The most common reason behind your furnace not being able to kick on once you turned up the thermostat, is that there has been a power failure someplace. If your furnace is very quiet (not doing anything), go check your breakers first.
Let’s say your thermostat is working and you adjusted it the way you like it (put on the heat setting and raised the temperature), now it’s time for the inducer fan to kick in. The first thing that should start within a furnace is an inducer motor (any furnace with an efficiency of 80% or higher should have it), and if it doesn’t start, you know that you have a power issue.
If your thermostat is working, it is very hard to suspect a power issue, but it may be fed separately via batteries. There are several things you can check for in relation to the power problem and make sure to go through them all before starting more fancy manipulations with your furnace.
Now we will go through different reasons why your furnace will not kick on.
No power to the furnace
So, what do you do when there is no power to your furnace? If you have power coming in from utilities and there is no blackout in town, let’s start troubleshooting:
#1. Furnace power switch
Is your furnace turned “ON”? Furnace switches look very similar to light switches, and someone could have turned it “OFF” by accident:
Even if you have a gas or oil furnace, it does require an electricity source for its different components. Your furnace will not even start if there is no power supply!
One of the first things you may want to check is the circuit breakers. You need to make sure they are flipped the right way and are supplying your furnace with electricity.
** Note! Furnaces are usually on dedicated breakers and if your circuit breaker keeps on tripping, call a professional to check it out!
#3. Furnace fuses
A fuse in a furnace is a guarantee that excess electricity will not damage your equipment or cause a fire in case of a short. It usually has one or two fuses, with one of them being a lower rating than your circuit breaker.
Let’s start with a big fuse. If your power switch looks something like this, it comes with a fuse box built-in. Here is one from Bussman (paid link) conveniently available on Amazon:
This is what the fuse inside looks like:
Before you start working on anything inside your furnace, make sure you:
Turn the power OFF!
Take the fuse out and if it’s blown (or just in case), replace it with the exact same type. They are not expensive, here a good UL-listed fuse (paid link) from Amazon.
There is also a fuse inside your control panel (it is usually about 3 or 5 amps), that can blow quite frequently (for any small reason). It could be located right on your control board:
This fuse could also be just hanging on the 24v hot wire coming from the transformer. Your control board is located in the lower compartment of the furnace and this is where all thermostat signals go.
If you changed your thermostat recently (or are still in the process of changing), you may have wires shorted out. If 24v hot wire (usually red) and common wire (usually blue) touch each other, you will blow your fuse immediately.
This fuse is placed between red power wire (R) coming out of your transformer and an “R” terminal in your control board:
Even though it looks like a car fuse, it is NOT a car fuse! Car fuses are designed for 32-volt systems and furnace fuses for 24-volt (low voltage):
Here is a nice video on where to find this fuse and how to replace it:
#4. Furnace door safety switch
Every modern furnace is equipped with a blower door safety switch, and it is usually located at the top of the lower access panel. Its main function is to disconnect power to the furnace when the panel is open and resume its operation when the panel is back in place.
It is designed this way to make sure that the panel door will never be left open while the blower fan is running. If the door cover is not completely closed or the safety switch is dead, then the furnace will never be able to start.
This is how you change the switch and test it:
#5. Pan float safety switch
Your power circuit could also be disconnected because of the device called “Condensation Pan Safety Switch” or “Pan Float Safety Switch” and it looks like this:
If your furnace produces too much condensation or is leaking, this safety switch will cut the power. It is activated by water filling up a pan to a certain point and brakes a circuit.
This is a simple repair, just look for the switch if you notice too much water. If it needs replacement, here is a model with good reviews (paid link) from Amazon.
No power to furnace or thermostat
You can easily notice if you get no power if your thermostat is powered by a control board. If it’s on batteries, it could give you a false assurance that there is nothing wrong with a power supply!
Thermostat primary job is to connect wires depending on your chosen setting:
- Red & White wires – activate a furnace
- Red & Green wires – activate a fan
- Red & Yellow wires – activate an air conditioner
If wires get connected properly and your furnace does not respond, you have a communication problem between your thermostat and the furnace. The problem could be in wiring or the furnace itself.
Another thing you could check for is if your furnace is able to start manually by crossing red and white wires on the thermostat end. If it works, you need to replace the thermostat (or change batteries).
No power to furnace control board
To start troubleshooting your furnace board, you need to temporarily tape the safety door switch with electrical tape (not aluminum!!!). This will give you the ability to test your control board.
If your furnace has a flashing led light ON, it means that the control board is getting 120v and 24-volt power. Not all furnaces have this feature, and you will need to use a multimeter for further testing.
Let’s first find out if there is high voltage power coming into your control board. The voltage between L1 (live voltage coming into the board) and a common (or neutral) terminal should be 120-volts.
Now let’s find out if your low voltage power is available. To do this, we will need to locate a little device called “a transformer”:
The primary function of a transformer is to supply your furnace with 24v power. It takes 120VAC power coming from your outlet and changes it into 24 volts.
There are two high voltage wires attached to the transformer (black and white) and two low voltage wires. Two low-voltage wires are usually red and blue.
So, how to check if the furnace transformer is bad? The voltage between “R” & “C” terminals should read 24-24 VAC. You can also measure the voltage at the transformer wires coming into the control board.
If there is no power coming out (while you have high voltage power coming in), your transformer is bad. After testing a transformer, you can start testing the control board itself:
If the control board is supplied with electricity (24v & 120v), but some of the furnace components don’t kick in, you could have problems with relays and loose (or burnt) contacts on the board. In this case, the easiest thing to do is to replace the board.
To replace the board, try to find the part number that should be written on it and get the same one:
Here are some furnace control boards (paid link) sold on Amazon to help you out.
Faulty inducer motor capacitor
If you hear your inducer motor humming and it does not start, you could have a problem with your capacitor. Most older furnaces inducer motors are PSC (or permanent split–capacitor) and require a run capacitor to give them a phase shift to start a motor.
It’s a permanent capacitor that will not be disconnected upon starting the motor. These are small capacitors (2, 3, or 4 UF), and this is what they commonly look like:
This capacitor should be safely checked for capacitance with a multimeter in the following way:
- Step 1. Shut down power to the furnace.
- Step 2. Short out your capacitor by touching both terminals with a screwdriver.
- Step 3. Unplug wires connected to the capacitor and take it out.
- Step 4. Set your multimeter to the capacitance setting (red arrow):
- Step 5. Check your measurement. If it is above or below the capacitor rating in microfarad (MFD, MF, UF, μF), replace the capacitor.
Here is an example of a 3.0 MF (250-volt) capacitor.
The capacitor should be replaced only with the same size microfarad rating and the same or higher voltage rating.
** Warning! Capacitors should be checked by a professional only!
** Important! If your motor has all the above symptoms (of a bad capacitor) and someone tells you that your motor is bad, get a second opinion!
Faulty inducer motor
Your draft inducer motor could actually go bad. The inducer motor is the first component of the furnace that starts when your thermostat calls for heat.
The purpose of the inducer motor is to aid in venting your furnace. At the start, it removes all the remaining gasses from the previous cycle in the heat exchanger.
This is what furnace inducer motor assembly looks like:
If it doesn’t start (assuming the power is actually getting to it), it could have got overheated and shorted out. In this case, you will have to replace your motor.
There are so many things that could go wrong with your furnace, that you may just want to leave the troubleshooting to professionals! If you are suspecting damaged wiring or any loose connections, contact an HVAC specialist ASAP!
** Warning! Working with furnaces and electricity, in general, is dangerous and you should not follow any advice without consulting a professional first!
Attention! This article is for informational purposes ONLY and is NOT a replacement for a professional advice! You will need to visit school's website for details and updates, as well as consult your local HVAC specialist for appropriate solution to your problem.
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