Understanding Thermostat Power
There are several reasons why your thermostat could be blank. Before diving into exactly what could be the cause, it is important to understand the anatomy of the thermostat and where it receives its power. Most traditional thermostats today have low-voltage wiring coming from the air handler to the thermostat, supplying both 24V and a common wire going back to the air handler. This completes the circuit, allowing 24V to flow through the thermostat.
This means the air handler’s transformer is essentially supplying power to not only the thermostat, but also all low-voltage circuitry throughout both the indoor and outdoor system.
Thermostats With Batteries and the Common Wire
However, it is important to understand that not all systems have an additional common wire, which would mean the thermostat itself requires batteries to supply voltage to the thermostat so that it may send signals when heating or cooling is required.
The easiest way to understand which system you have is to carefully remove the faceplate from the backplate of the thermostat. This will expose the wiring, and you should be able to identify whether your system has a common wire.
This wire should be placed in the C terminal. The most commonly used colors are blue, brown, or black; however, in most cases, blue is the preferred color. If you do not see a common wire in the C terminal, then either you do not have the additional wire available, or the installing contractor did not hook up the additional blue wire at the indoor unit or at the thermostat.
This would render the thermostat powerless without batteries. In this case, if your thermostat is blank, it is a good idea to replace the batteries annually. I always recommend that my customers change their batteries in the thermostat on the same cycle that they change their smoke detectors and fire alarm detectors throughout their home.
Check the Indoor Breaker
Another cause for the thermostat to have no power displayed is that there is no high-voltage power going to the indoor breaker. This is an easy identification and correction. Locate the inside breaker panel, usually located in a utility room, closet, or garage. When you open the breaker panel, your breakers should all be labeled.
Your indoor unit, otherwise called your air handler, should be labeled AHU, HEAT, or, in some cases, FURN.
Once you have located the breaker for the indoor unit, check to see if it appears to be in the trip position. A tripped breaker will not appear to be all the way in the off position, nor will it appear to be all the way in the on position. If the breaker is tripped, you must turn it all the way off, then turn it all the way back on.
Once this is complete, you should go back to the thermostat and see if the thermostat is beginning to reboot. Sometimes this takes several seconds, depending on the type of thermostat that you have. If power is restored, the system will likely come back on in five minutes or less. This is due to a safety feature built into the thermostat called a five-minute time delay.
If the breaker will not reset and keeps tripping, it is recommended that you contact an AC professional and leave it in the off position. If the breaker was in the on position when you checked it the first time, the power supply is likely not the problem.
Check the Primary Float Switch
The next step in this diagnostic is to locate the indoor unit where you likely change the filter. Once standing at the indoor unit, locate the drain line, usually on the lower left-hand side of the air handler if the air handler is standing up in a vertical position. Some brands may have it on the lower right-hand side.
Next to the drain line, you should see a primary float switch. There are different models of float switches. The easiest way to identify this float switch is that it looks like a small cup with a wire coming out of the top of the cup.
Do not pull on the wires where they are attached to the float switch, as you may cause damage to the wiring. Remove the cap from its plastic body by twisting the cap off.
Once the cap is removed from the float switch, look inside the cup. If there is water, then the drain line is restricted/clogged. This float switch is designed to prevent the system from flooding your house, and the float is severing the 24V connection and power supply to the thermostat, preventing the system from running.
Please refer to our drain line cleaning techniques in other areas of our website, or call us to schedule a service call.
Check the Secondary Drain Pan
If the primary float switch is clean and dry, the next step would be to locate and identify whether you have a secondary drain pan. This is commonly used on second-floor closet installation applications, or if your air handler is located in the attic space. The easiest way to locate your float switch on a secondary pan is to look for the same type of wire as your primary float switch.
Please note that the float switch will look different in this application. This float switch is usually a clip-on. Check to see if the float is in the downward position, making contact with the magnetic reed located at the bottom of the float.
If the float switch is upside down or on its side, restore it back to its normal position where the float switch and the bobber are in the downward position.
Recheck your thermostat to see if power is rebooting. If you have located your float switch and identified that there is water in the pan, you will first need a wet/dry vacuum to remove all the water. Once all the water has been removed, the thermostat should come back on.
| User Caution There is usually an underlying reason why there is water in the secondary drain pan. Further diagnostics will be required. Common things to look for would be: |
- Dirty filter
- Dirty evaporator coil
- Frozen evaporator coil
- Cracked drain pan
- Cracked drain line
- Drain line pitch problem
This is just to name a few. It is recommended to contact a professional to understand what has caused the water to drain into the secondary drain pan.
Check for 24V at the Thermostat
If all these areas have been identified and checked off as in good working order, and no issues are present, the next potential issue could be a faulty thermostat. Before replacing a thermostat, it would be helpful to understand if you are getting 24V to the thermostat using a simple volt meter.
Testing between the R terminal and the C terminal, the volt meter should read a nominal 24 volts.
If no voltage is present, it is recommended that you contact a professional. If 24V is present at the time of diagnostic, then proceed to replace the thermostat.
| Buyer Be Warned You must replace the thermostat with a compatible thermostat that matches your system. Most thermostats are programmable, which means you will have to configure the thermostat to match what type of heating and cooling system you have. Follow all safety instructions, including turning off power before removal and installation of the new thermostat. |
| Caution Guardian Heating and Air supplies this information as fact-finding information and does not recommend that anyone do any electrical work or repair on their HVAC system. Performing repairs on your HVAC can and will void any warranty you may have with your HVAC contractor/builder and manufacturer.Repairing your HVAC system can be dangerous and complicated. Guardian HVAC LLC recommends that you seek out a licensed contractor to work on your HVAC system. |