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As the demand for energy-efficient and sustainable heating solutions rises, air-to-water heat pumps are gaining significant traction in the hydronic heating industry. This innovative technology is not only helping reduce carbon footprints but also offering a cost-effective alternative to traditional heating systems.

“Demand is steadily increasing, especially in markets that already have large amounts of hydronic heating systems,” notes Mike Dion, development engineer for Stiebel Eltron. “States in the Northeast and Northwest are showing the largest demand so far. As electrification efforts in some states ramp up, the customers and installers are looking to heat pumps to provide home heating, cooling and DHW. Electrification efforts in some states have installers looking for alternatives to gas/oil/propane for heating and DHW. States like Washington are re-writing their building codes with a preference for energy-efficient heat pumps over fossil fuel heat sources. Other states, like Vermont and Massachusetts, are directly giving rebates to homeowners who install an air-to-water heat pump.”

Chris Decato, product manager at U.S. Boiler Co., says he is seeing several key trends driving the demand for air-to-water heat pumps.

“First, there's a growing emphasis on energy efficiency and reducing carbon footprints, which is leading more homeowners and businesses to explore greener heating and cooling solutions,” he explains. “As governments introduce more stringent regulations and offer incentives for low-carbon technologies, air-to-water heat pumps are becoming increasingly attractive. Additionally, the shift toward electrification in heating systems is gaining momentum. With advancements in technology, these systems are now capable of delivering higher efficiency, even in colder climates, which is expanding their appeal in regions where they previously weren't considered viable.”

Another trend Decato points to is rising energy costs. “As traditional energy prices continue to rise, consumers and businesses are seeking more cost-effective and energy-efficient alternatives for heating and cooling. Air-to-water heat pumps, which can significantly reduce energy consumption, are seen as a viable solution to offset these costs over the long term.”

According to Elliott Willey, senior manager, Climate Conscious for Weil-McLain, the company is beginning to see significant interest and inquiry into air-to-water heat pumps for hydronic heating applications from both HVAC and hydronic contractors through broader heat pump education and training along with homeowner interest from emerging incentive programs.

“Air-to-water heat pumps have been established and proven in Europe, and are now beginning to be applied to North American applications,” he says. “This is largely driven by the governmental regulations supporting decarbonization with the main catalyst being the Inflation Reduction Act. A significant public private partnership has been established to further catalyze technology readiness and adoption through the U.S. Department of Energy (DOE) Cold Climate Heat Pump Technology Challenge as part of the Initiative for Better Energy, Emissions, and Equity (E3 Initiative). The DOE has partnered with the U.S. Environmental Protection Agency (EPA), Natural Resources Canada (NRCan), and heat pump manufacturers to accelerate the development and commercialization of next-generation CCHPs that meet consumer comfort and efficiency needs in cold climate regions of North America.

“Heat pumps in general, have now crossed the chasm and are being applied to all market segments,” Willey continues. “The benefits of adoption are now being observed with broad industry coverage and supported by detailed analysis published by key stakeholders. As heat pump awareness, education and training efforts are deployed throughout the industry, it allows for more targeted, or niche, application understanding, such as air-to-water heat pumps for hydronic heating.”

Daniel Torres, North American regional product director for NTI Boilers, an Ariston Group company, says air-to-water heat pump demand is still a little low, but the trend is definitely growing.

“There’s a lot more manufacturers entering into the air-to-water market,” he says. “To me, that just means the market trend is really gaining a lot of traction. The segment is starting to gain a lot more popularity. The push away from fossil fuel technology — such as gas boilers and water heaters — and toward the whole trend of electrification really lends itself to this type of technology and hydronic-focused markets.”

An industry game changer

Many industry experts see the air-to-water heat pump as a game-changer for the hydronics market.

“They are a ‘game-changer’ for the hydronics industry as you can now achieve a step-level improvement in total system efficiency (COP far exceeding 1.0) for the first time when applied into a traditional residential ‘high-temp’ gas boiler hydronic heating system through a ‘hybrid’ (dual-fuel) approach,” Willey notes. “Furthermore, when taking the ‘hybrid’ approach, you can lead with the air-to-water heat pump providing significant decarbonization opportunity while still having your traditional boiler as an always-ready backup providing peace of mind. We believe this pairing also extends the life cycle of each appliance. You can achieve this while maintaining your existing system with minimal near-boiler piping changes to ensure a lower cost of adoption.”

Another game-changer is installation flexibility, Willey adds. “The installation flexibility allows you to install the hybrid solution all at once via a phased approach (boiler now, heat pump later), or a retrofit upgrade (adding the heat pump to your existing boiler). Lastly, the sizing and application tool that Weil-McLain is launching along with the hybrid solution ensures the comfort, efficiency, and durability benefits of the Hybrid System. The application must be properly sized. Weil-McLain has created the ECO Calc Application Sizing Tool, a tool to guarantee the correct sizing incorporating: Manual J (heat load), heat pump capacity, heat emitter capacity, DHW consideration, localized weather ‘bin’ data, localized utility rates, and rebates and tax credits.”

Dion points to the fact that heating, cooling and DHW can all be provided from a single heat pump. “In many cases, there's no need for additional heat pumps or mini-splits for supplemental cooling. Also, with our monobloc air-to-water heat pumps, the entire refrigerant circuit is outside. This is ideal for next-generation refrigerants like R-290 where there may be concerns about flammability. With a monobloc air-to-water, there is significantly less risk compared to a heat pump that is installed inside the home.”

According to Decato, air-to-water heat pumps’ ability to deliver high efficiency even at lower outdoor temperatures is transforming how the hydronics industry approaches energy usage.

“Unlike conventional hydronic systems that typically focus on heating, air-to-water heat pumps can provide both heating and cooling,” he says. “This dual functionality allows for year-round comfort with a single system, simplifying installation and maintenance, and offering a more integrated solution to consumers. And, as the push for decarbonization intensifies, the hydronics industry is shifting away from fossil fuel-based systems. Air-to-water heat pumps, which operate using electricity and have the potential to be powered by renewable energy sources, are perfectly aligned with these sustainability goals. This shift is redefining industry standards and practices.”

Air-to-water heat pumps can also be easily integrated into existing hydronic infrastructure, making them an attractive retrofit option for upgrading older systems to more energy-efficient ones, Decato adds.

“This flexibility is a significant advantage in the industry, where many buildings already have established hydronic networks,” he says. “Our Ambient Air-to-Water Heat Pump Dual Fuel controller enables the combination use of a boiler with the air-to-water heat pump. Additionally, air-to-water heat pumps have the ability to efficiently deliver heat at lower temperatures which aligns well with the growing trend toward low-temperature hydronic systems, such as underfloor heating. This compatibility enhances comfort while reducing energy consumption, further solidifying their role in the future of hydronic heating and cooling.”

New products flooding the market

As Torres previously mentioned, many manufacturers, including boiler manufacturers, are entering the air-to-water heat pump market. Many of these new products were on display at the AHR Expo earlier this year. NTI Boilers will soon be launching its own air-to-water heat pump in the coming year.

“I joined the Ariston Group about nine months ago, and part of the reason in bringing me on was my air-to-water heat pump experience with my previous company,” Torres explains. “NTI has really focused on boilers in the past. That’s what we do — we manufacture boilers, and we do a really good job of that. We’re one of the top boiler manufacturers in North America. However, we have seen the request and the push away from boilers. Within the past probably three years, more boiler manufacturers are starting to get into this space, when before, it was just like niche-type products. These other manufacturers are seeing it as a threat to their boiler business, and not just taking away boiler business, but taking away customers. In other words, if I'm going to go into a wholesaler and I say, ‘Mr. Wholesaler, I want you to sell my boilers.’ They're going to say, ‘Well, this company has a similar boiler offering, but they also have this air-to-water heat pump, so we're going to go with them.’ It's also protecting the boiler business so we're not missing out on opportunity. It's not just because we think the market's going to explode and we want a piece of it early on. It's really because we're trying to protect our current business and current wholesalers as well. They will flip to other manufacturers if we don't provide a similar offering.”

Torres says NTI’s air-to-water heat pump is slated to officially launch in first quarter of 2025 with test sites being launched this fall and winter.

“We’re going to feature R-32 refrigerant, and it’s going to be very comparable to the other R-32 products that are out there as far as efficiencies go and performance goes,” he says. “We're going to be launching a family of products for residential applications. Our main goal is to be able to pair our outdoor air-to-water heat pump monoblock units with our boilers on the inside. As a boiler manufacturer, we know boilers, we know piping and we know plumbing. We also know there's a limitation to air-to-water heat pumps, and a lot of projects are going to need some sort of backup heat, whether it's electric or gas. Our focus is to try to make our product as seamless, as easy to pair up with an NTI boiler as possible.”

Sandra Folleville, marketing manager for Veissmann, notes that more products are flooding the market due to the increased demand for air-to-water heat pumps, mostly driven by requirements for energy efficiency heating, cooling and DHW solutions. Eco conscious building owners are also driving demand in this space.

“[More manufacturers are entering the market] based on changing market demands and the need for electrification and decarbonization product options,” she says. “Veissmann has rich experience with heat pumps in the European market, and we are now introducing that technology into North America.”

Veissmann's monobloc air-to-water heat pump achieves up to 4.9 COP and 16.8 EER, Folleville notes. The product uses R-32 refrigerant and features heating performance down to -13° F. It can generate up to 165° F supply water.

Stiebel Eltron’s WPL air-to-water heat pump has been available for about a year and a half, but Dion says the company would have liked to have introduced it to the U.S. even earlier.

“We have sold a similar product in Europe for over 10 years now,” he notes. “In the U.S., regulations relating to monobloc air-to-water heat pumps are behind Europe. Our product currently has to be tested as ‘a chiller that can also heat’ under the AHRI 550/590 standard. A robust performance standard written for residential air-to-water heat pumps would make development and testing a lot smoother and quicker.”

Stiebel Eltron has two sizes of heat pump: the WPL 15 A2W Premium does nominally 2-ton in cooling, and 23 kBtu/h in heating and the WPL 25 A2W Premium nominally does 4-ton in cooling, and 50 kBtu/h in heating.

“The WPL 25 has a COP of 2.77 at the A5/W110 test point, which is what many states are using as an efficiency test point,” Dion says. “This is the highest of any monobloc air-to-water unit sold. Our cold-climate performance is one thing that makes our product stand out. The EVI compressor technology boosts capacity at lower ambient temperatures, which is when a building actually needs higher heat production. In addition, both WPL models output water at up to 149° F, making them suitable for many retrofit installations in addition to new construction using low-temperature heating.

“We have designed the WPL to perform best at low ambient temperatures,” he adds. “Many customers don't need peak heat capacities at 40° or 50° ambient temperatures. If our heat pump can provide a building's heat at -4° F with more efficiency than a fossil-fuel heater, then we feel like we are doing our part to reduce overall global energy consumption. Furthermore, the fact that a single WPL can provide for all of the thermal needs of a single-family home, less mechanical equipment is needed in the home, further reducing expended resources, and also saving money.”

Stiebel Eltron is committed to transitioning its heat pumps to natural refrigerants (R-290) in the coming years, Dion adds. “This refrigerant has a significantly lower GWP compared to the currently marketed refrigerants. We will be transitioning from R-410a to R-290, with an A2L refrigerant in the interim, but our long-term goal is an environmentally-friendly natural refrigerant.”

According to Decato, U.S. Boiler’s decision to enter the air-to-water heat pump space at this moment was driven by a combination of market dynamics, technological advancements, and strategic alignment with our long-term goals.

“Over the past few years, we've observed a significant increase in demand for energy-efficient, sustainable heating and cooling solutions,” he explains. “The market for air-to-water heat pumps is growing rapidly, driven by consumer interest in reducing energy costs, government incentives and a global shift toward decarbonization. Entering this space allows us to tap into this expanding market and meet the evolving needs of our customers. At AHR, it was clear that the industry is moving decisively toward electrification and sustainable energy solutions. By introducing our air-to-water heat pump products, we’re positioning ourselves at the forefront of this trend, ensuring that we stay competitive and relevant in a market that is increasingly prioritizing low-carbon technologies.”

Additionally, expanding into the air-to-water heat pump market complements U.S. Boiler’s existing product lines, particularly in the hydronics space, Decato notes.

“It allows us to offer a more comprehensive product portfolio that meets the demands of modern energy-efficient residential homes,” he says. “This strategic move also enables us to cross-sell and bundle products, enhancing value for our customers and strengthening our market position. The timing of our entry is critical. We’re at a point where market conditions, technological readiness and regulatory frameworks are all converging to create a perfect opportunity for growth in the air-to-water heat pump sector. By moving into this space now, we’re positioning ourselves to lead in a rapidly evolving market, rather than playing catch-up later.”

U.S. Boiler’s Ambient Hydronic Heat Pump has a 5-ton capacity (rated at 60 MBH) and provides reliable space heating with outdoor temperatures as low as -13° F.

“Offering a COP of up to 3.95 (395% efficiency) and delivering supply temperatures of up to 140° F, the Ambient Heat Pump offers significant energy savings over conventional electric heating systems,” Decato says. “Installation is simple, whether the monoblock heat pump is installed in a new residence or is retrofitted into an existing hydronic system. It’s also easy to install as a stand-alone heat source or in dual-fuel applications. The heat pump uses environmentally friendly R-32 refrigerant for high efficiency and low global warming potential. The Ambient Heat Pump is available with an optional dual-fuel controller for applications in extremely cold climates. The controller, as part of a kit, allows the installer to seamlessly integrate the heat pump with a new or existing fossil fuel or electric boiler. No additional venting or re-sizing of existing hydronic piping is necessary.”

Weil-McLain’s hybrid solution is up to five times more efficient than traditional boilers, providing significant energy savings and reducing environmental impact, Willey explains.

“Our heat pump uses state-of-the-art, eco-friendly R-32 refrigerant,” he says. “Our ECO HP heat pump with boiler backup ensures homes remain warm even in the coldest climates. The heat pump operates during milder temperatures to maximize efficiency and carbon reduction, while seamlessly switching to the boiler as the always-ready backup heating source on the coldest days. Homeowners enjoy peace of mind with our reliable heating system that does not require freeze protection with our innovative split design, ensuring hassle-free maintenance and long-lasting performance. Operating as dual fuel and positioning the boiler as a backup, extends both appliance’s life expectancies.

“Our heat pumps leverage leading refrigerants and advanced technology proven in European markets, and can be applied to support both residential and commercial goals and initiatives,” Willey adds. “According to the IEA, to align with all existing national energy and climate pledges worldwide, heat pumps will have to meet nearly 20% of global heating needs in buildings by 2032. We believe with our approach; our addressable market and adoption rate can support this. We make every future product decision guided by our past performance — to ensure best-in-class product longevity. We design for the unique characteristics of existing ‘high-heat’ residential hydronic heating applications.”

The future of hydronics

Though well-established in European countries, the air-to-water heat pump market is just getting started in the U.S.

“It [the market] will grow for sure,” Torres says. “Our initial focus will be on the territories that are traditionally hydronic and embracing/pushing toward the trend of electrification. Now, in the next five to 10 years, we'll start to see the impact and see what it does to the non-traditional hydronic markets. There's also some political impacts that could hurt or help the future of this style of product as well. Depending on the type of administration that could get in within the next five to 10 years, there could be huge support toward electrification and carbon reduction, or it could be a complete non-starter. Watching that trend, especially here in the U.S. is extremely important for us on the political side. Either way, our market predictions say it’s going to grow substantially within the next five years.

“As a boiler manufacturer, it’s not just about growing our business — which of course, we always want to do — but it’s also about protecting our current customers and our current footprint in our market,” he continues. “We want to be known as a progressive brand, so we feel like this is the right way to do it. We could have easily brought on an air-to-air product and taken a little piece of that market, which is massive, but it's not our customer base. We wanted to stay true to who we are as a brand.”

Dion says that the market has to transition to the lowest GWP refrigerants possible.

“Within five years, R-290 must be made legal for installation in usable quantities for monobloc air-to-water heat pumps,” he says. “These products are marketed as energy-efficient, but we must also be able to market them as environmentally-friendly if they are to truly be successful. In the U.S., we should be willing to heed the lessons that other regions have learned, and also avoid their mistakes. In Europe, the transition to A2L refrigerants was cut short due to revelations on their association with TFAs and PFAS. We need to have the courage as an industry and a country to make informed decisions with our future in mind.”

From a product perspective, Willey anticipates a three-stage evolution over the next several years.

“The first defined as where we are today, with majority of residential hydronic heating systems served by gas boilers,” he says. “The next stage is the transition period — a ‘hybrid approach’ defined as integrating an advanced air-to-water heat pump with a traditional gas boiler. The final stage will adopt next-generation air-to-water heat pump technology.”

Folleville agrees that heat pump technology will continue to advance in coming years. “This will have the way for widespread application of air-to-water heat pumps in new and retrofit applications. We will see higher performance in extreme climates.”

Over the next few years, Decato anticipates significant evolution in both the market for air-to-water heat pumps and the underlying technology.

“The market for air-to-water heat pumps is expected to grow substantially as more areas shift toward electrification and energy-efficient solutions,” he says. “With government and regulatory bodies continuing to push for decarbonization and offer incentives, adoption rates will rise, particularly in regions where heat pump technology is still emerging. We can expect to see air-to-water heat pumps becoming a mainstream choice for both new builds and retrofits. Continued innovation will lead to more efficient, reliable, and versatile heat pumps. Future systems will likely offer improved performance at lower outdoor temperatures, expanding their applicability in colder climates. Additionally, advancements in variable-speed compressors, inverter technology and refrigerants will further enhance efficiency and reduce environmental impact.”

The future will see more hybrid systems that combine air-to-water heat pumps with other renewable energy sources, such as solar thermal or photovoltaic systems, Decato predicts.

“This will create highly efficient, low-carbon solutions for heating, cooling, and hot water production,” he says. “The integration of energy storage solutions, like thermal batteries, could further enhance these systems, making them more resilient and flexible in response to grid demands. We can also expect more stringent regulations around energy efficiency and emissions, which will drive further innovation and adoption of heat pump technology. Governments and municipalities may introduce new building codes and standards that mandate or favor the use of heat pumps in residential and commercial construction. Retrofitting older buildings with energy-efficient heating and cooling systems will become a major focus, particularly in regions with aging infrastructure. Air-to-water heat pumps will be a key technology in these retrofits, offering a way to upgrade existing systems while meeting modern efficiency standards.”