We just built a new home this summer. We have 2x6 walls and plenty of insulation. However, our gas bill for the month of Dec.-Jan was $460.00!! Gas company has checked and there in no leak. We have a Comfortmaker furnace, not sure of model, and last night with temp. outside of -8, the furnace ran 15 minutes until temp. was met on thermostat, but then in less than 5 minutes was back on and running again for 12 to 15 minutes. Any idea why we are unable to maintain temp. for longer period?