I have an air conditioner/furnace combination that is many years old (the house was built in 1974 and could be an original unit, but I don't know). Recently the furnace has exploaded on ignition so I turned it off--thankful I live in the Southwest. I considered replacing it because the cooling isn't especially efficient, but was quoted a $7,000 price from Carrier. I may move in 3 years when my kid graduates. Any recommendations on keeping the cost down or is the risk too great and it should be replaced?