WARWICK — National Grid is asking permission to increase natural gas rates to cover a spike in the cost of the fuel during the cold snap in late December and early this month.The company filed a request with the state Public Utilities Commission on Monday that would raise the heating bill for a typical household by 15.6 percent from March 1 to Oct. 31, which averages out to an additional $11.87 per month, or $94.84 in total over the eight-month period. If approved, the filing [...]