The Case for Combisystems

Homeowners in Sun Prairie, Wisc., reap the benefits of combing solar and propane energy for better performance and lower operating costs.

Sun Prairie, Wisc., may not be the epicenter of green building, but an existing, 2,000-square-foot farmhouse in this town just northeast of Madison provides one of the best examples of combining efficient energy sources to reduce reliance on fossil fuels and lower on-going costs for homeowners.

Cardinal Heating & Air Conditioning, a local specialty contractor, devised a clever solution to help make the existing house and its propane-fueled space and water heating equipment deliver even better performance and efficiencies. "The owners already had a 95 percent efficient propane furnace, so we keyed off of that," says Keith Ouimette, the solar division manager at Cardinal.

Specifically, the Cardinal team ground-mounted an array of six, 4x10-foot solar thermal collector panels on the site and plumbed them—using a buried, rolled copper pipe, insulated and sleeved in a perforated drain tile—to a new, 119-gallon solar storage tank in the home's basement 250 feet away.

Ouimette chose to mount the collectors at ground level on a tilted racking system rather than on the roof to optimize their performance. "The orientation of the roof wasn't ideal, and it was shaded," he says. The owner also had plenty of land and the panels—despite being in a position to collect the most solar energy—are hidden from view by a small grove of trees. The tilt of the collectors also sheds water and snow; the rack is slightly raised to keep snow from blocking the panels.

Depending on how much solar energy the array collects (usually more than enough on a sunny day), the cold water in the tank can be preheated or fully heated for delivery to the home's existing propane-fueled water heater. If the water dips below the set temperature of the water heater's thermostat, it is supplemented by propane energy to make up the difference.

The system also circulates water from a solar storage tank through a copper coil heat exchanger located in the return duct of the propane furnace. Like the water heater, the propane-fueled burner on the furnace automatically ignites should the solar system fall short of the desired indoor temperature setting. The same combination system also heats the swimming pool.

By leveraging the sun first and having an efficient, affordable, and reliable backup energy source (propane) at the ready, the owners of the Sun Prairie home have realized a $700 per year savings on the energy bills—despite record-breaking snowfall and winter temperatures. The equipment installed by Cardinal, including the solar collectors and storage tank, is all commercially available, as is on-site propane service providers.

This combination of technologies and energy sources exemplifies the next generation of affordable and environmentally smart domestic space and water heating. No longer must builders or homeowners have to rely on just one energy source, or invest entirely in newer  renewable technologies that require longer payback periods.

In fact, these systems qualify for federal energy tax credits under the American Recovery and Reinvestment Act of 2009 and often for state and local tax credits and utility rebates, thus making their return on investment even more attractive.

For Cardinal Heating & Air Conditioning, this project represents a new niche of work that is quickly becoming the standard spec. "We've installed at least a half-dozen other combination systems with propane as the auxiliary energy source," says Ouimette.

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