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Old 03-25-19, 10:56 AM   #10
MN Renovator
Less usage=Cheaper bills
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I agree that it is primarily from keeping your indoor air conditioning(evaporator) coil from getting plugged up. It's an expensive PITA to pay someone to clean the evap coil, to the point where you should avoid ever needing to have it cleaned. Fiberglass filters just don't do the job, getting a cheap pleated(fanfold) type filter is a decent choice. They last a decent amount of time and clean out the bigger stuff. I get the 2 for $3 filters when they are on sale at Menards.

I've got a 34 year old HVAC system in my house, 80% efficient natural draft furnace with an 8.5 SEER AC that I've upgraded to around 9.5 SEER. It could crap out at any moment and buying a $40 filter doesn't seem to be a good idea at this point but when it comes time to replace the system, it seems that contractors automatically swap out 1" filters like the 16x20" in my 800 CFM system with much larger filters 16x25x4".

With the newer, larger filters, I'd probably swap it out once a year right at the start of the cooling season. Cooling CFM is key for preventing the indoor coil from freezing and to keep the coil thermal exchange at the maximum for efficiency. For natural gas heat with the new 90%+ systems, it doesn't matter as much but it is also much easier to measure temp rise at the start of the heating season and check it once a month to make sure it isn't creeping up over the winter. If it starts to, swap out the filter, but it likely won't unless you've got pets or have some active cause of extra dust that needs to be filtered out. For 1 inch pleated filters, I swap mine out at the start of July(most expensive cooling month), second month of the heating season(November), and January(most expensive heating month). If the temp rise has had zero change at the end of the heating season, the filter stays there until July because the AC doesn't turn on until June and usually doesn't run much that month. Once I've had a filter in place during the heating season for more than a month, I check temp rise every week or two and once the rise is 5 degrees higher than the default 70 degree rise, I swap the filter.

So amortizing a $40 filter versus tossing a cheap pleated one, eh. I can't justify it. Just like how I can't financially justify replacing a good working HVAC system for $8000 to save a maximum of $50 a year in heating costs and maybe(but probably not) $100 a year in cooling costs.
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