Furnace filters get replaced at different intervals based on style, here are some general guidelines
Standard 1" Throwaway filter- Every 30 days of fan runtime, or if it’s dirty before then.
Media Filter-(4", 5", or 6,) All Brands- Most Manufacturers recommend filter media replacement every 6 months to a year, however it is best that you visually check the filter every 3 months to be sure it is not plugged, use your overall best judgment on whether or not it will last until next check Electronic Air Filters- Models- Most Manufacturers suggest cleaning intervals of every 6 months to a year. As with the media, it is best to visually check the furnace filter every 3 months to be sure it is not plugged.
Most filters can be washed with a mild dish detergent in warm water, be gentle, the wires can break causing an unwanted repair. Be Sure to let the filters DRY THOURGHLY before you reinstall the filter. If operation of your equipment without a filter is a concern, a standard 1" throwaway or a media filter can be placed in the EAC cabinet during the minimum 24 Hours that your EAC should dry
The largest is the amount of money that goes up the chimney. Your old furnace is probably running at 60% efficient, meaning that 60% of the energy produced by the fuel actually heats the home, rest goes up the chimney. That is a lot, basically for every dollar spent on heating 40 cents goes up the chimney. New high efficiency furnaces go as high as 96% efficient, 4 cents up the chimney, that's what we like.
As much as we would like to think that physical size matters, in the heating industry that is not always the case. Technology has come a long way, we have learned to make machines smaller and more efficient. Heating equipment is measured by heat output and not size. Often times, 2 of todays 140000 BTU furnaces would envelope the same physical size of 1 75000 BTU furnace of the past