A furnace tune-up ranks right up there in the list of "Things Minnesotans don't want to think about in September."

But there's a good reason for scheduling it this month rather than waiting until November. A handful of Twin Cities heating contractors are known to "red tag" or disable a furnace because they claim carbon monoxide is being released through a cracked heat exchanger. If a person's furnace is shut down on a cold day, they have to scramble to get a second or third opinion and use space heaters to keep pipes from freezing. Unscrupulous heating and air conditioning contractors know that many consumers will just give in to what seems like an emergency and have the heat exchanger or furnace replaced at a cost of $1.500 to $7,000.

Rick Welter of Ray Welter Heating in Minneapolis said if a heat exchanger crack is discovered, it does not necessarily mean that carbon monoxide is leaking into the house, "If no carbon monoxide is leaking into the home, you have more time to find a contractor for a second opinion," he said.

When calling around, tell the HVAC receptionist at the outset that a second opinion is needed on a red tagged furnace. That usually prioritizes the call. Most companies charge for the second opinion, but companies such as Ray Welter Heating in Minneapolis and Schwantes Heating and Air Conditioning in Stillwater do not.

Furnace tune-ups should generally be done annually or every other year. Older furnaces usually require more attention to run as efficiently and safely as possible.

Consumers looking for a reputable heating and air conditioning company can find advice and local ratings at the site for Twin Cities Consumers' Checkbook, a nonprofit consumer organization. The article and ratings are available free until Oct. 8 at Checkbook.org.startribune/HVAC.

Contractors that advertise a low price of $0 to $69 for a furnace tune-up may be low-balling the customer so they can come into the home and claim the furnace needs expensive repairs or replacement. "Anything under $79 and they're losing money," said Welter.

If a consumer believes that a furnace has been red-tagged needlessly, contact the Better Business Bureau to register a complaint at 651-699-1111 or www.thefirstbbb.org.