CHICAGO (CBS) — A lot more people have been complaining this winter about what it costs to heat their homes.

The Citizens Utility Board said it has seen a 65% increase in complaints and questions this winter about unregulated natural gas suppliers. Some of those companies are charging 40% to 116% higher rates than People’s Gas, according to the utility watchdog group.

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“A lot of times, people don’t even know they’re with an alternative gas supplier until their bill goes through the roof. We’ve talked to people who had no idea they were with an alternative supplier, and suddenly they’ve overpaid by hundreds of dollars,” said CUB spokesman Jim Chilsen.

Chilsen said you should be careful not to give out your gas bill account information, especially your account number, to alternative gas supplier sales people. Some of them have been known, in the past, to take account numbers and switch the consumer’s natural gas supplier without the consumer knowing.

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Rates for unregulated natural gas suppliers tend to be higher now than the rates charged by Peoples Gas and Nicor, according to Chilsen.

“Somebody comes to your door, saying they’re from the gas company and they want to lower your bill. Be very, very careful. We would recommend not giving out your gas bill or your account number to just anybody,” Chilsen said.

CUB also cautioned people to be wary of low introductory offers by unregulated companies. Chilsen said some people have found, after a couple of months of low bills, bills from alternative suppliers go “through the roof.”

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Chilsen said you can find more information at