe of this product help you reach the impossible corners for cleaning. Check Latest Price
Innovera 10016 Compressed AirThis product is 100% Ozone safe and removes all dirt very quickly. Check Latest Price
Dust-Off Plus Compressed AirA reliable compressed air you can use which is also very reasonable in price. Check Latest Price

Step 4: Use a damp cloth or sponge to wipe down the outside of the thermostat, being careful not to get water inside. If there are any tough stains, use a mild soap solution to clean them off. In the same way, clean the cover of the thermostat as well. Make sure they are completely dry before reassembling them. 

Step 5: Reassemble the cover by snapping or screwing the cover back into place once you are done cleaning. 

Step 6: Turn the power back on to the thermostat. 

Reason 4: Low Battery

If you have a battery-generated thermostat, the low battery can be a cause for getting low voltage. 


Replace the battery with a fresh one. Make sure to use the right type of battery for your thermostat. 

Reason 5: Transformer is Not Grounded

If the 24-V transformers of your thermostat are not grounded properly, there is a chance of getting less voltage. 

Grounding the common side of the transformer provides a reference point for the circuit’s hot or positive side to the ground. This reference point helps to establish a stable electrical potential or voltage.

Without a proper ground reference, the circuit’s voltage can become unstable and fluctuate, potentially leading to low voltage output


Different transformers have different grounding configurations. So, check the manual of the transformer or consult a technician to ensure proper grounding of the transformation. 

How To Save Energy & Money With a Thermostat?

Here are some easy tips you can follow to reduce pressure on your thermostat and the whole HVAC system and save energy and money. 

  • Keep the optimal temperature setting in summer and winter. According to the U.S. Department of Energy, you should keep your temperature at 68o Fahrenheit during the winter season. And in the summer season keep the temperature at 78o Fahrenheit. 

This is the most optimal temperature that will help you save energy. However, sometimes you might require to go below 68. In this case, if the thermostat doesn’t work, for example, if your thermostat doesn’t go below 68 degrees, it indicates a problem. 

  • Consider investing in a programmable or smart thermostat, which can adjust the temperature automatically based on your preferences and schedule. This can help you avoid unnecessary heating or cooling when you are not home, saving energy and money.
  • Use ceiling or portable fans to help circulate air and your home will feel more comfortable. Also, in the summertime, you can keep the window open for proper ventilation rather than relying solely on your HVAC system.
  • Regularly clean or replace your HVAC system’s air filters to ensure that it is running efficiently and effectively.

Frequently Asked Questions (FAQs):

What Are The Signs Of A Thermostat Not Working?

If your thermostat is not working, you will notice signs like getting fluctuating temperatures and erratic readings from the thermostat. Also, you might hear unusual noises from the thermostat and see the coolant leaking. A faulty thermostat will also increase fuel consumption. 

How Do I Test My Thermostat With A Multimeter?

To test a thermostat with a multimeter, set the multimeter to read ohms and remove the thermostat from its mounting plate. Touch the multimeter’s red connector to the thermostat’s red cable and the black probe to the thermostat’s black wire. Match the reading with your thermostat’s user manual. 

What Happens If You Wire A Thermostat Wrong?

Wiring a thermostat wrong can cause several problems such as the system not turning on or off, short-circuiting, or tripping the circuit breaker. It can also cause damage to the HVAC equipment. It may also pose a risk of electric shock or fire hazard.


Here go the solutions for only getting 15 volts at the thermostat problem. Consider the reasons gradually and try the fixes. I have no doubt you’ll find the solution. However, if you are not comfortable with fixing this by yourself, call an expert technician for the job.

That’s all for now. Have a great day! 

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