The Solutions Already Exist: Session Three of the Heat Pump Solutions Lab

By Jay Fox

The Reimagine Buildings Collective’s Heat Pump Lab started with a comment that Heather McKinstry of Dattner Architects made while Passive House Accelerator Founder Michael Ingui was interviewing her back in the spring. She mentioned, almost as an aside, that Dattner had worked out a heat pump solution in a large multifamily building, and Ingui’s reaction was immediate. As he noted then, that kind of knowledge is something that needs to be disseminated far and wide because so many teams are encountering the challenge of designing heat pump systems from square one. According to McKinstry, the solution was already sitting in the room, ready to be shared.

That brief exchange led the Accelerator and the Reimagine Buildings Collective to build a lab around the premise: three sessions, sorted by building size, with every presenter showing practical solutions rather than theoretical ones. The first, on July 29, took on small buildings and produced a running list of what goes sideways in single-family Passive Houses along with fixes practitioners had already paid for once. The second, on August 12, moved to mid-size multifamily and light commercial. The third and final session, held August 26, took on the largest buildings in the set, and included talks from McKinstry, Robb Aldrich of Steven Winter Associates, Matt Desmarais of EnergyCatalyst, and Charlie Bushby of Ecoforest.

Holes in a Very Good Wall

McKinstry directs Dattner’s housing studio, which designs 100- to 400-unit buildings in New York City. Passive House adoption has been rapid among affordable buildings, and between two-thirds and three-quarters of Dattner’s affordable portfolio is now Passive House. By contrast, almost none of the market-rate work is going for Passive House certification.

However, McKinstry’s presentation was not structured as a victory lap, but rather as an examination of how the firm’s thinking has evolved with respect to the design of mechanical systems in these huge buildings. The firm’s first Passive House project, 425 Grand Concourse, has 277 units and it was designed around VRF: compressors on the roof, a branch controller on each floor, cassettes and floor-mounted pancake units in the apartments. The loads came in so low that the team could hang six cassettes off a single compressor, which Mitsubishi had never done and had to rewrite its programming to allow. It worked—mostly. Units experienced a delay in service for the first few weeks of occupancy. Once the code was fixed, that issue went away.

There should be a third option, which is why Dattner is looking for somewhere else to get heat. Ground source is the gold standard, and where the ground is unavailable the answer has been wastewater. At Alafia, the enormous East New York development Dattner master-planned in 2018 and has carried through Phase 1, Phase 3, and now Phase 4, a single building loop ties the pieces together. Apartment heat pumps draw off the loop; wastewater heat recovery injects heat back into it; that heat serves domestic hot water and circulates onward; ground source makes up the balance; the whole thing runs in reverse come summer. There is no refrigerant piping threading the building and no additional penetrations in the façade.

There are complications, of course. Ground source systems are prohibited over critical city infrastructure and can be expensive without subsidies. Dattner learned this the hard way. Several projects chose ground source for the Inflation Reduction Act tax incentive, then had to pivot when much of the act was gutted. There is perhaps a larger lesson here, particularly for large-scale projects. Timelines for these buildings run six to eight years; administrations turn over every four; and members of the House of Representatives are elected every two years.

Don’t Put It in a Closet

If McKinstry’s focus was more macro, Robb Aldrich of Steven Winter Associates steered the conversation toward the micro. His subject was the integrated-tank heat pump water heater: a tank with a hood, a heat pump underneath drawing warm air from wherever it happens to sit, and cool dry air coming out the other side. That last part is the part nobody plans for. The machine moves 100 to 150 CFM and drops it 15 to 20 degrees, which means roughly 100 CFM of 50-degree air discharging into someone’s home. Rated UEFs run 3.5 to 4.0. Measured COPs run 2.0 to 2.5, mostly because the resistance elements in the tank keep waking up.

In a basement without a living space, none of this matters that much. In multifamily construction, it can go wrong very quickly. Aldrich put up a photograph from a Hudson Valley retrofit that he called representative rather than exceptional: a water heater wedged so tightly against an electrical panel that the cover could not come off, a drain pan crushed flat in the process, and a space whose temperature collapsed the moment the unit ran. Someone had responded by running an extension cord to a small resistance heater. The building manager was horrified. The utility, he told Aldrich, had paid him more to install the things than they cost. Within a short time, each one came out and resistance tanks went back in.

As for the problem of stealing heat from the heating unit to cover hot water, the metering settled it. The units offset about 43 kWh of cooling in summer and cost about 116 kWh of heating in winter, a net HVAC increase of 73 kWh against roughly 500 kWh a year saved over a resistance tank. Aldrich was careful with the caveat: these were 1,000-square-foot apartments, not 400-square-foot studios, so this is not a universal solution.

Trading High Temperature for High Duration

Matt Desmarais of EnergyCatalyst spoke about the one retrofit in the session, and he opened by conceding that Aldrich’s closet problem exists in the ground source world too. He also wanted one myth dead on arrival. Desmarais says he hears ten times a day that heat pumps cannot work with high-temperature baseboard. “That is objectively false,” he said. They just need to be adapted to work with the baseboards.

And now on to the retrofit.

Hawthorne Valley School is a roughly 40,000-square-foot Waldorf school in the Hudson Valley. The total square footage is spread across four buildings: the Main School (26,520 sq ft), Kindergarten (3,914 sq ft), Assembly Hall/Lower School Building (5,407 sq ft), and the TuRose Administration Building (3,114 sq ft). The buildings had a full catalog of ordinary problems: several small fuel boilers scattered around the complex, maintained for years by a local plumber who retired and left the puzzle to somebody else; high-temperature baseboard as the only means of distribution; effectively no controls; almost nowhere to run duct; a $2 million budget that was modest for the size; and a power budget that would have forced an electrical upgrade had the team decided to fully commit to an air-source system. Ground source became a very attractive option because it dodged the electrical upgrade and met the client’s demands for filtration with some cooling.

Converting the largest building at the school to forced air would have required ductwork for 16,000 CFM. There was room for barely 5,000, and most of the ductwork had to be run in hallway ceilings because Waldorf schools favor unconventionally shaped rooms that eschew 90° angles. There was also limited room for the heat pumps, as the existing boiler room was 100 square feet—large enough for maybe one. To overcome this challenge, the team tucked five 8-ton units into whatever voids they could find, including a closet that was little more than a gap in a wall, and reused the existing baseboard.

Each of the units can produce hot water up to 130°F, while also producing hot air. Both are used to heat the building. What’s interesting is that the system is designed to trade high temperature output for high duration output. The previous boilers had run 10 minutes per hour at 180°F. The EnergyCatalyst system prolongs the runtime of the units, allowing them to run for 50 minutes or more at a lower temperature (typically 110°F to 122°F). Within the school, this change (in addition to minor envelope upgrades that do not come close to Passive House) allows the system to be far more responsive and to operate below capacity without sacrificing comfort. As Desmarais explained, the school never needed to hit that 130°F during the first winter with the EnergyCatalyst system. The closest they got was 128°F. Instead of a system that can blast out tens of thousands of BTUs for a short period of time, as was the case with the previous one, the new system consistently produces a few thousand BTUs of heat.

Desmarais also noted that any system that follows this example will have to be zoned to accommodate the needs of individual rooms (as was the case with Hawthorne Valley). Within some classrooms, comfort can be achieved by running the system 20 or 30 minutes every hour. Other classrooms have needed runtimes of 50 minutes or more. Luckily, the school’s baseboard system was set up so that each room was already individually zoned.

The trick underneath is a refrigeration cycle Desmarais called a double hybrid heat pump design. Hot gas off the compressor makes the high-temperature water; downstream, an air coil cools the liquid refrigerant before it reaches the expansion valve, which improves the cycle. The air warmed by that coil becomes the supplemental heat feeding the baseboard rooms. The one move solves two problems.

To connect the building to the boreholes and to avoid the destruction of the school’s playground, the team relied on a combination of vertical and horizontal drilling. They also used 17 Twister loops, which required about 120 feet of loop per ton—about 35 percent less than a conventional system.

As for real-world performance, Desmarais said that the entering water temperature didn’t fall below 40°F all year, even when the air temperature dropped to -15°F. In terms of efficiency, the seasonal measured COP came in a little under 5.0. On account of the exceptional performance and clean install, the project won NY-GEO’s Top Job award for 2026.

Desmarais concluded by noting that the system has been so efficient that they plan to start heat season with water temperatures at 100°F. They’ll then ramp up as the season demands more heat and may find that the school can remain comfortable with water temperatures that target a slightly lower range.

Water In, Water Out

Charlie Bushby of Ecoforest, returning from the first session, closed out the event by revisiting some of the points he had discussed earlier. Bushby’s firm builds geothermal heat pumps and insists on calling them water-to-water heat pumps, because water goes in and water comes out and the machine has no opinion about where either end connects. It is source and load agnostic.

His example sits in the center of La Coruña, in northwestern Spain. Using the ground as a source was not an option. Two water-to-water heat pumps handle heating, cooling, and domestic hot water for a mixed residential and commercial building, sourced and sinked entirely through rooftop dry coolers used in both directions, which is not how American projects typically deploy them. Iberdrola, the utility that operates in the States as Avangrid, is behind it.

The striking part is the storage: four 5,000-liter buffer tanks, more than 5,000 gallons. That looks like a lot of capex until you notice the solar array and ask about payback rather than first cost. A kilowatt of surplus pushed into a battery is a kilowatt, and the battery degrades. The same kilowatt pushed through a heat pump into water becomes three, four, or five kilowatts of stored thermal energy, and the payback drops to five years instead of ten.

What Happens in October

The three labs were always designed to be the raw material rather than the final product. The product will be the Heat Pump Solutions Symposium. Hosted by the Accelerator, the symposium will take place on October 23^rd from 12–3 p.m. ET. This is a free and virtual event where the lessons learned across the three labs will be presented to a much wider audience. As Ingui stressed, it is designed to provide specific and practical solutions to challenges designers and engineers encounter in the real world.

Attendees of the labs will see the agenda before the final program is set in stone, and anyone who spots a missing piece is invited to say so.