The most commonly used heating system in the U.S. residential market is the forced-air furnace (usually gas). In fact, many estimates indicate that more than two-thirds of all American households rely on this type of equipment to meet their space-heating needs.

Given the forced-air furnace’s firm foothold in the U.S., it seems unthinkable that Americans would ever consider any other type of space-heating system. However, several manufacturers are looking to challenge the ubiquitous furnace by offering a different option: the hydro-air system, which they claim is a cleaner, more efficient, and more comfortable alternative to conventional furnaces.


Hydro-air systems, which can provide domestic hot water as well as space heating, usually involve pairing a hydronic air handler with a tankless, gas water heater. The tankless water heater acts as the heat source for air, thus meeting both hot water and heating demands for the home simultaneously, said Shaun Thomas, hybrid applications engineer, Rheem Mfg. Co. “This pairing provides soft, comfortable heat while maintaining comfortable humidity levels, regardless of the climate.”

Vic Waskiewicz, president of JV Mechanical Contractors Inc. in Webster, Massachusetts, has offered hydro-air as an upgrade option to customers for 23 years and usually installs between eight and 10 systems a year. “We started offering them as an upgrade to conventional forced-air furnaces. While hydro-air is typically more expensive up front, it offers homeowners more flexibility to use an oil- or gas-fired boiler as a source and allows a user to incorporate the domestic hot water needs as part of the same system. We are also able to add radiant floor warming in areas of the home to complement the air system, and the equipment offers an exceptional life expectancy.” A hydro-air system can also be extremely efficient, said Thomas, especially when a hydronic air handler is paired with an ultra-efficient tankless water heater.

“Our Prestige® Series condensing tankless water heater operates at 94 percent efficiency, so when used in a hydro-air system, the space-heating portion is about the same efficiency,” Thomas said. “Plus, the system consumes very little electricity.”

Hydro-air systems also reduce the total gas load, because just one gas-fired appliance is required for both domestic hot water and space heating, said Brian Fenske, specialty channel sales manager, Navien Inc. “Eliminating one appliance not only offers potential cost savings in equipment but may also eliminate secondary installation requirements, including, but not limited to, combustion venting and gas piping.”

Having a small footprint is another benefit, as a hydro-air system takes up very little space, noted Freddie Molina, product manager, Bosch Thermotechnology Corp. “Additionally, our new hydronic air handler’s integrated control board has the logic to time the start and stop of the blower and pump to provide optimum comfort and efficiency and to help prevent bacteria growth in the hot water system. For homeowners, it’s an all-around comfortable and clean HVAC option.”

Hydro-air systems can be used in just about any geographic area, with two important caveats, said Arthur Smith, product manager, specialty residential, A.O. Smith Corp. “In Northern climates, the incoming ground water is colder, so a larger tankless water heater may be required to meet both domestic hot water and space heating needs. And, in hard water areas, tankless water heaters are more susceptible to scale formation than tank-style water heaters and that can lead to premature failure. In areas with hard water, an anti-scale product, like our Product Preservers, should be used or else the tankless heater should be regularly flushed.”


Installing a hydro-air system is fairly straightforward, noted Smith, but contractors should take into account several considerations. “First, there has to be a sufficient gas supply; second, an electrical supply is necessary because electricity is required to power the controls and spark ignition of the condensing tankless water heater; and third, a drain is needed because high-efficiency tankless water heaters, like our Ultra-Low NOx condensing model, produce condensate, which must be drained. If a floor drain is not available, a condensate pump can be used.”

Until recently, it was much more difficult to install a tankless water heater in a retrofit situation, because a home’s gas line had to be upgraded to ¾-inch, due to the volume of fuel the water heater required. This made replacements challenging for contractors and costlier to homeowners, said Thomas.

“However, the National Fuel Gas Code (NFPA54. ANSI. Z223.1) now confirms that a 200,000-Btu gas appliance can be installed on a ½-inch gas line up to 40 feet in length,” Thomas said. “This shift makes tankless water heaters — and, in turn, hydro-air systems — viable replacement options.”

And because hydro-air systems can replace a gas furnace one-for-one, installers do not have to retrofit the ducting of the house, said Molina. “When properly installed, the hydro-air system communicates with the household thermostat the same way a traditional furnace would. However, contractors do need to know how to properly configure the system in order for it to deliver the desired results. Factors that need to be considered are flow rate, Btuh, pump speed, and the water temperature in the water heater.”

While hydro-air systems are a great choice for some homeowners, they are not always ideal for larger square foot homes with bigger heating loads, said Fenske.

“Hydro-air systems will continue to meet the needs of smaller and tighter homes,” he said. “They have also grown in popularity for use in additions and converted living spaces as well as master bedroom add-ons, finished basements, garages, and bonus rooms. The tankless water heater’s lower heating Btu combined with the ever-important space-saving opportunity make the hydro-air system the perfect choice for many applications.”

Publication date: 11/28/2016

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