The increasing frequency and intensity of heat waves are one of the many direct effects of global warming. Longer and harsher summers increasingly compel us to use cooling devices such as air conditioners. However, such devices come at the price of high energy costs and resulting emissions – which effectively increase outdoor heat. The AEC industry has had to develop creative, scientific solutions to address the global challenge of thermal comfort while accounting for the practical constraints of cost, awareness, and acceptability. Heat pumps are a viable technology that helps mitigate thermal discomfort while being kinder to the environment than the good old air conditioner.

The promise of heat pumps

Heat pumps literally pump heat into or out of an enclosed space. This function makes it the perfect tool for heating and cooling indoor spaces as the season demands. As discussed later in this blog, their versatility, cost-effectiveness, and sustainability give them an edge over air conditioners or gas-powered systems.

Heat pumps are all-purpose air conditioners with better yearlong energy efficiency. When operated for cooling, heat pumps are the same as air conditioners in their working principles and efficiency characteristics; however, there remains a minor difference in their mechanical components. The ‘reversing valve’ in a heat pump is an ingenious game changer – it switches the direction of the refrigerant flow to transform the cooling device into an efficient heating solution.

In cold climates, an air conditioner is typically coupled with a gas-powered boiler or furnace to provide year-long thermal comfort through cooling and heating. In most cases, heat pumps can be a viable replacement for the two separate devices. Given the completely electrical operation, they emit fewer equivalents of CO2 compared to the two devices in tandem. To better understand the nuances of using heat pumps, it’s important to understand their inherent components.

How does it pump heat?

As mentioned before, a heat pump is identical to an air conditioner for cooling operation, while the device reverses the direction of the refrigerant flow for heating operation. The heating or cooling cycles are enabled by these key components of the heat pump:

Indoor and outdoor units

Both the indoor and outdoor units contain a coil and fan each. For the outdoor unit, the coil functions as a heat source (for heating) or a heat sink (for cooling), while the fan supports the convective heat exchange by increasing the air movement. Similarly, the coil functions as a heat source or sink for the indoor unit based on the use case. The fan, in this case, enables heat exchange while prioritizing indoor airflow for occupant comfort. In HVAC terminology, the heat source is synonymous with the ‘condenser’ and the heat sink with the ‘evaporator.’

Compressor

As the engine of the heat exchange phenomenon, the compressor pushes the refrigerant through a network of valves, ducts, and coils at high pressures. In the cooling mode, the refrigerant’s flow can be understood as:

       → Compressor → Outdoor Coil → Expansion Valve → Indoor Coil → Compressor →

In contrast, in the heating mode, the refrigerant flow is reversed – this converts the refrigerant cycle to:

       → Compressor → Indoor Coil → Expansion Valve → Outdoor Coil → Compressor →

Valves

As highlighted in the above cycles, the expansion and reversing valves remain central in the cooling and heating processes. The expansion valve rapidly expands the refrigerant, enabling thermodynamic processes that cause accelerated heat exchange. This valve also helps to monitor and regulate the refrigerant flow and pressure in the system. On the other hand, the reversing valve is engaged to reverse the refrigerant flow for heating operation.

These three key components make a typical heat pump; to estimate the device's efficiency, we commonly use two metrics – SEER and HSPF. SEER stands for ‘seasonal energy efficiency ratio’ and measures the cooling efficiency of an HVAC system. Therefore, air conditioners can be compared with heat pumps based on their SEER, with a preference for a higher value. Similarly, HSPF stands for heating seasonal performance factor and measures the heating efficiency of a heat pump. Similar to SEER, a high HSPF is preferable.

Essentially, these metrics compare the magnitude of the desired effect (cooling or heating) and the electrical energy input. It is important to note that these metrics should be referred to with an understanding of the local climate, use case, and the type of heat pump, as described in the next section.

Types of heat pumps

Heat pumps can be categorized based on the nature of the heat source/sink that enables the heating or cooling process. The four broad types of heat pumps are:

Air source heat pumps

Like air conditioners, these devices use the atmospheric air as the heat source and sink and are the most commonly used heat pumps. However, in extremely cold climates, the heating operation of heat pumps is often augmented using additional components or systems like electrical resistance coils, for example.

Ground source heat pumps

Ground source heat pumps are the most efficient type of heat pump; therefore, they are one of, if not the most efficient HVAC systems. These devices use the earth's relatively constant temperature as a heat source (for heating) and a heat sink (for cooling). Their installation requires the underground laying of heat exchangers (in the form of high-density PE tubes) and therefore needs high initial capital investment.

Water source heat pumps

These devices use the thermal mass of a nearby water body as a heat source and sink. Their operation is dependent on the temperature of the water body and therefore is limited to specific use cases, climate zones, and microclimate.

Hybrid and alternative heat pumps

These devices combine multiple heat sources/sinks to yield the desired heating or cooling effect. Some alternative approaches also combine solar photovoltaics with the device to increase energy efficiency. In certain peak heating use cases, the device is operated in tandem with gas-powered systems for enhanced thermal comfort. Below are key differences between each of the systems described:

Myths about Heat Pumps

Despite the distinct advantages of heat pumps over other HVAC technologies, they have not been widely used due to several myths surrounding their usability, maintenance, and efficiency characteristics. Some of the top myths are:

Myth 1: Heat pumps are used only for heating

No, they are used for both heating and cooling across a broad range of outdoor temperatures, as established above.

Myth 2: Heat pumps are more expensive than ACs

The initial cost for heat pumps (specially ground-source) can be higher than conventional ACs; however, over the life cycle, heat pumps prove more cost-effective and sustainable.

Myth 3: Heat pumps require more maintenance than ACs

Ground-source heat pumps do require careful installation; however, over a longer period and with recent advancements in technology, the device can be maintained and serviced as frequently as a typical AC.

Myth 4: Heat pumps need to run for longer than ACs

No, they produce the desired thermal effect similar to the air conditioners. However, in extremely cold climates, they may need to be coupled with an additional heating system to create optimal warmth.

Myth 5: Heat pumps need significantly more space than ACs

The outdoor unit of a typical heat pump may be ~1.5 times larger than that of a typical AC; however, the indoor unit and ducting are not significantly different from that of ACs.

Myth 6: Heat pumps cannot be used as retrofits in older buildings

No, heat pumps can be viably used as retrofits just as ACs. Further, they can also help power the underfloor heating and radiant systems.

In conclusion, heat pumps are an efficient and versatile HVAC solution that promises thermal comfort at reduced costs and are sustainable for the planet. Building design teams and end-users must acquaint themselves with the tangible benefits of this simple yet effective technical solution.