Ground source heat pumps and air source heat pumps use very similar, effective refrigeration technology. However, depending on your climate, one could be far more efficient than the other at heating and cooling your home.
What is the difference between a ground source and air source heat pump?
Both air source and ground source heat pumps operate on the same premise: transporting heat into or out of the home. Unlike traditional heating systems like a fossil fuel furnace, heat pumps don’t generate heat – instead, they just move it.
Comparing ground source vs air source heat pumps, the main difference is where they collect heat. Don’t worry, it’s all in the name.
Air Source Heat Pump
Much like a traditional air conditioning unit, an Air Source Heat Pump (ASHP) pulls heat from the air inside of the home and rejects it into the outdoor air. In winter, an ASHP can reverse its function and heat the home by absorbing thermal energy from the outdoor air and transporting it indoors.
For a more detailed explanation of how an ASHP works, check out our last blog post, Need to Know: Air Source Heat Pumps.
Ground Source Heat Pump
A Ground Source Heat Pump (GSHP) draws stored heat from the ground and transports it into the home. In summer, a GSHP extracts heat from the air inside the home and expels it into the earth.
GSHPs use a system of ground loops installed in the homeowner’s backyard to transport thermal energy.
For a more detailed explanation of how a GSHP works, check out our page Geothermal 101.
GSHP vs ASHP Comparison Chart
Let’s talk air source vs ground source efficiency
At peak operational efficiency, from 1 unit of electricity powering a GSHP, the pump can have an output of up to 5 units of thermal energy. In comparison, 1 unit of electricity powering an ASHP has an output of 3 units at most.
When compared to traditional heating systems like gas or oil furnaces which have 1: >1 energy ratios, both ASHP and GSHP systems perform exceptionally well. However, the GSHP does have the highest potential peak efficiency.
For most of the year, an ASHP is going to operate nearly as efficiently as a GSHP. What sets them apart is how they operate in temperature extremes. When the systems needs to work harder to extract or reject heat, efficiency drops.
For example, an ASHP can easily maintain indoor comfort when it’s 0°C outside. The heat pump has no trouble warming the home when the temperature difference between indoors and outdoors is reasonable.
But when its -25°C outside, the ASHP must account for significant temperature difference. At -25°C most ASHPs drop below 200% efficiency which means it’s energy output isn’t much greater than the electrical energy consumption.
In comparison, a GSHP could still be operating at 350% or greater efficiency at that temperature. This is because it sources heat from the ground, and year-round, at about 6ft below the surface, the earth maintains a stable temperature of between 10 and 15°C.
So, even when it’s -25°C outside and an ASHP is trying to accommodate a significant temperature lift, the GSHP is only performing a thermal exchange from 10 or 15°C below-ground.
Plus, because an ASHP unit is outside and exposed to the elements, when ice or snow accumulates substantially on the coil, the heat pump must melt the build-up. To do so, it switches into cooling mode, rejecting heat from the coil until the ice melts and it can switch back to heating the home.
While the process is ongoing, the home’s heating is supported entirely by electricity before the heat pump takes over again – the melting process usually only takes a couple of minutes, but this requirement does impact the overall efficiency rating of the ASHP.
It’s for these reasons that the optimal operating temperature for an ASHP is moderate climates while a GSHP can maintain exceptional efficiency in climates with greater temperature extremes.
Our energy specialists can help you decide which system better suits your needs.
Installation: Excavation or Drilling?
GSHPs use buried ground loop system, there are two types: vertical and horizontal.
Vertical ground loops require a drilling process similar to installing a water well. The loops are inserted into the ground perpendicular to the earth’s surface. Vertical loops can be installed on smaller properties or when the homeowner wants to limit the disruption of landscaping.
Horizontal ground loops require a larger area of unobstructed land (about 1000ft²-1500ft² per ton) because they are installed parallel to the earth’s surface but are less expensive to install than vertical loops.
ASHPs require no excavation or drilling as a small unit attached to the side of the home. An ASHP is a great alternative for a home with a very small backyard or land that’s unfavourable for drilling, or for homeowners that don’t want to disrupt their landscaping.
However, the long lifetime of ground loops (if that’s a suitable option) makes the excavation process worth it – plus, over time and with proper levelling and grass regrowth, the homeowner shouldn’t notice anything more than a small mound where the loops were installed.
Ground Source vs Air Source Heat Pump: Lifetime and Maintenance
An ASHP requires similar maintenance to a standard air conditioning unit: changing the filter regularly, having a contractor clean the coil and vacuum the unit.
The GSHP system requires less regular maintenance because the heat pump, installed inside a basement or furnace room, does not collect debris through pollen or snow like an outdoor ASHP unit.
Plus, the ground loops (what transports the heat) are buried safely underground in the backyard so they’re unaffected the elements and unlikely to require any maintenance until late into their lifetime.
Both should have regular maintenance check-ups, but it’s likely an ASHP will require more frequent service especially as the system ages.
The GSHP system remains sheltered throughout its lifetime and its ground loops are closed, more durable, and made of long-lasting, high-density polyethylene piping. That means a GSHP system can last up to 30 years before the heat pump needs to be replaced – but ground loops are projected to last 100 years before needing replacement.
In comparison, an ASHP will have a typical useful life of up to 15 years before the unit will need replacement.
Ground Source vs Air Source Heat Pump: Cost
The prices of heat pump systems can vary significantly. A GSHP is going to be more expensive upfront than an ASHP. This can make the ASHP a most attractive option for homeowners. However, GSHP homeowners will enjoy a more substantial return on investment through energy efficiency savings.
Let’s look at the annual ROI of a homeowner switching to a ground source vs air source heat pump:
If a homeowner spends an average $3500 on propane heating
*See Annual Heating and Cooling Cost bar chart for reference
A GSHP might cost them $25,000 to install, then have an annual operating cost of $1500. The homeowner is saving $2000 a year, generating an annual ROI of 8%. That’s an average payback time of 8-10 years based on today’s climbing fuel prices.
An ASHP might cost $15,000 to install and have an annual operating cost of $2400. With the $1100 savings from switching off propane, the system has an average annual ROI of 7%.
With similar ROI rates, it’s very important to note the GSHP will last 20-30 years and the ground loops an estimated 100 years, while the ASHP averages 15 years before needing to be replaced.
Ultimately, when a homeowner chooses geothermal, they are investing in access to their own energy resource. A GSHP harnesses a stable, sustainable source of energy in their backyard.
An ASHP is also sustainable, but the difference in upfront cost is the difference between a system that will reliably deliver heating and cooling for 15years, and one that will deliver for 100 – without being affected by rising global temperatures.
With either system, a homeowner is choosing energy independence by investing in access to a hyper-local, unlimited sustainable energy source, insulated from the impacts of volatile global energy markets.
So, which is better?
Air source heat pumps are a great alternative to ground source when your property isn’t favourable to excavation, drilling, or doesn’t have ductwork or a radiant system. But the ground source heat pump is a clear winner in comfort, efficiency, and durability.