Greenbuild Blog
District Geothermal + Passive House: A Match Made in Heaven?
By Nick Vener
March 3, 2025
In sustainable building, a combination of District Geothermal and Passive House would seem an ideal approach. But are these energy conservation strategies synergistic or antagonists?
In Greenbuild’s 2024 session, “District Geothermal + Passive House: A Match Made in Heaven?” the following speakers discussed how they incorporated district geothermal fundamentals combined with passive house principles in their project at the Barry Farm development in Washington, DC.
Speakers
- Brandon Harwick, President, Principal Engineer at Engenium Group
- Dave Hermantin, SVP Geothermal Development at Brightcore Energy
- Celine Damide, Senior Geothermal Analyst at Brightcore Energy
- Michael Hindle, Owner / Passive House Consultant at Passive to Positive
Barry Farm, in Washington, DC, is a mixed-use neighborhood redevelopment that will create 800 housing units. All its buildings have been designed to achieve Passive House certification. The DOE awarded a grant to support a district geothermal loop serving a 139-unit apartment building and another 98 rental apartments in an adjacent block of townhomes, making this one of the largest district geothermal installations in the country. Additional apartment buildings and townhomes may be added to the district loop in future phases.
Inside the Concept
Beginning with the fundamentals, the district geothermal concept is comprised of district systems that use a shared network of pipes to circulate fluid to provide heating and cooling for two or more buildings.
Buildings can transfer heat between themselves before tapping into the ground, which can be scaled by a factor of two or more buildings (tapping the shared loop before anywhere else).
Inside the Mechanical Systems
This type of system can vary in size and scale for buildings, campuses, and neighborhoods.
Diving a little deeper: In addition to the single-unit buildings with the Barry Farm project, The Edmondson is a multi-family building consisting of 139 units, where the speakers examined some of the optimizations improving overall energy efficiency and carbon reduction.
HVAC System
Typically, a multi-family building would have an air-cooled split system (condensers on the roof, sections in a closet etc.), it would be code compliant. Instead, they VRF to get to passive house goals. After they won the geothermal grant, they converted the whole system over to the water source heat pump (third image). The ductwork all stayed the same; they just updated that one piece of equipment.
Ventilation
traditionally a multi-family building has rooftop units (air cooled). For passive house, they chose to go to a neighborhood system (1 unit servicing 8-9 dwelling units in the building) and one ventilation unit for 8-9 units. This helped them reduce the air leakage which helps with the passive house requirements, and the energy performance of the building.
Domestic Water Heating
Upgraded the water heating with a heat pump domestic water heating system, which has a much better COP and performance and helps with the project’s energy goals. They left the system air cooled. As noted, if done differently, they would have combined the geothermal system and air-cooled system, as it would have helped with their load balances.
Key Takeaways
As mentioned in the session, some of the key takeaways include involving the design team early in the project planning process, as it provides the best chance for reaching the project’s goals. Also, iterative modeling and collaboration will set up the project team for success with a constant stream of open communication. It tends to help when owners and consultants are onboard and put in the effort to make sure the sustainability initiatives succeed.
Although there were many other specific examples, this high-level synopsis mentions some of the other key themes, including passive design matters and the need to reduce the load profile (the load profile of a passive house is drastically different from that of a regular building), which affects how the project team can balance the loads in the geothermal system.
In addition, load diversity yields a balanced system; for example, the different types of domestic water heaters used in different buildings combined with the building's use and occupancy.
Conclusion
It’s a match with a mindful design. Each bubble represents how the energy flows were working.
A passive house poses unique challenges for ground source district geothermal and real benefits—you have to be mindful of both for a successful design.
