Condensing Water Heaters

By Jessica Rivas, E Source Research Analyst -- Foodservice Equipment and Supplies, 1/1/2008

Hot water consumes a significant amount of energy in a commercial foodservice operation. In a typical restaurant, water heating can represent 20 percent of the total annual gas consumption, making this system a good target for energy savings. Although operators can choose from many types of efficient water heaters, there have been few studies illuminating the energy-savings potential of these units in the foodservice industry. Until now, that is.

Researchers from Pacific Gas and Electric’s Food Service Technology Center (FSTC) conducted a study to compare the energy use between a gas-fired standard efficiency tank-type water heater and a high-efficiency, or condensing, tank-type water heater in a restaurant setting.

Condensing tank-type water heaters, which have been commercially available in the United States for many years, represent the highest efficiency tank-type units available. Their elaborate heat exchangers extract more heat from combustion gases, which allows them to achieve typical thermal efficiencies of 95 percent compared to 80 percent for standard efficiency units.  Both types of water heaters burn fuel in a combustion chamber, and the resulting combustion gases pass through a heat exchanger, which transfers energy from the gases to water. Much of the operating loss in conventional units comes in the form of heat carried out of the unit with the flue gases.

Condensing water heaters recover much of this energy by using a larger heat exchanger or a second heat exchanger. In a conventional water heater, combustion gases rise up a straight flue through the center of the tank. In a condensing water heater, the flue spirals around inside the tank so that the combustion gases spend more time releasing heat until the water vapor in the gas — a natural by-product of the combustion process — condenses, releasing even more energy. Where the flue exits the tank it is barely warm to the touch. Additionally, condensing water heaters have powerful burners that are well-suited for large-capacity applications.

The FSTC researchers conducted their study at a casual-dining restaurant in California, which had an average daily hot water use of 2,100 gallons. Originally, researchers had intended to directly compare a 100-gallon 300,000-Btu-per-hour (Btu/h) 92-percent-efficient condensing water heater with a standard 100-gallon 400,000-But/h 80-percent-efficient water heater by installing stand-alone units. However, concern that the condensing water heater’s heating capacity was lower than the water-heating capacity of the current standard efficiency system impacted the installation of the units. Installed upstream, the condensing water heater handled the majority of the water-heating load; and during instances of high water flow rates, the standard efficiency unit handled the rest.

Researchers found that the condensing water heater operated at an efficiency rate of 8 percent to 15 percent higher than the standard water heater. Overall system efficiency was calculated as the total energy transferred to the heated water divided by the gas consumed by the heater. Based on an average natural gas price of $1.20 per therm, this is equal to annual gas savings of 10 percent to 20 percent, or cost savings of $850-$1,700 annually. What’s more, condensing water heaters have external ventilation piping made of polyvinyl chloride (PVC), and the vent can travel a long way, which makes the heaters easier to install than conventional units. In fact, the participating restaurant found that the reduced installation cost alone offset the incremental cost of purchasing a higher efficiency unit. However, even with an incremental purchase cost as high as $2,000, the payback for a condensing water heater is only two-to-three years.

The FSTC researchers were also able to trend water heater efficiency. Researchers found that overall system efficiency increased with increasing daily water usage (gal./day). This data suggests that over-sizing the capacity of a water heater system can result in lower system efficiencies and reduced energy performance.

For restaurateurs that cannot justify the upgrade to higher efficiency condensing water heaters, the FSTC researchers found that significant savings could be achieved with standard efficiency water heaters by incorporating a few best practices. These include adding insulation to the hot water line, using automatic flue dampers, and installing a timer on the hot water circulation pump. Researchers found that these simple upgrades were able to save $952 annually.

The complete report can be downloaded from here.