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For many homeowners, one of the most satisfying aspects of a renovation is replacing the old heating system. It may not deliver the eye-candy appeal of new kitchen cabinets, but the payoff can be important in a subtler way with lower utility bills and increased indoor comfort. And as builders and remodelers look for equipment that meets the growing demand for energy- and space-efficient homes, combi boilers can be a multifunctional option that meets both goals.

Combi boilers have been used in the United States for the past 15 years — and for over 25 years in Europe — and offer the benefits of energy and space savings. The propane- or natural gas-fueled technology combines an on-demand condensing water heater with a condensing boiler to supply both space heating and domestic hot water through one unit. Because they are compact and lightweight, combi boilers can be hung on a wall, and multiple venting options allow them to be installed in an interior closet or on an exterior wall.

David Lohnes, president of Chambers Plumbing and Heating in Loveland, Colorado, likes their flexibility for a range of applications from new homes to retrofits. “We replaced an old boiler in an early 1900s home here in Loveland,” he says. “It was down in a very small cellar, and getting a standard boiler back in was almost impossible. Space was a problem with both the water heater and the old boiler.” The buyers chose to replace the run-down equipment with a Rinnai combi boiler, which they vented through the crawl space. It supplies domestic hot water and baseboard heating for the whole house.

Lohnes installed a similar propane combi unit in the first-floor closet of a Big Thompson Canyon home when a flood destroyed the furnace in the crawl space. “The owners were able to remove the old water heater and create more living space with the entire system in the closet,” he says. These boilers can be a good fit almost anywhere, he adds. “Between the high-efficiency heat and the combined domestic hot water system, the combi units far outperform conventional systems,” he says.

Randy Kingsley, heating business director at Rinnai, says that the company’s combi boilers are suitable for a variety of projects, from light commercial spaces and condos to single-family homes, and have been installed in homes up to 5,000 square feet. State, local, and industry incentives may be available, especially for energy-efficient combi boilers with an annual fuel utilization efficiency rating of 95 percent or higher.

If there is a limit to where combi boilers work best, it is in the household’s hot water requirements. “Many combi boilers are limited to four gallons per minute (gpm) of hot water, which is suitable for two to two and a half baths,” Kingsley says.

Jim Blake, district manager at Eastern Propane and Oil in Danvers, Massachusetts, agrees. “Normally a combi boiler is a good replacement for an old oil-fired boiler with an internal hot water coil,” he says. But larger homes may prove to be an exception. “A lot of homes today have a crazy number of bathrooms, and hot water requirements can at times be greater than the heating load. That changes the dynamics, because the domestic hot water needs temperatures above the typical condensing level.”

In those larger projects, the most cost-effective solution is to install a separate tankless water heater, Blake says. Heating the domestic hot water separately allows the boiler to operate at a consistently low temperature so it can condense and use energy sparingly.

Bosch’s Greenstar Series combi boilers offer direct hot water flow rates from 2.7 to 4 gpm and are ideally suited for one to two bathroom homes, says Craig Lazinsky, marketing program manager. “The Greenstar floor-standing unit (FS) has many innovative features, like an integrated manifold that can be used for direct supply and return connections or be field-converted into a low loss header, making it ideal for multi-family applications,” he says. With their high performance and minimal carbon footprint, propane combi boilers offer state-of-the-art solutions across many kinds of installation scenarios.

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Cheryl Weber