Massachusetts Attorney General Martha Coakley on Friday criticized the Department of Public Utilities’s decision to let Columbia Gas of Massachusetts to increase its rates by $7.8 million.

Columbia, also known as Bay State Gas, originally requested at $29.2 million rate increase. Coakley’s office objected to the request, saying rates should instead be reduced by $28 million.

“We argued from day one that Columbia Gas’ unreasonable request never passed the ‘common sense test.’ Consumers should not have to foot the bill for the company’s failed promises and excessive costs,’’ Coakley said in a statement.

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She said the company received a significant rate increase in 2009 and failed to control costs.

The DPU defended its decision, saying that the effect of the ruling and another rate decision affecting Bay State will save heating customers in Springfield, Brockton, and Lawrence about $19 a month this winter.