Heating Air with Hot Water Is an Efficiency Double Play

By borrowing heat from a tankless water heater to deliver steady, comfortable space heating throughout the home, hydronic furnaces save space and energy.

In homes at the energy-efficient Byers Place by Arcadia Properties in Denver, the second-story furnace needs no vent and uses no heating fuel.

Instead, each home is equipped with a hydronic air handler that borrows hot water from a tankless water heater, moves it through a coil to pick up the heat from the water, and blows it through the ductwork and into the house.

The propane- or gas-fueled tankless water heater supplies unlimited hot water — unlike a traditional tank with a capacity of just 50 or 75 gallons — so the system pumps a constant flow through the coil and delivers steady, comfortable heat to the home without the need for a fuel-fired furnace or an extra vent to the outside. At the same time, the tankless water heater performs its primary job without interruption: heating the water that is piped to showers, sinks, dishwashers and washing machines.

Because the water heater is the source of heat for both the air and the water, hydronic systems are as efficient as the tankless water heaters they run on. That means pairing a hydronic air handler with a high-efficiency condensing tankless water heater can achieve efficiency as high as 96 percent.


Rheem's Integrated Heating & Water Heating System includes a tankless water heater, a hydronic air handler, and a cooling coil.

Byers Place developer Mark Bethel says the two-appliance system is part of his project's sustainable design that features energy-efficient smaller homes.

Still, notes Tracy Young, a product management specialist with Rinnai, which manufactures the community's heating equipment, hydronic furnaces are catching on slowly among single-family home builders. The technology appears more commonly in townhomes, condominiums and apartment buildings, all of which can utilize the compact size of the tankless water heater/hydronic furnace combination.

But Charlotte, N.C., HVAC contractor Scott James predicts more builders will embrace hydronic heating as homebuyers already familiar with the benefits of energy-efficient tankless water heaters learn that those energy- and water-saving devices can also help heat their homes.

After his local newspaper ran an article about a 1,200-square-foot bungalow that James equipped with Rheem's Integrated Heating & Water Heating System, several of his builder clients brought their customers to see it. "It's an easy sell when you sit down and explain to the homeowners how much they have to pay to heat the water while they're sleeping" if they use a traditional water heater with a tank, says James, operations manager at TCS.

Hydronic furnace systems deliver a number of other cost, comfort, and convenience advantages:

  • A tankless water heater is more expensive than a traditional model with a tank, but some manufacturers offer the hydronic air handler for less than a comparable gas-powered condensing furnace.
  • Some homeowners report that their water-warmed heat is more even and less dry than heat radiated via a traditional gas furnace.
  • The system can be installed in a new building or added to an existing home.
  • Homeowners can control the air temperature by setting a thermostat, just as they would with a traditional furnace.
  • The dual technology has prompted some plumbers to get into the heating business and some mechanical contractors to add tankless water heater installation to their line of services, notes Sal Brunetto, Rheem's corporate manager for national accounts. "This crossover product certainly fills a need" during a sluggish economy when jobs are harder to come by, he says.
  • Hydronic systems create no waste water. They do so by taking the water used to heat the air and recirculating it back to the tankless water heater.

If it's your first time working with a hydronic system, take note of two common installation concerns:

  • Although some hydronic furnaces are compatible with traditional water heaters or with any brand of tankless water heater, some manufacturers have designed their systems to require both pieces from the same maker.
  • The hydronic system is for heat only, but an air-conditioning system can be incorporated if needed.

Choosing the right heating system for your homes can translate to money saved for your project and for the home's current or future owners. It can also help reduce the amount of harmful carbon emissions released into the atmosphere. To learn more about your options for home heating and compare systems for your client, check out A Comparative Analysis of Residential Heating Systems at the Propane Training Academy. The course is based on a study performed by independent research firm Newport Partners that evaluated and compared 15 heating systems in both new and existing homes in 20 locations across the United States. Download the fact sheet at Buildwithpropane.com.


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