When it comes to residential heating and cooling systems, few types of systems are as energy efficient as heat pumps. But what exactly are they and how do they work? In short, a heat pump is simply an electrical device that is capable of transferring heat from one place to another. Though it may seem counterintuitive at first, heat can actually be extracted from cold air and added to warmer air.
To explain this in a more practical manner, let us step back a little bit to understand the basics. First of all, for the sake of an analogy, let’s imagine heat as a big body of water. Logically, water would flow from a higher place to a lower place. Similarly, heat naturally flows from a hotter place to a colder place.
So how is it that we can extract heat from a cold place and send it to a warmer place? Well, it is a little bit like moving water from a low place to a high place. All you need is a pump! Heat is actually defined by the movement of the molecules that make up matter. So in essence, all of the air in the world that is warmer than absolute zero (-273 °C, the temperature at which molecules actually stop moving) contains some amount of heat. So technically, it is possible to extract heat from air of any temperature and send it somewhere else, all it takes is a little energy.
Heat pumps are actually quite commonly used in our everyday lives. In fact, this process is happening almost all the time in your home right now, as refrigerators are actually a typical form of heat pump.
Heat pumps are able to extract heat from air by using a liquid refrigerant (more on this later) to absorb and remove heat and what’s known as the vapour-compression refrigeration cycle, which is a fancy name for a thermodynamic process that is commonly used for heat transfer applications.
There are four basic steps to this:
- The process starts where the refrigerant is in a state known as a saturated vapour. This saturated vapour enters a compressor where the pressure is increased and, consequently, the temperature as well.
- Hot vapour is then passed through a condenser, where it is condensed back into liquid form. The result of this condensation is that the refrigerant will lose its heat. This is what is what is occurring right now in your household refrigerator, and why it is warm in the back.
- The liquid refrigerant then passes through an expansion valve, where the pressure drops and the liquid becomes much colder. At this point, the fluid is typically colder than the space that needs to be cooled.
- Lastly, the cold fluid, which is now partially evaporated due to the pressure drop, passes through an evaporator, which typically consists of a coil or long tubes. A fan then blows air over the coil or tubes, cooling the air. This causes the refrigerant to evaporate within the tubes, returning it to its original saturated vapour state.
Basically, what is going on is that a refrigerant is being forced to go through cycles of condensing and evaporating where the temperature and pressure rise and drop significantly. These temperature fluctuations are then used to heat or cool a stream of air or water, depending on the application.
So that explains a bit about the science behind heat pumps, but what exactly is it that makes heat pumps so interesting for residential heating and cooling applications? The advantages of heat pumps are quite numerous.
First of all, the amount of energy typically required to run the compressor and the fans or pumps is usually significantly less than the amount of heat that can be moved, or in practical terms, 'generated'. Heat pump performance is usually measured by something called the ‘coefficient of performance’ or COP. The COP is the amount of heat delivered, or moved, divided by the amount of energy required to move that heat. The COP of an average residential heat pump is usually somewhere around 3, which means that for every one unit of energy you put into the system, 3 units are transferred. When you compare that to an electric baseboard heater, which has a COP of 1 (every unit of energy you put into your electric baseboard comes out as heat), heat pumps suddenly become quite attractive!
Another interesting advantage of heat pumps compared to other residential heating or cooling systems is that a heat pump can actually be configured to do both heating and cooling. The thermodynamic cycle described above can actually be reversed in order to switch it from one function to the other. Instead of taking heat from outside to heat your home on the inside, you can also take heat from inside and throw it outside in order to cool your home.
This can help you save on purchase price and maintenance costs, as you would have one machine doing the job previously done by two. This can also help you save basement storage space, as heat pumps are typically smaller than an equivalent gas furnace and air conditioner combo.
Other interesting advantages include increased indoor air quality, since there are no fuels being burned and exhausted and the system will always add fresh air into your home. Heat pumps are also quite versatile; they can be used to heat incoming air from the outside, or as an air-to-water heat pump for generating residential hot water. Heat pumps are also used in conjunction with geothermal heating and cooling, where the heat is either taken from, or added back to the ground.
Heat pumps may sound like a pretty magical device that can accomplish anything; however they do have a few important downsides. First of all, their performance is very much dependent on the climate. In very cold climates where the temperature often drops below -10 degrees Celsius, heat pumps can become less effective.
Although heat can still be extracted from cold air, heat pumps are generally not well-suited to very cold temperatures for two reasons: 1) the COP tends to drop significantly in very cold weather, thus negating the efficiency advantage; 2) In colder climates such as most of Canada, the heating demand of a home tends to be much higher than the cooling demand. So much so that in moderately insulated homes it can be impractical to have a heat pump as the sole source of heat.
Generally speaking, in colder climates, it is usually recommended to back up your heat pump with electric baseboards or some other form of heating to ensure the house stays warm during the coldest days of winter. There is an argument that can be made, however, for investing in additional insulation in new homes rather than additional heat generation, so that the heat pump can operate at a more optimal efficiency since less capacity would be required.
Also, with the right amount of insulation and a properly designed home for passive solar heat gains, it is even possible to safely heat your home during the coldest days of winter with only a heat pump. Take note that although the system may save you money in the long run, the initial costs of heat pumps tend to be a bit higher than other systems, especially if a second backup system is also required.
Lastly, the heat generated by a heat pump is typically less intense than a conventional gas furnace. For example, a heat pump typically generates heat at temperatures between 32 and 37 degrees Celsius, which is slightly lower than your body temperature. By comparison, a typical natural gas furnace will generate heat at closer to 50 degrees Celsius, which is much more comfortable on a cold winter day. Some people find this lower temperature heat a little uncomfortable in colder weather, particularly in a poorly insulated house.
So if you’re in the market for a new central heating and cooling system for your home, you should definitely consider a heat pump for your heating and cooling needs. How much sense it makes will depend on how well insulated your house is.
In a super-insulated home a heat pump could provide all the heat and comfort you need, but in a house built to code you may need a backup. So as a homeowner and builder, this is where you will need to decide between investing in insulation or an additional heating system. Either way, heat pumps are definitely a very efficient heating and cooling source for your home and in many cases the advantages can outweigh the disadvantages.
Jason Ng Cheng Hin, Jr. Eng, M.A.Sc.
Jason is a junior consulting engineer who specializes in energy efficiency in buildings and building energy simulations. Jason completed his master's degree at Concordia University in building engineering, where his thesis centered around the optimization of a solar combisystem installed in an energy efficient house in Quebec.