Selecting the Correct Size for Your Furnace Filter
Has the time come to replace your furnace filter? If so, you’re definitely going to want to make sure that you’ve figured out exactly the right size and style that you need. Otherwise, your furnace won’t be able to properly clean the air to remove airborne contaminants. Using the wrong filter in your furnace can shorten the overall life of your furnace itself, and reduce the energy efficiency of your home heating and cooling system, causing you to spend more than you should on utilities each month.
Furnace filters aren’t exactly a topic that comes up in casual conversation very often, so you’d be forgiven if you haven’t given much thought to the different sizes and styles of furnace filters that exist. However, when the day comes where you need to replace your furnace filter with a new one, it’s very important to make sure that you’ve got the knowledge you need to spend your money on exactly the right filter for your furnace.
Sorting through the hundreds of different brands, sizes, and models of furnace filters that are available can seem like a confusing task. Fortunately, choosing the best replacement furnace filter for your home can be made much easier. That’s why we’ve compiled this handy guide to help you find out precisely what to look for when you’re shopping for replacement furnace filters. We’re going to run through everything you need to know about identifying proper furnace filter sizes in a variety of different ways, and what you should look for when you start shopping. Let’s jump right into it!
Where Can I See What Size Filter I Need for My Furnace?
Perhaps the easiest way to find out what size furnace filter you need to get is to simply examine the existing filter. The vast majority of furnace filters sold in Canada will have the dimensions and sizing details printed clearly on the frame of the filter for easy reference. What you’ll be looking for is a marking on the outer edge of the filter that follows this example format: 16x25x3. This marking is a combination of three dimensions; length in inches, width in inches, and depth in inches. However, you might see two different measurements on the filter that follow this format. This is normal, as one represents the ‘nominal size’, and the other will usually represent the ‘actual size’. What’s the difference? Glad you asked!
Nominal size is essentially a standard size indicator that simplifies the process of selecting the right filter. Using the example we mentioned, any furnace that uses a 16x25x3 furnace filter would be able to accept that same size of filter from different manufacturers. What we mean is that a 16x25x3 filter made by one filter manufacturer will fit the same furnaces as 16x25x3 filters made by other manufacturers. The reason that filters are mainly sold according to nominal size is because the actual size of furnace filters will vary slightly from one filter to another, even if they are made to fit the same furnace.
Actual size is just that; the actual tight dimensions of the filter. These measurements will usually be in fractions of inches, as there needs to be just a little bit of extra space on each side of the filter to accommodate for the different manufacturing tolerances of furnaces that use the same size filters. This allows the filters to fit properly and not be too tight in the filter slot.
Going back to our example of a 16x25x3 size filter, if you were to take a tape measure and examine the filter closely, you’d likely see the actual size coming in at 15 ¾” x 24 ¾” x 2 ¾”. This allows for just enough ‘wiggle room’ for the filter to install properly into the filter slot on the furnace. You want enough space so that it fits without getting stuck or jammed, while also still being tight enough to provide optimum filtration performance by not letting excess air escape around the sides of the filter.
What If I Can’t Find the Size on the Filter?
If the existing furnace filter doesn’t have any clearly visible size markings on it, there are other ways to find out what size you’ll need. The next best place to find the proper filter size information for your furnace is in the owner’s manual. The manual will have a list of parts and components, including the filter, and should clearly indicate what size you need. If you have perhaps lost or misplaced the physical copy of the owner’s manual, you can usually download a digital version from the furnace manufacturers’ website. You’ll need to locate the make and model number of your furnace on the unit, and search the manufacturer website for the corresponding manual.
Still not having any luck finding the filter size? Fear not, because you can also grab your trusty tape measure and take matters into your own hands.
How Do I Properly Measure a Furnace Filter?
Measuring a furnace filter is very easy, but there’s one key thing you need to keep in mind; the difference between actual size and nominal size. As we talked about earlier, the actual physical dimensions of a furnace filter will be slightly smaller than the nominal size that you’ll use to shop for replacements. With that in mind, here’s the method to use when measuring your furnace filter.
First things first, you’ll definitely want to switch your furnace off before going any further. Once it’s been shut off, take the existing filter out of its slot and get your tape measure. Measure each of the main three linear dimensions and jot down the values for width, length, and depth (thickness). The actual size measurements you write down will usually be in fractions of an inch, so to find the nominal size you’ll just need to round up to the nearest whole inch. This means a length measurement of 15 ¾” should be rounded up to 16”. Do this for all three measurements, and you’ve got your nominal size.
What If I Don’t Have the Old Furnace Filter Anymore?
Well, this makes things a bit trickier, doesn’t it? Consider this a worst-case scenario, where you don’t have a filter to refer to, you don’t have the owner’s manual, and you can’t find the information anywhere online. What you can do is measure the internal filter slot opening. Using your tape measure and a bit of patience, jot down the dimensions of the internal space where the filter will go, making sure to place the tape measure against the metal surfaces where the filter will actually make contact. This is important, because sometimes there are metal tabs or ridges that are designed to hold the filter in place.
When measuring the internal space of your furnace filter slot, you’ll likely get incremental inch measurements like you would when measuring the actual filter size. However, in this case you will want to round down to the nearest inch instead of up. For example, if the internal space of the furnace filter slot has a length of 16 ¾”, this measurement will be rounded down to 16”. To get your nominal size, round down for each of the internal filter slot dimensions.
Where Can I Learn More About Shopping for Furnace Filters?
There’s many other factors to think about when shopping for replacement furnace filters, other than what the right size is. You’re also going to want to consider the level of filtration efficiency you might need, what brand you might want to buy, and how many replacement furnace filters you’ll want to keep on hand for future filter changes. We recommend you check out other guides here at Dan the Filter Man, such as our Guide to MERV Filter Efficiency Ratings, Guide to Furnace Filter Change Frequency, and our Filter FAQ page. There’s plenty of handy information there that can help you get the most bang for your buck when you shop for furnace filters. Plus, we’re always here to help if you need us! Simply get in touch with us via our contact page, and we’ll get back to you as soon as possible!
