As of 2008, our system had not been serviced probably since 1992 when the house was built. The previous owner wasn't into maintenance of equipment. I figured servicing our furnace and a/c would cost an arm and a leg but it didn't. We signed up for the year contract when they worked on the furnace and they came out quickly when we called to work on the a/c in the spring. There was a lot to be done on both items but they took their time and we've had lower bills since. They do quality work and are responsive and friendly. I doubt that they are the cheapest but you get what you pay for.
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