TAPPING INTO THE GROUND: EXPLORING GEOTHERMAL HEAT PUMP SYSTEMS
Did you know that you can use the earth to heat and cool your home?
Yes, indeed, you can tap into the constant temperatures existing underground to fully or partially heat your home in the winter or cool your home in the summer using a geothermal heat pump. This system may also be referred to as a ground-source heat pump or geo-exchange system. You already have the ground of your property that your house is built on (or planned to be built). Since the soil temperatures below the freezing line are relatively constant all the time, this system pretty much runs year-round. And ground temperatures are a free, sustainable resource!
This sounds pretty good and the technology is quite mature, so let's dive in to learn more.
It all starts with the ground
As you may know from setting foundations for a fence, for example, you need to dig to a certain depth into the ground. This is to prevent the foundation (and anything it carries) to shift when it freezes in the winter. The general direction usually given is around 4 feet deep. The reason behind it is that above 4 feet from the surface, the ground doesn't freeze anymore. The depth up to which the ground freezes is called the frost line.
Of course, the exact frost line depends on where you live and can vary greatly across North America and in Canada. For our location in Toronto, the frost line is anywhere between 3.5 to 5 feet.
Below the frost line, the ground temperature stays relatively stable throughout the entire year, anywhere between 6 to 11 degrees C. And this is precisely the environmental circumstance geothermal heating and cooling systems are taking advantage of.
A Geothermal HVAC System
This is where it gets a bit technical but bear with us...
Geothermal heating and cooling systems essentially use the thermal energy from the ground and via a heat pump to distribute heating in the winter and cooling in the summer. A heat pump is a heat exchanger.
So, when the ground is warmer than the outside air in the winter, the system extracts and compresses the heat from the ground and sends this through your house using your heating system. This could be either via the air ducts of a forced-air heating system or the radiant heaters (actual radiators or in-floor radiation systems). In the summer, when the ground temperature is cooler than the ambient air temperature, the process is simply reversed and the heat pump extracts the heat from the air in your house and gets rid of it to the ground.
Here is a diagram of the basic mechanics of a ground source heat pump:
While the heat pump sits just beside or just inside your house, most likely the utility room, the system also uses ground loops outside. These ground loops are pipes running through the ground in an open or closed-loop system. Open-loop systems carry water back and forth between the house and a water source (underground or pond). The closed-loop configuration pipes typically carry a mix between water and glycol (also commonly used for antifreeze).
The ground loops can come in different configurations, as shown in the image below. The configuration will largely depend on the size and geology of your property. For anyone who is realistic about implementing a geothermal system in a city environment, only the looped ground collectors and the vertical collectors are valid options. The horizontal collector and pond connection would require a lot of space or an actual pond on your property.
The below diagram shows the system in a heating cycle. Coldwater is pumped from the house (blue end of each loop) into the ground collectors and returns to the house warmer than it left (red and of each heat loop). Again, this is because the ground still carries a certain temperature between 6 to 11 degrees C no matter the season.
The horizontal collectors or ground loops are generally installed in trenches dug between 6 to 10 feet deep and then backfilled. The area of expansion of these collectors depends on the space available on the property and the desired system capacity.
Vertical collectors require a well to be drilled. The depth depends on the geology of the particular site and the desired system capacity. You could also have two or more wells with connected vertical collectors.
Geothermal HVAC systems are highly efficient when compared to conventional HVAC systems and even compared to an air-source heat pump. Efficiencies of anywhere between 40% up to 70% are possible depending on the system design and location. Many heat pumps are also Energy Star rated, promoting their overall energy efficiency.
The only energy the system uses is the electricity needed to operate the pump that moves the fluid through the ground collectors and the condenser on the heat pump. This is significantly less energy, about 45%, than other HVAC systems.
So, the efficiency combined with the energy savings will translate into lower utility bills for heating/cooling and electricity.
An added benefit of geothermal heat pumps is that they can support your hot water demand too. Whenever the system is not required to heat the house, it can be used for domestic hot water. Some systems are designed to automatically switch back and forth between these two functions. The reduction in energy demand for your water heater will further add to the energy savings of the system overall.
System maintenance requirements are pretty basic and similar to the maintenance needs for a standard air conditioner, for example. This is great news because you won't need to plan for an additional maintenance budget.
The life expectancy of the system is also a very strong feature. The heat pump itself typically lasts for around 20 years and many of the underground installations are warranted for 25 to 50 years. This speaks well for the longevity of the technology itself.
Geothermal systems are considered to be sustainable as they use existing resources to do the majority of their work, i.e. the stable underground temperature. The minimal electricity these systems require to operate may as well come from renewable or other sustainable sources.
And finally, any system that will reduce the energy efficiency of your house, has low maintenance costs and a long life expectancy will inevitably increase the resale value of your property.
Your Personal Considerations
While we have just seen a large list of benefits from such a system, the question is: Does it make sense for you?
As is usually the case when looking into options, the decision ultimately comes down to financial viability. So, you will need to work out what the return on investment on a geothermal HVAC system is for your particular project.
System costs range significantly as they depend highly on the system design. What you will find is that the main costs are not in the actual equipment that makes up your system, but rather the installation cost. Any system will require some level of excavation and/or well drilling. This can only be done after getting a geological or soil assessment of your site completed and having required permits in place. The installation must be done by a qualified geothermal contractor. All this adds up quickly, but you will get payback over time. How soon you will want to see the return on investment is up to your own circumstances and how long you want to stay in the home. For our own personal planning, we are typically calculating with 10 years as a target.
Luckily, some contractors or utilities are starting to offer financing solutions. This means you can rent or lease your heat pump, rent-to-own, or finance over time. This may open up possibilities for individuals who cannot or don't want to finance the bulk investment upfront. This may also be a great option for retrofit projects of existing homes.
Another consideration is the overall energy efficiency of your home. To make your home most energy-efficient, where does it make the most sense to spend your money? While putting in a system that is efficient in heating and cooling and reduces the energy needed to do so, it might be better to build a house that requires less heating and cooling energy, to begin with. Many experts in the field of energy-efficient housing, and in particular Passive House designs, advocate for spending your money on a better thermal building envelope first, then look at additional sustainable energy systems afterward.
If there is still room in the budget and it makes sense to you, going geothermal is worth the consideration.
When we started to actively discuss geothermal options with our builder, it was just a little too late in the game for this option, unfortunately. It may have been a good addition knowing that we could have benefited from a direct heat transfer into the in-floor radiant heating and our hot water heating. However, our property would have required a vertical collector and drilling a well.
A geothermal heat pump system can act as your furnace, your air conditioner, and your hot water heater, all in one. This makes it a great option.
While heat pumps are quite common these days, geothermal systems are not as widespread --- yet. It appears there are three main reasons for the slower adoption of this technology:
Limited awareness of the technology compared to other systems like solar;
The high upfront installation cost for the underground portion of the system;
Many properties use geothermal HVAC as a supplement only and still need a conventional or additional system to fully cover the total heating or cooling demand.
Under the right conditions, a geothermal system can make sense for your project and with energy prices continuing to increase over time, we are sure there will be more and more of these systems being implemented across the country.
Some experts regard geothermal systems as a solution better suited for larger commercial buildings or multi-residential developments, where you have more land available to put in horizontal ground loops at relatively shallow depths and can share the demand and supply across a larger user base.
Geothermal heat pump systems are not to be confused with:
Air-to-air heat pumps, which are much less efficient but more common because of the lower cost due to the lack of the underground portion of the system;
Ground-to-air heat exchange systems, which have underground pipes that air gets circulated through under the same principles as above (also known as earth tubes);
Geothermal energy production, which is the process of using the heat in the earth to produce electric energy on a large scale to feed power to the grid (geothermal power plants).
For further reading, we recommend the official Natural Resources Canada website:
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