I think we need to get our gas furnace (central heating) looked at/cleaned, can anyone give me any tips on what needs to be done and doesn't need to be done? We've been in this house for 4 years and I haven't had it cleaned yet (changed the filters regularly, but didn't know the rest needed cleaning regularly.) What is involved with a furnace cleaning and how much do they cost? I mostly want to be aware of what is wheat and what is chaff before the service people arrive. For example, every garage door repairman I've ever used tries to convince my my rollers are bad, but I've yet to have one go out on any house I've ever lived in (hence rollers = chaff.) Any tips, oh wise ones?