I use DMI LLC because they are a local company, and they do a good job at keeping my furnace working. I have them come once a year. They are quick to respond when I call them and always very thorough. They usually send the same person, but the entire staff is well-trained. Similar to when I get a servicing on my car, I don't know when something is wrong, so DMI keeps me informed when things come up in order to extend the life of my furnace.