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Water Heaters With Space Heating Capability

A single unit can heat water for domestic washing and provide a source for the home's conditioned air heating

There are many types of commercially available systems that combine water and space heating functions. These systems use one primary heat source shared between conventional pieces of equipment, such as, an air handler and a tank water heater, to serve dual heating purposes within the home.

One system consists of a high output water heating tank connected to a separate air handler by a coiled pipe loop. The pipe coil, through a heat exchanger, provides auxiliary heat to the air handler of a heat pump system. Heated air is then forced out by a blower and circulated through the house via ductwork. At the same time, heated water is available in a traditional storage tank for domestic washing uses.

Another system consists of a boiler that provides heated water to both the radiant heating loop(s) and a hot water storage tank. Instead of a boiler, a tankless water heater can provide hot water as demanded, and service both domestic hot water demand and a hydronic heat loop. A tankless unit would generally be placed near the delivery target such as the bathroom or kitchen, and provide space and water heating on a localized scale, whereas a boiler can be sized to supply the whole house.

Energy Efficiency

The energy efficiency of the boiler or tank hot water storage system is derived from the dual use, minimizing the standing water losses and typically offsetting electric usage with natural gas. The Department of Energy estimates that systems utilizing one heat source for both air and water can reduce domestic energy consumption by 27%.


Water heaters with space heating capacity require sophisticated controls, so installation requires specialized skills with each particular system. These systems are appropriate for both new construction and retrofit or remodeling, however, the location of the equipment in proximity to each other is a factor of the system’s efficiency.

Some systems, such as an electric forced hot air handler serviced by a hydronic heat coil from the water heater, may be cost neutral when specified at the outset. Typically, however, the cost of heating space with water heated for domestic use will require a higher output, quick recovery unit that might not otherwise have been selected.

Applying the U.S. Department of Energy’s projected savings of 27% to an average annual home energy cost of $3,159, of which 54% covers heating, indicates that average monthly cost of heating or cooling air and heating water runs around $104 compared to $142.

Most systems are UL (Underwriter's Laboratories) and AGA (American Gas Association) certified. However, check with the local building permit department before installation. Since the water in the coil mixes with the domestic hot water, some inspectors might request a pump timer because the water could become stagnant.

NextGen Final Report. Steven Winter Associates. 6/13/2000

The modular or integral systems are usually designed for new construction, and instead of having a separate water heater and furnace, only one unit is installed. There are some retrofit units available that can use an existing water heater, but check with the specifications and consult the supplier. Supply and return runs of copper pipe are required for the pump attachment to the heat distribution line. Recently, plastic piping has been approved for use in some areas, as an alternative to the more expensive copper pipes. The air distribution system is conventional galvanized ductwork, and should be sealed to prevent leakage of air or allow dirt/dust to enter the system.

The tankless water heater system is designed for installation near the water source. Pipes are then run from this unit to either baseboard heaters or to a radiant floor system. For the average 2,000 sq. ft. home, more than one unit may be required.

Installation can be accomplished by a licensed HVAC trade contractor working in partnership with a licensed plumber if a water heater is part of the system. Systems that employ boilers can be installed by either the plumbing or the HVAC contractor. Units are installed and delivery systems are roughed-in in sequences similar to those of conventional systems, so system choice should not affect critical path schedule.

Equipment warranties are the same as comparable equipment i.e., the air handler with a hydronic coil has the same 5-10 year warranty as a forced hot air version. Tank water heaters will carry warranties ranging from 5-12 years, also varying by manufacturer. Some tankless water heaters carry longer warranties (see PATH Technology Write-up.) Boilers, on the other hand, tend to carry warranties of 40-50 years.

The combination unit costs more than a typical system with separate water heater and furnace, but installation costs are reduced due to the elimination of the furnace. Some state and local jurisdictions offer rebates and tax credit programs that could help offset these higher costs. Also, fewer moving parts allow easier maintenance and less service. Since only one source of heat is required, multiple utility hook-ups are not required. If the tankless water heater is used, only a small space near the sink is required, thus eliminating an entire area dedicated as a utility room. Some systems can be more efficient than the traditional methods, thus resulting in lower utility costs. Some super efficient water heaters can produce 125 gallons of hot water per hour.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.