MANKATO — Kathy Brynaert cautions that patience is required for anybody thinking about installing a geothermal heating and cooling system.

Brynaert was talking about the large well-drilling rigs that will be operating in the yard for hours on end, the ripped up lawn, the installation of the related heat pump and other equipment. But she could have been talking about the financial payback of geothermal, which can take a decade or so of lower heating bills to cover the up-front cost of the system.

And she could have been talking about the patience she and her husband, Tony Filipovitch, practiced in the decades since they first bought their home and the time when they finally took the plunge.

“Broadly speaking, we’ve been environmentalists all of our life,” said Brynaert, a state lawmaker.

A couple that had been recycling since the 1970s, they bought their 130-year-old home in West Mankato a decade later. They never installed central air conditioning and only used their bedroom window AC units on the hottest of nights.

And they’d been thinking about geothermal for years.

“Geothermal is an amazing use of the earth’s resources, second probably only to solar,” Brynaert said.

“Part of it for us is a commitment to the environment,” said Filipovitch, a professor of urban and regional studies at Minnesota State University. “It’s both a pocketbook issue and a stewardship issue for us.”

They disagree about how fast the payback will be for the system, scheduled to be switched on on Tuesday. Their gas bill this year is averaging $128 a month and is expected to drop to as little as $6 a month after Tuesday. Two weeks of using their window air conditioners has bumped up their electrical bill by $100 in the past.

An unknown part of the equation is how fast natural gas prices will rise in coming years.

Brynaert is more optimistic about the payback period, thinking the savings in energy costs might cover the up-front costs in less than eight years.

“I’ll be happy if it’s 15,” Filipovitch said.

Part of it will be determined by the specifics of how federal stimulus dollars are used to promote renewable energy projects, a rule-making process still being conducted by a state agency. There’s already a federal tax credit that will cover about a third of the $38,000 cost of the system installed at the Brynaert-Filipovitch home on Carney Avenue.

The price tag they’re facing isn’t a typical one. The well-driller, based on where they live, warned that he’d hit rock very early on and that the cost would be high. And they didn’t spare expense when it came to the efficiency of the heat pump and other equipment involved, including geothermal heating of their household water.

The end result is a house that’s going to likely be “the most efficient for heating and cooling in this town,” said Matt Soucek, who coordinates geothermal installations for the Schwickert Company of Mankato. “... It’s an older house, but they’re going to have a Ferrari in the basement.”