We promised more detail about how a heat pump works, and we are making good on that promise right now.
If you are looking for more general information about heat pumps, [visit our archive] to browse more articles. If you want a thorough explanation of the technology of heat pumps, then you are in the right place.
What Is a Heat Pump?
Heat pumps, while a relatively new method to heat your home, are not a new technology. Old, proven, and reliable, it is the same technology that air conditions your car and keeps your fridge and freezer cold.
The same process can be engineered to heat or cool your home as a heat pump. The heat pump system is very efficient, especially when compared to traditional furnaces and boilers.
Just like a computer, you don’t need to know all the specifics of how a heat pump works to choose, install and enjoy this system. But we will get into all the details so you know exactly how it works. No need to have the HVAC guys talk over your head; read on to understand what they’re talking about when installing, maintaining, or repairing your heat pump system.
All About Heat Pump Efficiency
All the talk of the day is ‘efficiency, efficiency, efficiency’. This comes down to how effectively the system runs and how much it wastes. 100% is the best score you can get on an exam, and it’s not a bad number for heating efficiency, either.
While not completely accurate, it can be a useful analogy to compare efficiency to money spent. At 100% efficiency, 100% of every dollar you spend on fuel is converted into heat for your home. Likewise, a 98% efficient furnace converts 98 cents of every dollar into heat for your home and 2 cents is waste. (The waste might be heat lost with exhaust, for example.)
Although modern furnaces have come a long way from the 60s and 70s, the top-performing models are still less than 100% efficient. This is where a heat pump system really shines: a heat pump can be as much as 300% efficient. That’s like getting $3 worth of heat from $1 of fuel.
The heat pump’s technology is what makes the difference. A furnace or boiler converts fuel, usually natural gas or propane, into energy through combustion and releases it into your home as heat. A heat pump simply moves–or pumps–heat from one place to another. Moving energy is easier than changing the form of energy. For relatively little electrical energy input, your heat pump can move significant heat into your home.
There are factors that affect how efficiently a heat pump operates. You might not see 300% efficiency all year round. The biggest factor that affects efficiency is the temperature of the place that heat is being pumped from. When heating your home, this is the outside air or ground. The warmer the outside temperature is, the easier it is to pump heat to the inside. The colder the outside temperature, the more energy it takes to pump heat into the inside.
Making Sense of The Terminology
Measuring and Comparing Heating Systems
There are a few different abbreviations you might see when looking into heat pumps. First, let’s cover some general terms that you might be familiar with already. These are used with any heating system.
The capacity of the system describes how much heat the system will provide. It can be measured in BTU, kW, or Ton. Different regions or fields will use one unit over the other. BUT is an imperial measure and common in North America, while kW is used internationally and Ton is usually used to with cooling systems.
The capacity of the system you install needs to provide as much heat as will escape from your home on a cold winter day–or you’ll be in for an uncomfortable Christmas. This is what is known as the size of the system. A registered energy advisor or HVAC technician will calculate the correct size for your home before you purchase a system.
BTU/h is British Thermal Units, and 1 BTU is how much energy–or heat–is needed to raise the temperature of one pound of water by one degree Fahrenheit. For reference, one BTU is about the energy output of a single birthday candle. Manufacturers will list the output of a system in BTU/h or BTU per hour. Often written as BTU on the labels, it actually means BTU/h. The rating will be somewhere in the thousand range. For example, a heat pump might be rated at 36,000 BTU.
Some models rate their heating and cooling capacity in Ton. 1 Ton is 12,000 BTU. The 36,000 BTU heat pump would be 3 Ton. A heat pump will typically be somewhere between 1 to 5 Ton of heating or cooling.
Watts (W) and kilowatts (kW) are units of energy. You might remember from high school physics that 1 watt is 1 joule per second. Your electrical bills charge you for kWh or kilowatt hours. A heat pump might use somewhere in the range of 1 to 5 kWh of electricity.
The simple efficiency (known as steady-state efficiency) of a system is the energy output divided by the energy input. For an electrical system such as a heat pump, you might use 100 units of electricity to pump in 300 units of heat. This system is 300% efficient. Fuel-based systems such as natural gas furnaces have a maximum efficiency of 100%. Taking into account real-world scenarios, a furnace that achieves 98% simple efficiency is an incredibly efficient system.
There are other factors that affect efficiency ratings, things like fans and motors, and indoor and outdoor temperatures, that each impact the output. The rating systems below account for these variables.
Heat Pump Terminology
There are some abbreviations that are specific to heat pump systems. There are 4 common acronyms: COP, HSPF, SEER, and EER.
EER is the Energy Efficiency Ratio. It calculates the cooling efficiency of a heat pump. It divides the BTU/h of cooling by how many watt hours (Wh) of electricity are used. The number listed by the manufacturer is measured at set temperatures (outdoor temperature of 35°C, indoor temperature of 26.6°C, and 50% humidity). It is a benchmark to compare across all models but in reality, BTU/h output and Wh input will vary based on the outside temperature. EER might not reflect the efficiency you will get in reality. EER also measures only the cooling efficiency of a heat pump system, not heating capacity. SEER (see below) is a better ratio of cooling efficiency as it more accurately represents real-world situations.
COP is the Coefficient of Performance. It is very similar to EER but can measure the efficiency of heating as well as cooling. It is calculated by dividing the amount of thermal energy moved–for either heating or cooling–by the amount of electrical energy used to do it. Both variables need to be in the same unit, either kWh or BTU/h. (1 BTU/h is approximately equal to 0.293 kWh.) If the COP is 1, the unit is 100% efficient. A COP of 3 would be a unit that is 300% efficient. COP is also measured at set temperatures, and does not reflect variations in efficiency due to changes in outside temperature.
The next two metrics try to account for seasonal variations in temperatures. Remember that the temperature outside will affect how efficient a heat pump system is. The season changes the performance of your heat pump.
HSPF is the Heating Seasonal Performance Factor. It is the same calculation as EER averaged over an entire heating season. It represents one particular climate region and uses historical daily temperatures for the calculation, so it might not be accurate for your location. The manufacturer may have a rating available for a different region if you contact them directly. The calculation is done by the total BTU/h of heat provided over the entire season divided by Wh used over the entire season. A good number is 8 and above, but some are models are rated up to 15.
SEER is the Seasonal Energy Efficiency Ratio. It is the same as HSPF but taken during the cooling season. It measures the cooling efficiency of the heap pump over the entire cooling season. The calculation assumes an average summer temperature of 28°C. Again, this ratio is region specific and that rating can be requested from the manufacturer. A good number is 15 and above, but there are models up to 24.
In Canada, the most commonly listed ratings will be COP or HSPF. No matter the metric, a higher number is a more efficient unit.
How A Heat Pump Works
A Heat Pump Uses High School Physics
Heat pumps operate on the physics principle of the phase change of fluids.
When liquid water boils, it has absorbed a certain amount of energy, enough to change state from a liquid to a gas, and it also expands in volume. When water vapour condenses, it releases a certain amount of energy and contracts to a smaller volume. The energy absorbed and released at state change is the heat that is pumped into or out of a house by the heat pump system.
Rather than water, a heat pump uses a coolant or refrigerant to transfer thermal energy between two spaces. The coolant is sometimes a liquid and sometimes a gas, depending on where it is in the system.
A heat pump’s exceptional efficiency can help achieve a high-efficiency, low-energy home, but it is worth noting that many refrigerants used are not environmentally neutral. It’s worth finding out which refrigerant your system uses, and what environmental impact it might have.
Pumping Heat Out of Winter
Even in winter, there is heat to be pulled from the outside. Remember your high school physics: any temperature above -273°C–absolute zero–holds energy. There is plenty of thermal energy for the heat pump to draw from, even in the winter. The current limitation to cold climate air-source heat pumps is not the availability of thermal energy but the outdoor humidity which will cause ice to build up on the outdoor heat exchanger and block the air flow preventing any heat exchange. A heat pump system needs a defrost cycle, often resistant heating, to remove the ice buildup. This is the main reason for the drop in efficiency in cold weather.
The Source and the Sink
Remember above we discussed that the heat pump can be more than 100% efficient because it does not create any heat, it only moves heat from a source to a sink. The heat pump takes heat from one environment and transfers or pumps it into another environment. The environment that heat is taken from is called the source. The environment that receives heat is called the sink.
The source and the sink changes depending on whether the heat pump is heating or cooling the building. In heating mode, the source–the source of the heat–is outdoors. In cooling mode, the source–the source of the heat–is indoors.
Though it is possible to get a heat pump system that only heats your house, the most common configuration is fully reversible. What sources and sinks you would like to use will determine what type of heat pump you install.
Thermal energy can be transferred outside the home from the air, the ground, or a body of water. Whatever the source, it must have enough residual heat for both the demand and duration of the heating season. We’ll discuss each type in more detail in separate articles.
Inside your home, thermal energy can be transferred and distributed through your home by air or water. This can depend on what your home is already fitted for.
Though the sources and sinks may change from system to system, the basic operation of the heat pump is the same.
The Heating Cycle
The heat pump is basically a compressor and an expansion valve with a heat exchanger both inside and outside. A heat exchanger is a very simple component. It is a coil or plate with fins that has a very high surface area. Its role is to allow as much thermal energy transfer as possible. A fan for air (or a circulator pump if water) will help exchange the surrounding air so that more heat will be transferred.
In heating mode, a refrigerant passes through the outside heat exchanger which is acting as the evaporator: the refrigerant will absorb heat and boil or evaporate into a gas (R410 is a common refrigerant and it has a boiling point of -48.5C). If the outside source is air, a fan blows air over the heat exchanger to increase the heat exchange. The outside source could also be the ground, in which case the heat exchanger is buried, or it could be submerged in a body of water which heat is transferred from.
The gas refrigerant then passes through the compressor. Here it is compressed to a high-pressure, which will increase the temperature of the refrigerant.
The refrigerant gas then moves to the indoor heat exchanger. The heat exchanger will often be inside your ducts close to your furnace. It transfers heat from the refrigerant to the indoor air (being the heat sink). The heat can now be distributed via the ductwork to the rest of your home. As it moves through the heat exchanger, the refrigerant becomes cooler and will condensate into a liquid (this phase change releases more heat).
Now as a high-pressure but cooler liquid, the refrigerant passes through an expansion valve, which allows the liquid to expand. As it expands and lowers the pressure, the refrigerant will lower its temperature while also becoming gas or a gas-liquid mix to return to the heat source and begin the cycle again.
The Cooling Cycle
The reversing valve switches the direction of the cycle, and the same process, in reverse, will cool a home. The compressor forces the high-pressure, high-temperature vapour refrigerant through the outdoor heat exchanger. The outside temperature, even hot summer air, is cooler than the refrigerant and so heat is transferred from the refrigerant to the outside. The outside air, ground, or water is now acting as the sink. The refrigerant becomes a high-pressure, cooler liquid.
Passing through the expansion valve, the refrigerant drops in pressure and temperature further. It becomes a part-liquid, part-vapour mixture. It next moves to the indoor heat exchanger where warm indoor air is blown across the coils.
The heat from the indoor air is absorbed by the very cold refrigerant. This both cools the air and boils the refrigerant. The low-temperature vapour refrigerant then returns to the compressor to continue the cycle.
Variations of Makes and Models
There will be small differences in design between manufacturers and models, but you now have a good understanding of how the main components of a heat pump works–without needing any mechanical design diagrams and reviewing specific components.
Helping You Build a Climate-Resilient Home
A low-energy, high-efficiency house is achievable with a well-thought-out plan. With an understanding of the efficiency ratings, you’ll be able to compare recommendations from HVAC professionals and help choose a system that provides both low operating costs and functions the way you want.
If you have any doubts or questions, contact us today. A Peace Energy registered advisor will give you customized recommendations that will suit your home.