The move from fossil fuels to heat pump water heaters is critical for sustainable hot water solutions, but it requires understanding the unique installation strategies, especially in retrofitting scenarios in multifamily buildings. This column summarizes crucial insights and lessons from a recent 400-unit project in Portland, Oregon, where HPWH installation company Great Northwest Installations, Ridgefield, Washington, replaced gas water heaters with in-unit HPWHs and ductless heat pumps for hot water and space heating.

The airflow challenge: Ducting and location

HPWHs function by extracting heat from ambient air and exhausting cold air. Unlike older electric resistance water heaters, which can be installed in small, airtight exterior utility closets on balconies, HPWHs require sufficient ventilation. If the cold exhaust air is not vented out, it creates a feedback loop that severely reduces system efficiency. This is why proper venting is essential.

For these HPWH installations in Portland, Great Northwest used some novel ducting strategies: 

Double ducting: Contractors successfully employed a double-ducting approach, running both intake and exhaust air to the exterior. They also found that using screen terminations works better than standard dampers, which often resist the HPWH fan’s low airflow.

Reusing infrastructure: A resourceful solution involved removing the old 3-inch gas vents and reusing the existing holes as conduits for new ductwork, eliminating the need for new penetrations.

Condensation control: It was observed that metal ducting can cause condensation when cold exhaust meets warmer air, which can lead to mold formation. Insulated corrugated ducting is the best practice to prevent this issue.

Taming the condensate: Innovative drainage

Unlike older gas water heaters, HPWHs produce condensation. Placing units in exterior closets can create drainage challenges. Traditional methods (running external piping and using heat tape to prevent freezing in the winter) can prove costly and labor-intensive.

As a result, here are the innovative drainage strategies used by Great Northwest.

Repurpose existing vents: Great Northwest repurposed the existing gas vents to accommodate new condensate drainage pipes, dropping them vertically through the building. The contractor then connected the condensate lines from all upper-floor units on the bottom floor into one single primary drain pipe, which exited the building.

Internal freeze protection: Instead of expensive exterior pipe wrapping on multiple condensate lines, Great Northwest ran the heat tape inside a single exterior drain pipe alongside the drain tubing. This solution met building code requirements while achieving significant cost savings.

The human factor and product choice

Consumer complaints about HPWHs often center on noise and cold air exhaust. Even moderate noise levels (40 to 50 dB) can bother residents accustomed to gas systems with lesser noise levels:

Great Northwest made it mandatory to use rubber spacers and grommets both behind the unit and on earthquake straps to prevent vibration from transmitting through walls. 

Furthermore, transparency with homeowners about potential noise and cold air exhaust is crucial for managing expectations.

Great Northwest has found that a successful retrofit requires aligning the system with existing infrastructure:

Unitary HPWHs may be affordable to buy compared to market-available split systems, but their total installed cost can be higher due to the complex requirements for internal ducting and noise mitigation.

Split systems (heat pump outside, storage tank inside), while more expensive upfront, can be cheaper overall because they eliminate the need for internal airflow management and noise mitigation equipment. However, market availability for split systems is currently limited.

The in-unit installation of HPWHs in the Portland 400-unit multifamily building highlighted thoughtful considerations for a successful retrofit. By reusing existing gas vent conduits, applying targeted ducting and drainage solutions, addressing noise and vibration concerns and maintaining clear communication with residents, installers can reduce cost and improve performance. 

This project also shows that choosing the right type of HPWH can significantly improve the overall installation process and associated economics involved.

Learning from this project adds to the growing set of best practices, guiding future multifamily HPWH retrofits to be more efficient, reliable and cost-effective. l

Vidhisha Moopnar, senior project analyst with the Building Innovations team at the New Building Institute, supports the team with technical research, policy analysis, and data analysis and visualization. She contributes to project initiatives by planning and creating roadmaps, engaging with stakeholders and assisting colleagues with technical convenings, memos and presentations. In addition to supporting other NBI program areas and divisions, Moopnar currently works on the Advanced Water Heating Initiative and the GridOptimal Building Initiative.