Furnace Filters 101

Everything You Need to Know About Furnace Filters

What does a furnace filter do? Which one should I buy? How do I install it correctly? When should I change it?

If, like many people, you don’t think much about your furnace so long as it’s keeping you warm in the winter and cool in the summer, you might find these questions a little tricky to answer. But, since it’s really important to change or clean your furnace filter on a regular basis to keep it working properly and avoid it malfunctioning, we’ve put together this handy guide.

Read on to learn how to keep your furnace filter working properly and avoid damage or potential breakdowns.

How Does a Furnace Work?

A traditional forced-air system – the most common heating and cooling system – draws air in via return ducts, warms it over a heat exchanger then, with the help of a blower fan, pushes the heated air through a series of ducts that branch off into rooms throughout your home.

As the warmed or cooled air flows into the rooms, the unit’s fan pulls existing air out of the rooms via a separate set of “return” ducts and toward the heat exchange (where it is again heated or cooled). This circular cycle continues until the desired temperature is reached.

What Does A Furnace Filter Do?

Did you know, a furnace filter’s primary job is not actually to clean your air as many people believe? In fact, the main purpose of a furnace filter is to protect the blower fan from all the dust, hair and other gunk the return duct pulls in. However, during this process, it does, of course, improve the quality of your inside air since it is removing contaminants from being recirculated.  Not changing your furnace filter when it becomes clogged can cause serious damage to your furnace and will also result in much lower inside air quality for your home.

Which Type of Furnace Filter Should I buy?

Merv Ratings

It’s important to understand MERV ratings when considering which is the best furnace filter to buy. Furnace filters are rated using the minimum efficiency reporting value (MERV) which was created by The American Society of Heating, Refrigerating and Air-Conditioning Engineers(ASHRAE) to allow consumers to compare air filters made by different companies. MERV ratings can range from a low of 1 to a high of 20 with the higher the rating, the more particles the filter can remove. Higher rated filters allow less air to flow through though and can force your furnace to work harder and possibly malfunction.

According to the United States Environmental Protection Agency, air furnace filters with a MERV between 7 and 13 will be adequate for most homes as they are likely to be nearly as effective as true HEPA filters at controlling most airborne indoor particles. To be safe, always check if your furnace manufacturer has a maximum MERV rating your model of furnace can use to figure out which is the best furnace filter for you.

Some stores, such as Home Depot, use their own rating system. While these ratings are similar to the MERV scale, they do vary. It’s best to confirm what their rating converts to on the MERV scale to ensure you’re using a filter safe for your furnace.

The Different Types

The most common type of furnace filter is the disposable pleated kind. These come in a range of standard sizes and ratings. Pleated filters are constructed out of paper and polyester and do a good job at filtering most household particles and allergens. The price of these filters varies from a couple of dollars all the way into the $30 – $40 range depending on the brand, size, and rating of the filter.

Disposable fiberglass filters are the cheapest filter on the market and are not the best furnace filters. They have an almost spider web appearance and are most often blue in color. They come in many standard sizes, but are generally more flimsy and have lower ratings than pleated filters.

Permanent reusable filters (also referred to as washable filters) are constructed with either a solid aluminum or plastic frame and are more efficient than a disposable filter. These filters can be vacuumed off and cleaned with water. They come in a range of sizes and ratings.

A Note About Electrostatic Filters

Both disposable and washable filters come in electrostatic versions. Electrostatic filters self-charge themselves as air passes through them. This process helps trap small particles, making them good for homes with pets or people who smoke inside. Check your furnace manual to ensure you can safely use electrostatic filters.

Filter sizes

Furnace filters are sized by thickness (depth), height and length. The most common thickness is 1”, with 4” also being a popular choice for larger systems. Height and length combinations range from 10”x10” all the way to 30”x30”. The most common sizes are 14”x25”, 16”x20”, 16”x25”, 20”x25”, and 25”x25”. To find out what size filter your furnace uses, remove and check the old filter (the size should be written on the frame of the filter) or refer to your furnace manual.

How Do I Install My Furnace Filter Correctly?

We’ve devoted an entire blog post on how to properly install your furnace filter, so check it out. Especially if you’ve got questions about the furnace filter direction (don’t worry, you are not alone!)

Here’s a great video that also explains the steps you need: https://youtu.be/-Y06YJpVZbw

When Should I Change My Furnace Filter?

This is the tricky part. Often, people rely on a random “when it feels right” approach. Others follow the rule that:

  1. Pleated filters should be checked monthly for blockages and replaced on average every 90 days.
  2. Disposable fibrerglass filters should be checked and replaced more frequently than pleated filters due to their inferior quality.
  3. Permanent reusable filters last an average of 5 years but need to well maintained with a proper cleaning at least every 90 days.

However, these guidelines are not necessarily reliable. FilterScan Wifi is an air filter monitoring system that tells you when it’s time to change your filter, eliminating the guesswork and saving you time and money. Learn more.

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