How to Fix Aux Heat on Your Thermostat: A Complete Guide

If you’ve noticed that your thermostat is using auxiliary (aux) heat too often, or the aux heat is malfunctioning, it’s important to address the issue quickly. Aux heat is a secondary heating source in heat pump systems, typically more expensive and less efficient to run than the heat pump itself. It’s designed to supplement the heat pump during extremely cold conditions or when the heat pump struggles to reach the desired indoor temperature. If the aux heat is running when it shouldn’t be or not working at all, you could face higher energy bills or insufficient heating.

In this guide, we’ll walk you through common issues with aux heat, how to troubleshoot, and what you can do to fix the problem.

Understanding Aux Heat on a Thermostat

Before diving into the fixes, it’s essential to understand what aux heat is and when it’s supposed to activate. In a heat pump system, the primary heat comes from transferring heat from the outside air to the inside of your home. However, in colder temperatures, the heat pump might not be able to extract enough heat efficiently. Aux heat, often an electric heat strip or gas furnace, kicks in to supplement the heat and maintain the indoor temperature.

Aux heat is only meant to be a backup; it should not be running all the time unless you’re in extremely cold conditions. Continuous use of aux heat can significantly raise your energy bills.

Common Reasons Aux Heat Isn’t Working Properly

If you’re having trouble with your aux heat, there are several potential causes:

• Thermostat Settings or Faulty Thermostat: Sometimes, aux heat can activate if your thermostat settings are incorrect. Alternatively, a faulty thermostat might not properly control when aux heat should activate.
• Heat Pump Issues: If your heat pump isn’t functioning correctly, it may force the aux heat to kick in more frequently than it should, or aux heat may not work at all if the system is over-reliant on the primary heat pump.
• Temperature Sensor Problems: Many thermostats use a sensor to determine when to switch to aux heat. A malfunctioning sensor might cause the aux heat to come on prematurely or not at all.
• Electrical or Mechanical Failures: In some cases, a failure in the wiring, control board, or heating elements of the aux heat system may prevent it from working correctly.

Now that we’ve identified some potential causes, let’s explore how to troubleshoot and fix these issues.

Step-by-Step Troubleshooting Guide to Fix Aux Heat

Check the Thermostat Settings

The first place to start is with your thermostat itself. Ensure that it is correctly set to “Heat” mode rather than “Emergency Heat” or “Cool.” Emergency heat will bypass the heat pump and use aux heat exclusively, which can lead to excessive heating costs.

You should also verify the temperature differential setting, which determines when the aux heat should activate. Most thermostats are set to trigger aux heat when there’s a 2-3 degree difference between the indoor temperature and the setpoint. If this setting is too narrow, aux heat may come on too frequently. Adjusting the setting can help prevent unnecessary aux heat use.

Inspect the Thermostat Wiring

A faulty thermostat or poor wiring connections can also be the culprit. Turn off your thermostat and remove it from the wall to inspect the wiring. Ensure that all wires are securely connected and in the correct terminals based on the manufacturer’s guidelines. Loose or corroded wires could cause communication issues between the thermostat and your heating system.

If you notice damaged wiring, it’s best to consult a professional HVAC technician to avoid any further electrical hazards.

Test the Heat Pump System

The next step is to check if your heat pump is functioning correctly. If the heat pump isn’t able to provide enough heat, the thermostat will automatically switch to aux heat.

1. Listen for Unusual Noises: If your heat pump is making unusual noises (such as grinding or banging), it could be struggling to operate efficiently, which may cause frequent aux heat activation.
2. Check Airflow: Ensure that the airflow from the vents is consistent and strong. Poor airflow can indicate an issue with the air handler, clogged filters, or blocked ducts. Restricted airflow can reduce the efficiency of your heat pump and force the aux heat to engage more often.
3. Examine the Outdoor Unit: Check the outdoor unit of your heat pump to ensure it is free of debris and ice buildup. An iced-up or obstructed heat pump cannot effectively transfer heat, which can cause the system to rely on aux heat unnecessarily.

Test the Aux Heat Components

If your aux heat isn’t working at all, it could be due to a malfunction in the aux heating elements themselves. Here’s how to check:

1. Check the Breakers: Go to your electrical panel and ensure that the breaker for the auxiliary heat system hasn’t tripped. A tripped breaker will prevent aux heat from functioning.
2. Test the Heating Elements: For electric aux heat, faulty heat strips or relays could be the issue. You’ll need a multimeter to test the heating elements for continuity. If they aren’t working, they’ll need to be replaced.
3. Inspect the Control Board: The thermostat sends signals to the control board in your HVAC system, which then triggers the aux heat. A malfunctioning control board can miscommunicate or fail to activate aux heat when necessary.

Replace or Upgrade Your Thermostat

If you’ve checked the settings and wiring but aux heat still isn’t functioning properly, your thermostat may be outdated or faulty. Consider upgrading to a programmable or smart thermostat that offers better control over your HVAC system. Many smart thermostats can be programmed to minimize aux heat use, helping you save on energy costs.

When installing a new thermostat, follow the manufacturer’s instructions carefully or seek professional installation to ensure compatibility with your system.

Conclusion

Fixing aux heat on your thermostat requires a combination of troubleshooting your thermostat, heat pump, and auxiliary heating components. By following the steps outlined in this guide—checking thermostat settings, inspecting wiring, testing the heat pump, and ensuring the aux heat system is functioning—you can diagnose and often resolve common issues.

If these steps don’t solve the problem, it’s best to contact a professional HVAC technician who can conduct a more in-depth inspection and repair. Addressing aux heat issues early can save you from unnecessarily high energy bills and ensure your home stays warm during the colder months.