Heat Pumps

What are heat pumps and do heat pumps break the laws of thermodynamics?

Heat pumps are often described as having efficiencies greater than 100%, which can sound counterintuitive. How can a system appear to produce more energy than it consumes? Does this challenge our understanding of physics, or is there a more straightforward explanation?

Understanding Heat Pump Efficiency

Unlike traditional heating systems, which transform some other form of energy into heat, heat pumps "pump" heat. This allows them to achieve an efficiency above 100%, that is if we only consider the energy we pay for when calculating the efficiency (the heat we pump from the environment is free).

The Laws of Thermodynamics and Heat Pumps

The First Law of Thermodynamics (Conservation of Energy): This law states that energy cannot be created or destroyed, only transferred or transformed. A heat pump uses electrical energy to transfer heat from the cold side to the warmer side.

The Second Law of Thermodynamics: This law says that heat naturally flows from a hot object to a cold one. That means in order to force heat to move in the opposite direction, some mechanical work needs to be done by the heat pump. In a well designed pump, the work required is much less than the amount of heat pumped.

Added Benefits

On top of being more efficient than other heating systems, the heat flow of heat pumps can also be reversed, enabling them to also act as an air conditioner in the summer.

What does BTU, SEER, and HSPF mean?

Deciding on a heat pump can be exciting, but deciphering the numbers in quotes might feel daunting. Here's a quick guide to help you understand these key figures:

BTU (British Thermal Unit)

BTU stands for British Thermal Unit, a measure of energy. Specifically, "BTUs per hour" indicates the heat pump's power output—essentially how much heat the pump can add or remove in an hour.

To put it in perspective, an electric baseboard heater typically has a BTU rating of around 5,000 BTUs per hour. For heat pumps, the BTU rating helps determine the size and capacity of the system. Larger homes or colder climates may require systems with higher BTU ratings to maintain comfortable temperatures.

SEER (Seasonal Energy Efficiency Ratio)

The SEER number represents the cooling efficiency of the heat pump and is expressed as a ratio of BTUs to watt-hours (BTUs/watt-hour). It is calculated by dividing the cooling output (in BTUs) over a typical cooling season by the total energy consumed (in watt-hours) during the same period.

Higher SEER ratings mean the heat pump is more energy-efficient, translating into lower electricity costs for cooling. For example, a system with a SEER of 20 will use less energy than one with a SEER of 14 for the same cooling output.

HSPF (Heating Seasonal Performance Factor)

The HSPF measures the heating efficiency of the heat pump and is also expressed as a ratio of BTUs to watt-hours (BTUs/watt-hour). It is calculated by dividing the total heat output (in BTUs) during the heating season by the total electricity consumed (in watt-hours) during the same time.

As a rule of thumb, an HSPF of 3.41 is equivalent to 100% efficiency (so the HSPF of an baseboard heater is 3.41), meaning the heat pump converts all the electricity it uses into heat. Modern heat pumps often achieve an HSPF well above this value.

In regions with cold winters, investing in a system with a high HSPF can result in significant energy savings. Systems with an HSPF of 8 or higher are considered energy-efficient and environmentally friendly, often qualifying for energy rebates or incentives.