Furnance Filters Question

My furnace uses a single 20x20x4 inch filter. All the local places (Home Depot, Lowes, etc.) save one, do not stock this size filter (nor can/will they special order it for me). The single source where I can find the filter charges an arm and a leg. I’m thinking of ordering a case from an online source because I can buy a case lot (six filters) including shipping cheaper than buying the equivalent from the local single source. And that single source normally carries only one or two of this size all year long.

Can I replace my fat furnace filter temporarily with one or two 20x20x1 filters? Can I get away with doing this permanently (a single 20x20x1 filter is roughly 10 percent the cost of a single 20x20x4 filter from the single local source)?

You shouldn’t replace the 4" with a 1". You have less filtration. Using the old 4" until you get a new one is probably better.

Actually, you will have the same filtration, potentially.

If you use a 1 inch filter that has the same MERV rating it will in fact be the same. A 4 inch filter however, will give you much more filter surface.

This will produce longer filter life as there is more filter media. The 4 inch filter may be 10 times the cost, but will last several times longer than the 1 inch filter. So the true cost differential is not 10 to 1 if the 4 inch filter lasts 5, or 7 or 8 times longer.

The biggest benefit, however, is that the 4 inch filter has a lower pressure drop accross it. That means better performance from both your A/C system and furnace, particularly the A/C system.

You may be able to buy the filter off the net much cheaper than from a contractor. It’s up to you, but the 4 inch filter is superior to the one inch.

ETA: you can do this both temporarily or permanently. How much did they want to charge you for the 4 inch?

Actually the manufacturers recommend changing the filters every three to 12 months. On average they say every six months.

Locally they want $30.00. I can find them as low as $15.00 in case lots of six. Shipping is another $30.00

Is saving $10 per filter worth laying out $120 dollars now and stock piling them for three years?

You might want to check out some of your local HVAC repair/install places. They may be willing to bring a case in for you at a lesser price, and if they get things from that company on a remotely regular interval, you might not have to pay for shipping if they can tack in on with one of their orders.

I have customers who get 2, 3 or more years out of a 4 inch filter, although I agree with the recommendation. Every home is different. The factors that will determine how often you have to change your filter are:

  1. Do you live on a busy street? Urban, suburban or rural?
  2. Kids?
  3. Pets?
  4. Smokers?
  5. Age of house?

Once or twice a year is indeed the average. If you change it once a year change it April 1.

A very quiet suburban dead-end street (we are practically at the end of a half-mile dead-end). No kids but cats. Smokers not allowed inside at all. (In fact, I do not allow smoking on the property.) The house is less than ten years old.

Unless you have 8 cats, I’d say you will get one full year easily. There’s a decent chance you’ll get more than that—maybe 2 years.

But unless you’re strapped for cash, why chance it? Thirty dollars a year is cheap insurance on your HVAC equipment, not to mention the overall indoor air quality consideration.

FYI: Just because a filter discolors doesn’t mean it’s ready to be changed. (I see that all the time) It’s doing it’s job. It needs to reasonably ‘pitted’ or clogged before it is spent. I advise my customers to put the date on the filter with a Sharpie so they’ll remember how long it’s been in. (and you’ll get a feel how often your particular filter needs to changed)

My .02: You’ve got a great filter that will protect your equipment and your health. Change it once a year on April 1.