Understanding Air Source Heat Pump Operation
The hum of an air source heat pump (ASHP) is increasingly becoming a familiar sound in homes across the globe. Promising both efficient heating and cooling, these systems have earned a well-deserved reputation as a greener alternative to traditional furnaces and air conditioners. However, a common question often arises from homeowners and those considering a switch: Why do air source heat pumps often come with a supplementary heating system, typically referred to as auxiliary heat? This article delves into the underlying reasons, providing a comprehensive understanding of how these systems function and why auxiliary heat is frequently a crucial component.
Air source heat pumps operate on a fundamentally different principle than traditional heating systems. They don’t generate heat directly like a furnace. Instead, they act as heat transfer devices, moving heat from one place to another. In the heating mode, the ASHP extracts heat from the outside air – even on cold days – and transfers it indoors. This is made possible by a refrigerant that circulates through a closed loop, absorbing heat from the air outside and releasing it inside the home. In cooling mode, the process is reversed, extracting heat from inside the home and expelling it outside.
Understanding the basics of the heat pump cycle is crucial to grasping the need for auxiliary heat. The core components include:
- Refrigerant: A special fluid that readily absorbs and releases heat.
- Compressor: This is the heart of the system, increasing the pressure and temperature of the refrigerant.
- Condenser: Where the heated refrigerant releases its heat indoors, warming your home.
- Expansion Valve: This reduces the refrigerant’s pressure and temperature.
- Evaporator: Where the refrigerant absorbs heat from the outside air (in heating mode) or from the inside air (in cooling mode).
The remarkable efficiency of ASHPs comes from their ability to move heat rather than generate it. They can often deliver more energy in the form of heat than they consume in electricity, resulting in significantly lower energy bills compared to electric resistance heating. This efficiency, however, is not constant and faces a notable challenge in cold weather.
The Challenges of Cold Weather Performance
The primary reason why ASHPs necessitate auxiliary heat stems from the inherent limitations of their heat transfer capabilities in cold climates. The colder the outside air, the less effective the heat pump becomes at extracting heat. This decline in performance is not linear; it becomes more pronounced as temperatures drop.
As the outdoor temperature plummets, several factors contribute to this decline in heating efficiency. The most significant is the decreased heating capacity. The “heating capacity” refers to the amount of heat an ASHP can deliver to a home under specific operating conditions. This heating capacity shrinks as the outdoor temperature decreases. In other words, an ASHP that provides ample heat on a mild day may struggle to keep up when temperatures dip below freezing.
The efficiency of a heat pump is often measured using the Coefficient of Performance (COP). COP represents the ratio of heat output to energy input. A higher COP indicates greater efficiency. A typical ASHP can achieve a COP of 3 or higher, meaning it delivers three or more units of heat for every unit of electricity consumed. However, the COP of an ASHP decreases as the outdoor temperature falls. At very low temperatures, the COP can drop below 1, meaning the ASHP may consume more electricity than it delivers in heat, at which point the system typically relies primarily on auxiliary heat.
Another significant challenge in cold weather is the formation of frost on the outdoor unit’s evaporator coil. Because the ASHP is extracting heat from the outside air, the coil can become colder than the surrounding air. This can lead to frost formation, especially when the humidity is high. Frost acts as an insulator, reducing the heat transfer efficiency of the coil. To combat this, ASHPs are equipped with a defrost cycle.
The defrost cycle involves briefly reversing the heat pump’s operation, essentially turning it into an air conditioner to melt the frost. This cycle typically lasts for a few minutes, during which the ASHP temporarily stops providing heat to the home. While the defrost cycle is essential for maintaining efficient operation, it can lead to periods of reduced heating and may contribute to temperature fluctuations indoors. The more frequent the defrost cycles, the greater the demand for auxiliary heat.
Finally, at extremely low temperatures, the ASHP’s heating capacity may simply be insufficient to meet the home’s heating demand. The home’s heat loss increases significantly as the temperature difference between the inside and outside widens. The ASHP, despite its best efforts, may not be able to keep up with the home’s heat loss, leading to a drop in indoor temperature. This is when auxiliary heat becomes critical.
The Role of Auxiliary Heat
Auxiliary heat plays a critical role in ensuring consistent and comfortable heating, especially during the coldest periods of the year. It’s designed to supplement the ASHP’s heating capacity when the primary system is struggling to keep up. Think of it as a backup system that kicks in when the going gets tough. The primary purpose of auxiliary heat isn’t to replace the ASHP entirely, but to provide additional heating when the ASHP’s capacity is limited.
The activation of auxiliary heat is typically controlled by the thermostat. Most ASHP systems are configured to activate auxiliary heat based on one or more of the following criteria:
- Outdoor Temperature: Many systems have a pre-set temperature (e.g., 30°F or lower) at which auxiliary heat is automatically engaged.
- Heating Demand: If the ASHP is unable to meet the heating demand requested by the thermostat, auxiliary heat may be activated to prevent a significant drop in indoor temperature.
- Defrost Cycle: During defrost cycles, auxiliary heat often provides supplemental heating to minimize temperature fluctuations inside the home.
There are various types of auxiliary heating systems. Electric resistance heating is a common choice. This system utilizes electric coils that generate heat when electricity passes through them. While reliable and relatively simple, electric resistance heat is typically less energy-efficient than the ASHP and can lead to higher electricity bills.
Gas furnaces also serve as effective auxiliary heat sources. They offer a more energy-efficient alternative to electric resistance heat, particularly in areas with affordable natural gas or propane. The type of auxiliary heat used often depends on the region, availability of fuel sources, and homeowner preferences.
Factors Influencing Auxiliary Heat Use
Several factors influence the extent to which an ASHP relies on auxiliary heat. Understanding these factors can help homeowners make informed decisions when selecting and operating their heating system.
The climate you live in is a significant determinant. Homes in regions with consistently mild winters will require less auxiliary heat compared to those in areas with prolonged periods of freezing temperatures. The frequency and duration of cold snaps directly impact the amount of time the ASHP operates in its less efficient mode, thus increasing the demand for auxiliary heat.
Proper sizing of the ASHP is crucial. An undersized heat pump will struggle to meet the home’s heating load, leading to frequent activation of auxiliary heat. A professional HVAC technician can perform a load calculation to determine the appropriate size of the heat pump for your home based on factors like square footage, insulation, and window efficiency.
The construction and energy efficiency of your home also significantly impact the need for auxiliary heat. A well-insulated and airtight home retains heat more effectively, reducing the heating load on the ASHP. Upgrading insulation, sealing air leaks, and replacing old windows can significantly reduce reliance on auxiliary heat, resulting in energy savings and improved comfort.
Thermostat settings and user behavior also play a role. Lowering the thermostat temperature at night or when the home is unoccupied can reduce energy consumption. However, excessively aggressive setback strategies can potentially cause the auxiliary heat to be engaged more frequently, especially in poorly insulated homes. It’s important to find a balance between energy savings and comfort.
Benefits and Considerations
The benefits of a heat pump system extend beyond energy savings. They provide consistent, comfortable heating by delivering a stable heat supply even in moderately cold weather. In the summer, ASHPs seamlessly transition to cooling mode, providing year-round comfort with a single system. They also offer improved air quality, as most systems include air filters that trap dust, pollen, and other allergens.
However, the balance between energy efficiency and comfort must be considered when operating an ASHP system. While auxiliary heat is essential for providing adequate heating in cold weather, excessive reliance on it can undermine the overall energy efficiency of the system. Optimizing thermostat settings, ensuring proper insulation, and regularly maintaining the system can help minimize auxiliary heat usage.
Conclusion
In conclusion, the need for auxiliary heat in air source heat pumps is a consequence of the physics of heat transfer and the varying efficiency of these systems based on outside temperature. As outdoor temperatures plummet, the ASHP’s heating capacity diminishes. Auxiliary heat, often provided by electric resistance or a gas furnace, steps in to supplement the heat pump’s output, ensuring comfortable indoor temperatures. While auxiliary heat may slightly reduce the overall energy savings, it remains a critical component of these systems, enabling them to function effectively and efficiently even in colder climates. Future innovations in heat pump technology continue to push the boundaries of performance, with advancements like cold-climate heat pumps extending their effectiveness in even the harshest conditions.
