Furnace Not Working After Changing Thermostat? How to Fix

Fixing your furnace at home is mostly a trial and error process. There may also be a few faulty components, and repairing or replacing one component may trigger another malfunction, especially if you don’t maintain the furnace regularly.

If your furnace isn’t working after changing the thermostat, possible causes include a lack of power, faulty thermostat battery, and issues with the furnace control board. If you replaced your thermostat to fix another issue with your furnace, you may have misdiagnosed the cause of the problem.

In this article, I’ll walk you through how to troubleshoot a furnace that isn’t working after changing the thermostat. I’ll also share the easiest way to fix this problem. (Check the bottom of the article.)

Let’s get started!

Rule Out the Thermostat As the Problem

Since the last change you made to your home’s heating system was changing the thermostat, you should start your diagnosis there.

• Confirm you installed the correct thermostat. Thermostats are model-specific; installing the wrong one can cause a communication issue between it and the furnace. Do your research and consult a professional before purchasing a thermostat.
• Be conscious of your thermostat’s location. You can’t expose the thermostat to sun rays or heat sources; they work by detecting heat changes, and if heat rays are constantly in contact with it, it will detect the wrong temperature. Any significant holes behind the thermostat also affect readings, so be sure to look for that as well.
• Ensure the thermostat can get power. A thermostat has a display that lights up and shows the set temperature; if the display is off, there is no power connection. Find out whether electric power in the house or the power supply from the circuit breaker and main switch is turned off and switch it on.
• Check the thermostat’s battery. Some thermostats use batteries, and although you just replaced it, you may have bought one with a faulty battery. Replace the battery and test it again.
• Check the thermostat’s fuse. If the fuse is blown, the thermostat won’t work, and the furnace won’t get the “turn on” command. You can easily replace fuses once you know the make and model. To be safe, carry the damaged fuse to the hardware store to get the correct replacement.
• Check if your room’s temperature is higher than your thermostat setting. When setting the thermostat to heat your room, ensure that you set a temperature slightly above the room temperature; otherwise, the thermostat won’t trigger the furnace to turn on. Reset your thermostat and wait a few minutes for the heat to kick in.

If the power supply works but the furnace still won’t work, you should investigate inside the thermostat. Here’s how you can do that:

• You will have to turn the power supply off at the circuit breaker
• Uncover the thermostat. You don’t need any tools for this, as the cover pops right up.
• Check whether you left loose screws or wires in the thermostat while installing it and tighten them.
• Close the cover back on, then switch on the power supply and test the thermostat.

Test Whether Your Thermostat Works

Thermostats have five wires, each connected to a terminal. The cables are color-coded with Red (R), White (W), Green (G), Yellow (Y), and Common (C). When you open the thermostat’s lid, take a photo of where every wire goes. You will need a screwdriver for this job.

Here are some step-by-step instructions:

1. Remove the wires from their respective terminals.
2. The red and white wires usually represent power and heat. Locate them and connect their b are ends.
3. Turn on the furnace and check whether the furnace starts working; if it does, you hadn’t installed the wires correctly.
4. The furnace should respond; if it doesn’t, there is a problem with the power supply between the thermostat and the furnace.

For a visual aid on troubleshooting thermostat issues, watch this video from AC Service Tech LLC:

https://www.youtube.com/watch?v=81_gCF4mJtQ

Check for a Problem with Your Furnace’s Control Board

The control board ensures that individual components in the furnace have a smooth working sequence. A typical furnace can have any of two control boards, including ignition control boards and furnace controls.

The easiest way to recognize a control board issue is by noticing a fluctuation in temperature inside your house. You keep adjusting the temperature, but it either falls short of your settings or uncomfortably exceeds your temperature settings.

When you realize the temperature falls short, warning lights on the furnace will be the next indicator. It is essential to find out what each error means. A sticker on the inside of the panel door has decoded what each LED light indicator color means. Control boards may malfunction because of:

• Loose wires on the furnace. Furnaces vibrate quite a bit, and this might cause the cables to come loose. To learn how to troubleshoot a furnace control board, watch this video:

https://www.youtube.com/watch?v=phsUYxo75jU

• Dirt and dust cover. When air filters are covered in dirt, dust, and debris, they tend to put a strain on the blower motor, which overheats and can eventually lead to costly repairs. If you hear an unusual sound coming from your furnace, then that is likely the problem. Make sure you’re cleaning your air filters regularly.

Dust buildup in the furnace surfaces may cause flame sensor malfunctions, foul odor, gas buildups, and eventual mini-explosions. To prevent this, perform regular furnace tune-up and maintenance.

Did You Misdiagnose The Problem?

The fact that you replaced the thermostat means that the furnace wasn’t working previously, and you may have mistakenly identified the thermostat as the problem. If none of the above solutions fixed your furnace, the thermostat probably wasn’t the issue. If you’re having this much trouble diagnosing your thermostat, I highly recommend hiring a furnace repair expert to take care of this.