ROCKFORD, Ill. (WTVO) — The colder temperature have furnaces working overtime to keep homes warm, and some residents in the “Forest City” are seeing a spike in their bills as a result.

Experts said that Northern Illinois Natural Gas customers are currently seeing higher rates in their bills. One resident said that their bills more than doubled in a month.

“Last month it was $155.19, this month it’s $217,” said Rockford resident Kerry Herring.

Herring said that his bill was $49 in November, but went up by $100 in December.

“My thermostat stays on 70, it’s been on 70 since November,” Herring said.

Some worry that gas bills will keep skyrocketing with the colder temperatures.

“What we’re seeing in Northern Illinois right now is kind of a perfect storm of factors that are resulting in very high gas bills,” said Sarah Moskowitz, deputy director of the Citizens Utility Board. “With these temperatures like this, I’m sure people are going to see higher bills as a result of higher usage, just to keep their living space comfortable.”

Moskowitz said that Nicor Gas’ delivery fees are high right now.

“All of these things are coming together right now and resulting in very high bills and pretty terrible situations for consumers that have to find themselves choosing between things like heat and food and heat and medication,” Moskowitz said.

However, there are ways that residents can keep their houses warm while saving money on their next bill.

“There are lots of things that folks can do around their homes to seal them up and make sure they’re not wasting energy,” Moskowitz said. “A couple of degrees with your thermostat can make a big difference. Use the sun to your advantage.”

It is also important to make sure and clean or change the HVAC filter, according to Moskowitz.

“It’s really important to look at the different parts of your bill to see what’s gone up,” Moskowitz said.

More tips on how to save money, as well as useful tools to help pays bills, can be found on the Citizens Utility Board’s website.