lennox whisper heat

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We have a 9 year old Lennox Whisper Heat furnace. When we put it in our house was 990 sq. feet. We put a 500 sq. ft. addition on and was told the furnace was big enough to handle it. We keep the thermostat around 70 degrees and the old part of the house stays warm enough although the furnace seems to run constantly--runs for 12 minutes then off for 7 minutes and then back on--the new part of the house is about 5 - 10 degrees cooler than old part of house. Could it be that the duct work to the new rooms (2 new rooms added) should have had their own run rather than coming off an existing run? Also, is the furnace running too often? I am worried that the furnace will conk out from overuse? How many years should we be able to get out of the furnace? How much would it cost to replace it? My dad's company installed this one, but he has since retired and I have no idea how much I would have paid? Thanks