WEB EXCLUSIVE: How to Design a Ground Source Heat Pump that Works
Ground source heat pumps are the talk of the town, with the appealing elegance of using the Earth’s inherent heating and cooling capacity. However, ground source heat pumps (in particular, the ground loops) are quite expensive, which means design, installation, or operational mishaps can be incredibly costly mistakes. Enermodal Engineering has designed and commissioned dozens of ground source heat pumps across Canada, and has found innovative ways to save money, ensure a strong initial design, and find and correct the most common problems with malfunctioning systems.
Innovative, Cost-Saving Designs
There are several configurations of ground heat exchangers (GHX) or loops commonly used with ground source heat pumps in Canada: horizontal trenches, horizontal slinky coils, vertical boreholes (many deep holes), and horizontal boreholes (one or two layers of shallower directionally-bored piping). While the vertical configuration theoretically accesses more uniform ground temperatures (hence greater capacity in winter and summer) and also requires the least land area, the cost of drilling usually makes this system the most expensive. Horizontal boring equipment is widely used in Canada for installing water, gas, and sewer mains, and so, given suitable soil conditions (e.g., clay) and sufficient land area, can be less expensive and less disruptive to the site than a vertical configuration.
Enermodal has used various ground loop designs to ensure cost-effective ground loops with optimal performance, including the following:
- At Saugeen First Nation Health Clinic in Southampton, Ontario, a hollow core soil auger, of the type used for geotechnical testing, was used to provide cheaper drilling than the usually mud rotary equipment, albeit to relatively shallow depths (~25 m).
- In Manitoba, at Northlands Parkway Collegiate in Winkler and Amber Trails School in Winnipeg, the GHX are built from two layers of horizontal directionally-bored loops, at 6 m and 4 m depth. The system was installed below the sports fields and walkways, so the bored piping poses less risk of backfill settlement than traditional open trench installations, and requires much less landscape remediation.
- The borehole thermal energy storage system at Drake Landing Solar Community in Okotoks, Alberta is comprised of 144 vertical boreholes which are grouted with a custom recipe of Portland cement and fly ash for a good thermally conductive grout between the piping and the ground.
- Where shallow bedrock was encountered at Balls Falls Conservation Area Discovery Centre, Grimsby, Ontario and at Fifth Town Cheese Factory in Prince Edward County, Ontario, the site was mass excavated, and a horizontal slinky heat exchanger was placed directly on the rock surface and grouted in place with cement grout for a good thermally conductive connection.
- For a Toronto Region Conservation Authority building in Vaughan, Ontario, a horizontal slinky placed in sandy soils was used. A sub-soil irrigation system was designed to maintain soil moisture and thermal conductivity around the heat exchanger piping.
Regardless of the type of ground heat exchanger, the key to good heat pump design is an accurate energy model. The GHX designer uses the energy model to calculate the total length and configuration of ground loop piping required. If the energy model underestimates the annual heating or cooling energy, the ground loop will be undersized, and will either freeze in winter or overheat in summer. Enermodal’s energy modelers bring to each project expertise gained from having simulated hundreds of buildings and verifying the results with actual monitoring data. In addition, we are experienced in designing buildings with very low heating and cooling loads, which minimizes the required heat pump capacity – another example of how investment in good envelope and lighting design can pay back with a smaller, less costly mechanical plant.
Common Problems and Commissioning Solutions
Despite their growing popularity, ground source heat pumps are not without their challenges. The most common are undersized GHX fields, excessive building energy use, and inefficient HVAC system designs, which imposes parasitic energy loads on the ground loop.
For example, our team was called to commission a ground source heat pump in southwestern Ontario. Ever since the system was installed five years ago, the return water temperature from the GHX in summer was progressively getting hotter.
Enermodal examined the energy model for the building’s loads and determined that the rise in biofield temperature was due to an imbalance in the heating and cooling laods. To correct this issue, it was recommended that modifications be made to add domestic hot water pre-heating to the system as an additional sink.
Other examples of solutions our team has implemented to fix faulty systems during design and after occupancy include:
- using Variable Speed Drive pumping to reduce the parasitic loads introduced into the ground;
- the elimination of a heat exchanger between the heat pump and the GHX to reduce the temperature difference between them, which reduces the cost of the entire installation and improves system performance;
- a change in the location of the heat injection for supplemental heating during winter so that the heat goes into the building only, and does not heat the ground.
Third-Party Installation Verifications
Another way our commissioning team minimizes headaches and expenses is to serve as a third-party professional engineer, verifying the requirements of CSA Standard 448 to ensure the system is properly installed and started up. Enermodal has served as this third-party verifier on several installations. Recommendations have included better thermal grouts, more rigorous pressure testing, and procedures to purge and fill the loop piping.
Enermodal Engineering, a member of MMM Group, is Canada’s largest green building consulting firm. With offices in Kitchener, Calgary, Edmonton, Halifax, Vancouver, and Toronto, Enermodal served as LEED consultant on over 200 LEED certified projects.