I bought brand new Lennox HE natural gas furnace 3 years back. It has been running perfectly. My dealer recommends to do a preventive maintenance because of 3 years time span. I regularly change the filters (Only 3M brand). I live in Southern Ontario, Canada, where temp goes in negative double digits (around -25 C) in winter. Question is does it require any preventive maintenance and what kind of ?
Of course you can do nothing but change the filters but that will affect the reliability and service life of the unit.
There is more to these things than just a filter. A furnace service usually includes a cleaning of the blower motor, an inspection of the burner and housing for any cracks or leaks (a HUGE safety concern!), inspection of the ignition system, inspection of the thermostat, as well as some other stuff depending on the unit.
Yes, it costs money but so does replacing a furnace prematurely or having the heat go out at the worst possible time and having to pay overtime or holiday rates for an emergency service call.
Changing the filter is a must and good for you for doing it. There are things that must be done to extend the life and efficiency of your furnace. Most important is cleaning the burners and checking the blower motor. Most are direct drive so they don't have belts but they can and do get real dirty. The blower blades have to be cleaned too. They will also check the venting for any signs of blockage. People like to skip this routine maintenance because it costs bucks but it's needed and could effect your warranty down the road if you don't do it.