A month ago we replaced both our furnace (gas) and AC. The old furnace was from 1988 so although the new one is only 80% efficient, we expected to use less gas. However, when I received my last gas bill that covered a period approximately 3/4 of which was covered by the new furnace and 1/4 by the old one I found that we used significantly more. Avg use for the same period of last year was about 6.1 Therms/day while this time around was almost 7. Our lifestyle was no different than last year. According to the gas company, the avg temperature last year was actually 1 deg lower than this year. We called the company that installed the new furnace and asked them to come over and check whether there are any leaks caused by the install and whether the furnace if functioning properly. They told us that they could come but if they don't discover anything wrong with the new unit, we'll have to pay for their visit. I am afraid this almost automatically means that they will find no problems. So first I want to try to check as much as I can myself. What should I be looking at? The unit is Trane XV80 2-stage var speed model TUD2C080A9V4 We also installed a Honeywell Air Cleaner that has a 4" filter. I replaced the original filter the company installed with one that is MERV 13 rated since we have allergy sufferers and pets at home. Any advice will be greatly appreciated!