If the powerful heating energy provided by a tankless water heater is ideal for creating domestic hot water, why can’t it be used for heating a home as well? It’s a question that manufacturers and pros in the heating and plumbing world are investigating more closely as the popularity of tankless water heaters continues to bloom.

Top-selling tankless brand Rinnai added velocity to the trend this year when it introduced a dedicated hydronic air handler that uses the manufacturer’s tankless water heaters and boilers as a heat source. The units can meet heating capacities ranging from 17,000 to 77,000 Btus, allowing them to serve as effective alternatives to propane or gas furnaces or electric backup heat, says Rahul Goyal, product director for the HVAC category at Rinnai.

Compared with a gas furnace, tankless hydronic heating requires only one gas appliance, meaning fewer vents and gas connections to install, he says. It also allows a home to leverage the thermal efficiency of a tankless water heater, which can achieve efficiency levels around 94 percent. And while homeowners typically love the hot air temperature provided by a furnace, a hydronic air handler provides a less-extreme heat to help avoid stratification, in which the air close to a duct is much warmer than space farther away.

Armistead Mauck, an owner and vice president of propane provider Cherry Energy in Kinston, North Carolina, installed one of the Rinnai air handlers in the company’s office as a pilot project. One side of the building is heated by the hydronic heat, while the other uses a heat pump. “At 8 a.m. on any day that was below 35 degrees, when you walk in the office, everybody wants to go where the hydronic heat is because it’s noticeably different,” he says. “Hydronic heat is probably the finest heat for colder climates because it’s not dry.”

With a hydronic air handler, a tankless water heater can perform two jobs, providing heat and domestic hot water to a home.

Hydronic forced-air heating can also address two common concerns with electric backup heat, Goyal says: the smell and the cost. The burning smell associated with auxiliary heat is typically caused by dust or lint burning off on the electric coil, a problem easily mitigated by hydronic air handlers, which aren’t exposed to the same high temperatures. And while electric backup heat can send electric bills soaring during cold weather, hot water from a propane tankless water heater is typically much more affordable.

Simplified hydronic installations

Using a tankless water heater as a hydronic heat source used to be a bit of a science experiment. Because air handlers weren’t tested with the Rinnai product, it could be difficult to get the installation just right, Goyal says. “Given the pressure drop, or not enough flow, or not setting the right temperature values, you would not get the desired heat output from the air handlers.”

Rinnai’s new air handler is tailored and tested to pair with the company’s condensing or noncondensing tankless water heaters, and it’s also available in a no-pump model to pair with Rinnai’s I-Series condensing boiler. Now, based on testing between the water heater and air handler, Rinnai can predict the heating output of a given installation scenario and recommend a water heater or boiler sized for your Manual J heating load as well as domestic hot-water load.

Turning hydronic water heating from a science experiment into a simple plug-and-play experience has been one of the goals through the product development and pilot process, Mauck says. “Nobody has a solution out there that’s got one-button simplicity,” he says. “That’s been one of our mottoes through this project.”

Installers can adjust the heat output of the unit by changing the blower speed, hot-water temperature, or flow rate, Goyal says. “There are various different combinations that are levers that you can pull to get the right amount of Btus for space heating.” Rinnai’s tankless water heaters and boilers run on propane or natural gas, he adds, “so they are ideal for applications that are using propane as a source of energy.”

Endless hot water and space-heating performance

In the past, installers who wanted to use Rinnai’s tankless water heater in a hydronic forced-air system could use a domestic priority switch (DPS), which allowed for simultaneous heating and hot-water generation. The DPS is still an active product and can be used with the new air handlers.

“The idea would be that you want to focus the energy towards the demand for domestic hot water and if it’s above a certain threshold pause centralized heating,” Goyal says. “Because we look at the domestic hot-water requirements and demand to be real-time. So we can provide that domestic hot water at the expense of central heating.”

Boilers offer a second layer of functionality. Tankless water heaters are connected to the air handler in an open-loop system, meaning the same hot water is used to heat the space and provide domestic hot water. A stagnation feature in the air handlers allows it to cycle every six hours to get rid of any stagnation.

But certain jurisdictions don’t allow an open loop for heating, which is where boilers come in. Boilers have separate loops for domestic hot water and centralized heating, allowing you to use glycol for the heating loop. And because the temperature of each circuit is set separately, the systems generally don’t require a mixing valve to bring down the temperature of the hot water.

Since the air handlers are compact and available in outputs as low as 17,000 Btus, Goyal sees them as an ideal solution for closet-style applications such as multifamily or townhome settings. They’re also a potential option for the increasingly common super-efficient homes where the heating load is low except on the coldest days. The air handlers currently operate on constant-speed PSE motors, but Rinnai is working on a variable-speed ECM solution that would drive efficiencies even higher.

Hydronic forced-air heating isn’t a new technology, but it’s one that could be entering its prime, given the building industry’s increasing focus on comfort and the evolution of both tankless water heaters and combi boilers. The product’s comfort promise is in line with Rinnai’s brand promise — creating a healthier way of living — a promise that’s increasingly relevant amid pandemic conditions. “Everybody’s focused toward health,” Goyal says. “And I think this is a product that is also focused towards the health of the consumer: making their lives healthier.”