If there is no power to the thermostat for your heat pump or HVAC system, that's just one reason the system won't turn on. It's one of the easiest to check, though, and that makes it a good place for homeowners to start in the process of troubleshooting their heating or cooling system.
A thermostat is an electrical device that functions like a switch, and the easiest way to test whether it's getting power is to bypass it and see if the HVAC equipment switches on. If you have a multimeter, you can also use that to test the thermostat for power, and this test is even easier to conduct than the bypass test.
Video of the Day
A standard heating/cooling thermostat operates at low voltage (24 volts), so if you get a shock, it won't be a bad one. Nevertheless, no shock is better than a mild one, so play it safe and turn off the power whenever you're handling the wires.
Easy Test for No Power to Thermostat
You might not even have to remove the thermostat cover to find out if the unit has power. One simple way to check is to turn up the thermostat as far as it will go and wait a minute to see if the furnace turns on. If the thermostat controls an air conditioning system, turn it down as far as it will go and wait for the air conditioner to start. While you're at it, toggle the fan switch from Auto to On, and if the thermostat has power, you should hear the blower start.
The fact that the LED on the thermostat screen is lit doesn't necessarily mean it's receiving power from the transformer. The LED and the programming functions may be powered by a battery instead of a common wire, or C-wire, coming from the transformer.
Bypass Test for Thermostat With No Power
If pretests are inconclusive, the next step is to remove the thermostat cover to expose the thermostat wiring. If you don't see wires when you remove the cover, it's because you're looking at the mounting plate for the temperature sensor and not the base plate to which the wires are connected. The sensor plate is usually held on by two or more screws that you can remove with a Phillips screwdriver.
When you get to the base plate, you'll see a number of colored wires attached to terminals designated by letters. The ones in which you're interested for this test are the wire attached to the R terminal, which is usually red, and the wire attached to the G terminal, which should be green.
1. Turn Off the Power
If your heating system has a plug, you can simply pull the plug to kill the power. Otherwise, flip off the circuit breaker in the main service panel (breaker box) that controls the HVAC system.
2. Connect the R-Wire to the G-Wire
Disconnect the wires connected to the R-terminal and G-terminal and twist them together.
3. Turn the Power Back On
Plug the system back in or go back to the breaker box and turn the circuit breaker for the system back on. If the thermostat is receiving power from the transformer, the blower will start when you do this.
4. Replace the Wires
Turn the power back off after conducting this test, connect the wires to the terminals from which you removed them, and replace the cover plate before turning the power back on.
If your home has a basic heating system with a blower, the thermostat may have only two wires: a red power wire and another wire — usually white — connected to the thermostat's W terminal. In this case, conduct the test by twisting the red and white wires together, and if the thermostat has power, the blower should start when you turn the home's circuit breaker back on.
How to Test a Thermostat Using a Multimeter
If you happen to have a multimeter, you can test the thermostat without turning off the power or disconnecting any wires.
1. Remove the Thermostat Cover
Pull the cover off the thermostat and, if necessary, unscrew and remove the sensor plate to provide access to the wires.
2. Set the Meter
Turn the meter dial to measure in the range of 100 volts AC (usually denoted by "VAC" or "V" with a squiggly line over it).
3. Take a Reading
Touch one of the meter probes to the R terminal (to which the red wire is attached). Touch the other probe to any other terminal that has a wire attached (for example, the G, W, or Y terminal). Note the reading. If the thermostat has power, it should be between 22 and 26 volts. If the reading is zero, the thermostat isn't getting power.
Troubleshooting Thermostats With No Power
If your thermostat isn't properly controlling your heating/cooling system, there are a few simple procedures you can do on your own before calling an HVAC professional.
- Replace the thermostat batteries. A thermostat with dead batteries won't work. Replace the old batteries with identical new ones.
- Check your electrical panel for a tripped circuit breaker. If you find one, reset it.
- Look for partially open doors on your air handler, furnace, or air conditioning unit. Most units have a safety switch that disables the power supply when the door is open.
- Clean or replace the air filters. Dust buildup might be severe enough to block airflow to the point that you can't tell if the blower has come on or not.
- Note any error code displayed on the thermostat's LED screen and look it up in the owner's manual. The manual will tell you whether (and how) you can fix the problem on your own or whether you need an HVAC technician.
- Review programming instructions. One of the most common problems with a new thermostat is that the thermostat settings aren't correct. This is particularly true if you have a new programmable thermostat or a Wi-Fi-enabled smart thermostat, such as a Nest thermostat or one of the many Honeywell products, which are all a little different from one another.
Ultimately, it may happen that your problems aren't being caused by the thermostat but by loss of power in the HVAC system. There could be a blown fuse in the control board, faulty wiring to the thermostat, or a bad transformer. If you have a heat pump that isn't providing warm or cold air, the cause could be a faulty condenser or loss of refrigerant. All of these are best handled by an HVAC technician.