A primer on residential geothermal heat pump system design

By Jeff Hunter

 

A successful ground source heat pump (GSHP) project starts with a well-planned system design. There are basic, yet critical elements to building out a practical design. These will ultimately lead to a finished product that meets end-user requirements; observes any site-specific installation considerations; ensures long-term reliability and performance of the equipment; and isn’t a burden on technicians providing service and maintenance.

The following steps can apply to both residential retrofits and new construction projects. An important note: this is a high-level perspective on the design process. For more detail, refer to a current copy of the CSA-448 standard for the Design & Installation of Ground Source Heat Pump systems, along with IGSHPA (International Ground Source Heat Pump Association) training courses/certifications, which in Canada are delivered through HRAI. Geothermal heat pump distributors will also provide equipment specific manufacturer training to support your design efforts.

 

Site assessment for ground heat exchanger

The first and most obvious difference between a GSHP project and any other type of system is the ground loop as the energy source/ sink. Perhaps sooner than you think, gone will be the days where we assume an infinite energy source is piped (by others) to our buildings for our endless consumption. With that luxury, we don’t have to think too deeply about the energy source. How often have you thought about what’s going on at the wellhead?

As HVAC designers/installers, we’ve only had to consider that energy source as it pertains to sizing the gas piping into the building and the equipment. With geothermal, we’ll have to consider how the balance (or imbalance) of the building loads, the variation in equipment selection, the ground conditions, and local weather conditions will among other things all impact our onsite energy source. While finite, the ground heat exchanger is a generational thermal energy infrastructure asset if managed properly. Whether we are assessing this for a new build project or looking to retrofit an existing home, these critical considerations can make or break a project, so do your homework.

Understanding the available land area upfront will help guide the first steps of your system design. Generally speaking, it will be reasonably clear what type of ground heat exchanger the project will use with a quick look at the site plan. Ultimately, the ground heat exchanger chosen for the project will be the most cost-effective of the options available to meet the project’s needs. However, there are nuances to the choice between each system type, be it horizontal trenched, horizontal drilled, vertical, or surface water (closed loop). Careful examination will determine the best ground heat exchanger type and placement. Dimensions of the field (circuit design and overall length) and length of lateral supply/return runouts are not static; every project is different. Each closed-loop system type can be manipulated based on site conditions. Designers may follow Annex D of CSA-448 for a multiple-measure method of ground heat exchanger sizing, which references minimum lengths as specified in table D.4. Or, more commonly, by using a manufacturer or third-party software with approved modelling algorithms. Watch out for making assumptions with the ground loop. Ground conditions can change dramatically from site to site (even in the same region). Researching subsurface conditions is an essential part of ground heat exchanger selection and design. Work with your resources, such as water well records and geological maps. The homeowner, local contractors, and geothermal drillers in the industry are a wealth of knowledge and are usually willing to help you. Open loop systems may also be an option depending on local regulations, available water quantity, and water quality.

 

Load calculation and energy analysis

We know how to calculate loads with HRAI/CSA F-280 or Manual J. It is imperative, whether it is new construction or a retrofit project, to have an accurate room-by-room load calculation to support a proper system design. Quick block load calculations may only be used to help get the ball rolling on a preliminary basis for a project. Still, I always want a full F-280 to confirm any assumptions we’ve made before committing to a final system design.

One thing to highlight here as something I’ve seen missed quite frequently by designers and contractors is the use of typical/textbook R-values/U-Values/ SHGC for envelope components and assemblies. This could be particularly impactful with higher-performance new builds where heating and cooling loads may be more balanced or, in some cases, cooling dominant. For example, glazing can be critical to the ultimate design and must be correctly accounted for. If the designer just used standardized U/SHGC values in their load calculation, this could influence ground heat exchanger length so be sure to get the values based on the manufacturer’s specifications for the actual window assembly used on the project. Errors here can have significant downstream impacts.

With a GSHP system, we are not stopping at the load calculation. We are feeding that data into energy analysis software to model the system’s long-term performance as we’ve specified (ground loop + building load + equipment selection). Residential-focused geothermal design software will typically use the “bin method” to estimate the amount of energy a system will use over a year and multiple years, considering outside temperatures throughout the year and incorporating those part-load conditions. This modelling is where the load calculation and the ground heat exchanger construction collide with the equipment selection to verify our design, plotted over the lifecycle of the system.

 

Equipment selection

A conversation with the end user usually drives the equipment selection. What are we trying to achieve in the house? What are the homeowners’ goals and constraints? Is it a straightforward forced-air retrofit or a new construction design combining hydronics, forced air and DHW?

Knowing what the client is looking for and where they sit with their budget will ultimately lead us to our equipment selection, together with any building specific or technical constraints that may impact our selections such as they don’t want ductwork but want to cool in certain spaces. Or, the client wants the highest efficiency mechanical and DHW system on the planet but wants it to fit into a broom closet. There is equipment to meet any need, but there is no magic here – there is the “ideal” application that nets the optimal results, and there are trade-offs for every degree off the ideal application we venture.

An example of this is “triple-function/combo” GSHPs. They are designed for tight spaces, combining the functionality of a water-to-air and water-to-water in one box. In new construction, where we often see radiant in-floor and forced air together, designers may default to these “triple function/combo” units. This can be a mistake as there are generally better ways, especially if mechanical space allows. Our goals should be to reduce the complexity of the mechanical system, allow for redundancy/emergency backup, and ultimately configure the system for the highest performance/greatest ROI potential within the customer’s budget.

There is much more to discuss beyond this overview of key aspects of the residential geothermal design process. Incorporating geothermal system design into your portfolio is not a giant leap for those already involved with HVAC design. A background in hydronic/ radiant design, load calculations, duct design, and energy modelling will significantly benefit the learning process. Find a local geothermal distributor to support your design efforts as they have a wealth of knowledge on the products they supply.

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