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I have an 80% efficient gas furnace at my house, that is seemingly working well and is configured with an air conditioner. What preventative maintenance should be done to it, and at what interval? Is replacing the air filter all that I need to do until I notice trouble?

Please provide details in general, so the answer can be be applicable to the most people.

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Yes, replacing the filter is basically the only maintenance that is necessary for natural gas and propane furnaces. You probably don't need to replace the filter more than once a season. There exists a "filter alarm whistle" that you can install. It whistles when your filter needs replacement. You should make sure that the filter is properly installed, so that air cannot bypass it.

Oil furnaces and boilers, however, do require regular maintenance.

Optionally, you can have the blower and heat exchanger cleaned, but it won't make much difference in terms of performance.

Despite the claims of duct cleaning companies, cleaning your ducts will not make much of a difference in terms of energy performance or indoor air quality (reference). However, if you have an air conditioner, cleaning the indoor evaporator coil can be a good idea.

With a forced-air system, the most important thing is to make sure the ducts are sealed and well insulated if they are located in an unconditioned attic, crawlspace, or basement.

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Thanks. The manual of my furnace actually had some long detailed instructions on how to do the heat exchanger cleaning, adjusting the gas pressures, etc.... But, it sounds like with modern furnaces I shouldn't need to do this unless there is trouble. I have a 5" thick filter, and I replace it yearly (per the manufacturer's instructions). The ductwork is in the semi-conditioned (really badly balanced) basement, so I'm not worrying about the ducts.... – Pigrew Sep 17 '13 at 1:17

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