By Jay Fox

The Infinite Monkey Theorem posits that a group of monkeys, given infinite time, typewriters, and paper, will eventually (and inevitably) produce the complete works of Shakespeare. Passive House Accelerator and Reimagine Buildings have long bought into a similar but more modest line of thinking: If you put enough Passive House practitioners in the same room for long enough, they will resolve even the trickiest design challenge. It is with this spirit of collective optimism that the Heat Pump Solutions Lab was born.

Heat pumps are not new technologies to most folks in the world of high-performance building. In fact, they have become relatively standard pieces of equipment in conventional construction in many parts of the country and the world. Teams have figured out a lot of solutions for a lot of different types of buildings. As design and installation are far from plug and play, that knowledge is invaluable to teams who are encountering novel challenges and have to design these systems from scratch—often with limitations on time and budget. If they can’t overcome those limitations easily, they end up reverting back to the familiar, which means systems that run on fossil fuels.

As a case in point, Passive House Accelerator Founder and Ingui Architecture President Michael Ingui told the audience about a recent conversation he’d had Dattner Architects Senior Associate Heather McKinstry. Ingui had been speaking with a developer a few months ago who really wanted to use heat pumps in an upcoming multifamily project but couldn’t figure out how to make it work. Consequently, they were getting ready to throw in the towel and go with gas. When Ingui described the roadblocks to McKinstry, she said that there were already solutions in place that Dattner had used in actual projects. On the one hand, this helped the developer make the switch from gas to heat pumps. On the other, it served as the genesis of the Heat Pump Solutions Lab.

The first session was held on July 29 and covered small buildings. The second will be held on August 12 and will cover mid-size multifamily and light commercial buildings. The final session of the lab will be held on August 26, and it will cover large buildings. These discussions will ultimately create the program for the public Heat Pump Solutions Summit hosted by Passive House Accelerator this October.

Throughout each one of these sessions, the focus will center squarely on specifics. We are past talking about the big picture or relitigating the pros and cons of heat pumps. The goal is to provide practitioners with practical and actionable solutions and, perhaps most importantly, to allow them to ask very specific questions about very specific challenges in a (virtual) room full of people who likely have the solution they’re looking for. These sessions are open to any Collective member and replays are/will be available in the Collective. Slides, recordings, and transcripts are posted in the Heat Pump Solutions Lab space.

The value of showing up is not the slides, though. It is meeting another practitioner who has already lost the winter you are about to lose because of a mistake you don't even know you're about to make. It's about learning how something as single as one wire can save you or your client an enormous headache and a season of sky-high electric bills.

Nine Years, Three Water Heaters

Brooklyn-based Ingui opened the event by talking about his own experience with heat pump water heaters. It was personal because the starting point was his own house, which is a certified Passive House.

While the house has performed beautifully overall, he is on his third A.O. Smith unit in nine years. The compressor keeps going out. True, A.O. Smith has honored the warranty every time the compressor has gone on the fritz, but it's inconvenient. Moreover, the recovery time has never met expectations—he estimated the tank gets him about three and a half showers, which is less than ideal for a family of four. He eventually unplugged the mixing valve pump because it was pulling hot water down through his four-story house faster than the unit could make more of it.

The configuration that has worked far better for him is two of the same units in tandem, side by side, in certified Passive House buildings with five or six people showering inside the same narrow window of morning. Across multiple projects and over several years, he’s received zero complaints.

Another configuration involves Sanden systems, which he’s realized need to be piped a specific way, with minimal ninety-degree bends and no spaghetti plumbing. When the piping doesn’t follow these recommendations, the systems tend to have issues. Additionally, Ingui recommends increasing the capacity for the Sanden systems, either by increasing the size of the outside unit or including a second tank on the inside.

Bob Divilio, who engineers most of these systems for Ingui's office, added that a common configuration for smaller townhouses is to have two modest outdoor condensers instead of one large one. As he explains, heat pumps are almost never manufactured in a capacity that matches a Passive House's genuinely small loads, so they run oversized, especially when it comes to heating (as an aside, Ingui likes to note that he typically only needs to turn on his heat a few nights per winter). Splitting the load across a one-ton and a three-ton unit buys turndown that a single four-ton unit cannot deliver. It costs some piping and some coordination, but it buys a system that modulates instead rather than short-cycles.

Ingui also provided two pieces of really crucial advice for designers. First, grill your clients about how they actually use water. Two adults who shower and two kids who take baths is a fundamentally different use profile than a house for four people who shower, and running out of hot water is, as Ingui put it clearly, “a bad thing.” Second, accessibility is key. Systems are designed to last for decades, but individual components do fail from time to time. You shouldn’t have to take out part of a wall to get to the problem spot.

The Changing Landscape of Refrigerants

During the Q&A session that followed Ingui's talk, NYC-based architect Peter Holtzman, founder Bespoke Architecture, brought up the subject of refrigerants. This could (and soon will) be an article onto itself, but it’s worth covering here in limited detail.

New York City's rules governing A2L refrigerants took effect on May 15, 2026. The change can be traced back to Local Law 77 of 2023, which amended the Mechanical Code's refrigeration provisions and left the Department of Buildings to work out what compliance would actually look like on a drawing set. The answer arrived as four new sections of Title 1, Chapter 7000. The first establishes design, installation, alteration, and repair requirements for systems charged with refrigerants that ASHRAE 34 sorts into the mildly flammable A2L category, which by now covers most of what ships in residential equipment, and it requires a refrigerant detector wired to trigger an FDNY alarm once concentrations climb past safe levels. The remaining three adopt and amend the 2022 editions of ASHRAE 15, ASHRAE 15.2, and ASHRAE 34 as they apply to residential work.

ASHRAE, AHRI, and Mitsubishi Electric all pressed DOB to adopt the 2024 editions instead of the 2022 versions, arguing they align more cleanly with the model codes and with UL 60335-2-40, the certification standard the equipment is actually built to satisfy. DOB held to 2022 and layered city-specific amendments over it, so designers in New York are now working from a rulebook that does not quite match the one manufacturers assumed when they engineered their products. Code consultants flagged a nearer-term headache in the comment record, noting that FDNY has been rejecting the detection systems built into packaged equipment and requiring separate detectors tied into the fire alarm panel, complete with a means of testing them that nobody currently sells.

None of this makes the work impossible. It does explain why architects who have spent a decade routing refrigerant to every zone are suddenly asking what it would take to route water instead or, as McKinstry noted, looking into packaged thermal heat pumps (PTHPs) as an alternative (though, as she pointed out, it is kind of “crazy” to build an airtight envelope and then poke hundreds of holes in it to accommodate hundreds of through-wall units for each apartment). McKinstry added that VRF systems can be used on individual floors (say, for example, the first floor in a mixed-use high-rise with a commercial space on the ground floor). There are fewer regulations when the system does not cross between floors.

To reiterate, this kind of hard-core building science geekery demands a more thorough examination. Stay tuned for that.

The Coil That Ran All Winter

Patty Chen of Patricia Chen Architecture was the next presenter. She focused her talk on her own Passive House in Ludlow, Vermont. It is gorgeous and cozy, offering magnificent views of the Green Mountains and easy access to Okemo Mountain Resort. In terms of being a homebase to host friends and family for a weekend ski trip, it’s hard to image a more welcoming setting.

Despite the idyllic setting and passive design, Chen did have a rude awakening when she received her first electric bill during her first winter. It was way higher than the model anticipated.

A Lesson from Spain

Charlie Bushby of Ecoforst, calling in from coastal Spain, offered a water-to-water solution that strips out the outdoor condensing unit and replaces it with a dry cooler consisting of a fan and a finned coil with hydronic connections: no refrigerant, no compressor, no circulators. Moreover, the components are inexpensive and can be easily replaced once they succumb to corrosive forces of the salty air. The system serves as source and sink for an indoor water-to-water heat pump (see Figure 1). Functionally it's a geothermal machine that's been handed an air-coupled loop instead of a ground loop.

Ecoforest adds a third heat exchanger dedicated to defrost the system. Three-way valves bypass the refrigerant circuit entirely, route buffer water through that exchanger, and send warm water out to the dry cooler coil. As a result, the system spends circulator energy instead of compressor energy to defrost, and it’s not robbing the space-heating load to do it.

Domestic hot water demands the highest supply temperature and therefore delivers the worst efficiency of any load in the building. Consequently, the sequence keeps the tank floating on desuperheater preheat as long as it can, and it only diverts the condenser to DHW when the tank falls below setpoint (120°F).

The Road Ahead

The next session on August 12 takes on medium-sized buildings, with large buildings to follow two weeks after that (August 26). If you have a specific challenge or can offer a specific solution to the problem of designing and installing heat pump systems for high-performance buildings, you should attend and share your wisdom or what has you perplexed. We don’t have the luxury of infinity, but we do believe that a (virtual) room full of engineers, builders, and architects can figure out how to successfully electrify and decarbonize our building stock.