What is auxiliary or emergency heat on a heat pump?

Understanding Auxiliary and Emergency Heat on Heat Pumps

Heat pumps are an increasingly popular and efficient way to heat and cool homes, offering a dual-purpose solution for year-round comfort. However, understanding the nuances of their operation, especially during extreme weather conditions, is crucial for homeowners. One aspect that often causes confusion is the concept of auxiliary heat, also often referred to as emergency heat. This article aims to demystify these terms, explaining what they are, why they are necessary, and how they function within a heat pump system.

The Basics of Heat Pump Operation

Before delving into auxiliary and emergency heat, it’s helpful to briefly review how a heat pump operates. Unlike furnaces that generate heat through combustion, heat pumps work by transferring existing heat. In heating mode, a heat pump extracts heat from the outdoor air and transfers it indoors, even when outdoor temperatures are quite low. This process is reversed in cooling mode, where it extracts heat from the indoor air and releases it outdoors.

The primary component responsible for this heat transfer is a refrigerant that circulates within a closed loop. This refrigerant changes state between a liquid and a gas, absorbing and releasing heat as it does so. The efficiency of a heat pump is measured by its SEER (Seasonal Energy Efficiency Ratio) for cooling and HSPF (Heating Seasonal Performance Factor) for heating. Higher ratings generally indicate greater efficiency.

When Heat Pumps Need Help

While modern heat pumps are remarkably efficient and can operate effectively in a wide range of temperatures, there are limitations. As the outdoor temperature drops, there is less heat available in the air for the heat pump to extract. Consequently, the heat pump’s ability to heat your home adequately diminishes.

This is where auxiliary and emergency heat come into play. They act as a backup heating source, ensuring your home remains comfortable when the heat pump alone cannot keep up. The key distinction between auxiliary and emergency heat lies primarily in their intended use and how they are activated, though in many residential systems, the terminology is used interchangeably. Functionally, they typically refer to the same supplemental heating element.

What is Auxiliary Heat

Auxiliary heat, often referred to as “aux heat” or “secondary heat,” is a supplemental heating element integrated into your heat pump system. Its primary purpose is to assist the heat pump when outdoor temperatures fall to a point where the heat pump’s performance is significantly reduced. It can also engage during the defrost cycle of the heat pump, which is a necessary process to remove ice buildup from the outdoor unit in cold weather, and during system start-up when the indoor temperature has dropped considerably.

The most common type of auxiliary heat is electric resistance heating. This works similarly to a toaster or an electric space heater, where electricity passes through resistive elements, generating heat. While effective, electric resistance heat is generally less energy-efficient and therefore more expensive to operate than the heat pump itself. Because of this, auxiliary heat is typically designed to engage only when absolutely necessary.

How Auxiliary Heat is Controlled

Your thermostat plays a critical role in managing the auxiliary heat. It monitors the indoor temperature and compares it to your desired setting. When the heat pump struggles to reach or maintain the set temperature, the thermostat will signal the auxiliary heat to turn on. This is often indicated on the thermostat by the word “Aux” or “Auxiliary” appearing on the display.

There’s usually a setpoint, often called the “auxiliary heat lockout temperature” or “balance point,” below which the system is programmed to rely more heavily on auxiliary heat. This temperature varies depending on the specific model of the heat pump and how it’s installed and configured. A professional HVAC technician will set this balance point during installation to optimize efficiency.

For example, a system might be set so that the heat pump operates on its own down to 30°F (-1°C). Below that temperature, the thermostat might still allow the heat pump to run but will also engage the auxiliary heat to supplement. At even colder temperatures, say below 20°F (-7°C), the thermostat might turn off the heat pump entirely and solely rely on the auxiliary heat. This ensures your home stays warm, but at a potentially higher energy cost.

What is Emergency Heat

Emergency heat is essentially the same backup heating system as auxiliary heat, but it is intended for scenarios where the primary heat pump system has failed or is unable to operate at all. When you manually switch your thermostat to “Emergency Heat” or “E-Heat” mode, you are instructing the system to bypass the heat pump and use only the auxiliary heat source to warm your home.

This mode is crucial for preventing your home from becoming dangerously cold if your heat pump breaks down. It’s a safety feature that guarantees you have a source of warmth, even if it’s less efficient than your heat pump. You would typically engage emergency heat if you notice that your heat pump isn’t producing any heat, or if the thermostat displays an error message related to the heat pump’s operation.

When to Use Emergency Heat

The key differentiator for using emergency heat is a complete or significant malfunction of the heat pump. This could include situations like:

• The heat pump is making unusual noises or has stopped running altogether.
• The refrigerant lines are frozen over and not thawing.
• The outdoor unit of the heat pump is damaged or not functioning.
• The thermostat is reporting errors related to the heat pump’s compressor or external sensors.

It is important to understand that running solely on emergency heat, especially if it’s electric resistance heat, can significantly increase your electricity bills. Therefore, it should be used only when necessary and as a temporary solution until your heat pump can be repaired. Once the issue with the heat pump is resolved, you should switch the thermostat back to its normal operating mode (auto or heat pump).

Types of Auxiliary/Emergency Heat Sources

While electric resistance heating is the most prevalent form of auxiliary heat in residential heat pump systems, other technologies exist, particularly in commercial or more advanced residential installations.

Electric Resistance Heaters

As mentioned, these are common coils that heat up when electricity flows through them. They are relatively inexpensive to install as part of the heat pump system but can be costly to operate due to electricity prices. They are straightforward, reliable, and don’t produce any combustion byproducts, making them safe from carbon monoxide concerns associated with gas furnaces.

Gas Furnaces (Dual-Fuel Systems)

In some cases, homeowners opt for a dual-fuel system, which pairs a heat pump with a gas furnace as its backup. In this setup, the heat pump operates as the primary heating source down to a certain outdoor temperature. Below that “balance point,” the thermostat automatically switches to the gas furnace for heating. This can be more energy-efficient if natural gas prices are significantly lower than electricity prices in your area.

Dual-fuel systems offer the benefit of a more powerful and potentially more cost-effective heating solution during very cold periods. However, they also introduce the safety considerations associated with gas appliances, such as the risk of carbon monoxide leaks or gas leaks. Regular maintenance and proper venting are therefore paramount.

On-Demand Electric Heaters

Some newer systems may incorporate more advanced electric heating elements that can deliver heat more efficiently than traditional resistance coils, though they still operate on the principle of converting electricity to heat. The efficiency gains are typically moderate compared to the fundamental efficiency of a heat pump.

Important Considerations and Safety

Understanding and managing your heat pump’s auxiliary and emergency heat functions is vital for comfort and potentially for avoiding unnecessary expenses.

• Thermostat Settings: Familiarize yourself with your thermostat’s settings. Know how to switch between “Heat Pump,” “Auxiliary Heat,” and “Emergency Heat” modes. Most digital thermostats will display when auxiliary heat is engaged.
• Energy Costs: Be aware that auxiliary electric resistance heat is significantly more expensive to run than your heat pump. If your thermostat frequently displays “Aux,” especially for extended periods, it might indicate that your heat pump is undersized for your climate or that there’s an issue with your system that needs professional attention.
• Defrost Cycle: During cold, humid weather, your heat pump’s outdoor coil can frost over. This is normal. The heat pump will periodically reverse itself to warm the coil and melt the ice. During this defrost cycle, auxiliary heat may engage to prevent cold air from blowing into your home.
• Professional Maintenance: Regular professional maintenance of your heat pump system is crucial. A qualified HVAC technician can ensure that your heat pump is working efficiently, set the correct balance points for auxiliary heat engagement, and inspect any backup heating components for safety and proper function. This is especially important for dual-fuel systems using gas furnaces, where carbon monoxide detectors and regular safety checks are non-negotiable.
• System Malfunctions: Never ignore signs of a malfunctioning heat pump. Strange noises, insufficient heating, or error codes on your thermostat should be addressed promptly by a professional. Using emergency heat is a stop-gap measure, not a permanent fix.

When to Call a Professional

If you’re unsure about your heat pump’s operation, if your auxiliary heat seems to be running too frequently, or if you suspect a problem with your system, it’s time to contact a qualified HVAC professional. They can diagnose issues, perform necessary repairs, and ensure your system is operating safely and efficiently.

For homeowners seeking to connect with local, independent HVAC and heating professionals, services like EZ Home Heating can be a valuable resource. EZ Home Heating is a free nationwide service that simplifies the process of finding qualified technicians in your area. They do not perform the work themselves but facilitate connections, providing a straightforward way to get the assistance you need.

When your home comfort depends on it, ensure your heat pump system has reliable support. Call EZ Home Heating today to get connected with a local pro who can help you understand and maintain your auxiliary and emergency heat systems.