The Essential Guide to Furnace Filters

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Last updated: 
August 9, 2016

When it comes to home improvements, furnace filters are not the most exciting of items. But that doesn’t mean they’re not necessary. Even vital, some experts argue, as furnace filters save money and improve the air quality in your home. Originally designed to keep all the moving parts of a furnace together, filters have evolved beyond that.

They are now responsible for preventing harmful particles from floating around in the air. If inhaled, they can cause asthma attacks and allergies. So, it seems as though the small and often neglect furnace filter is actually a powerful tool. But how do you choose the appropriate one for your home? Here is your essential guide to furnace filters that will answer that question.

How does a furnace work?

To begin to understand how a filter works, you must first have a working knowledge of your home’s furnace. It’s the only way you’ll be able to shop for the perfect filter afterward. Traditional forced-air furnaces draw in the necessary air through return ducts. They warm it over a heat exchanger and, aided by a blower fan, push it through some ducts that branch out throughout your house. The furnace normally keeps running until your house is warm enough. If you have a thermostat, when the furnace will stop when it reaches the desired temperature.

What does the furnace filter do?

The furnace filter actually connects to the blower fan. Its main job is to keep dust, hair, and other types of gunk away from the fan. The return duct pulls the air in, as shown above, and, therefore, has the tendency of also drawing in said debris and possibly harmful elements. Seeing as the air recirculates via the furnace, the filter ensures all that air does not circulate through your house dirty. However, contrary to popular belief, although it does improve the air quality significantly, the furnace filter’s job is not to clean your air.

How are furnace filters rated?

Furnace filters are rated based on an element called MERV. It stands for ‘minimum efficiency reporting value’ and its ratings range from 1 to 16. The higher the rating, the better the furnace filter is. Simply because a very high rating means it can remove a great deal of possibly harmful particles from the air.

However, there is one vital thing worth mentioning here. High rated filters usually allow a small amount of air to flow through them. This could lead to your furnace working harder, overheating, and possibly malfunctioning. Most homes require a rating between 8 and 11. Just to be on the safe side before shopping, find out if your furnace has a maximum MERV rating your model of furnace needs to take into account.

Furnace filter types

The most common furnace filter type, which you might have seen in shops, is the disposable pleated type. They come in a whole array of ratings and sizes for you to choose from. They are traditionally made of paper and polyester, which allows them to filter the unwanted particles from the air. They are usually not expensive, with prices ranging from $2-$3 to $30-$40. The price, of course, depends on the filter’s size, model, brand, and rating. These filters need to be checked on a monthly basis because they do then to clump the gunk they collect. Also, to keep them clean, you need to replace them every three months.

Apart from the paper and polyester type, there are also filters made of fiberglass available on the market. They are the cheapest furnace filters you can buy. They come in a traditional blue color and almost look like they’re made of spider web. They also come in a whole range of sizes, but you should know they are considered rather flimsy and have lower ratings than the paper-polyester ones. Because of these reasons, they need to be checked and replaced a lot more often.

The third category comprises disposable and washable filters. They both come in electrostatic versions. This means that they have the ability to charge themselves while the air flows through them. This is what enhances their power of sucking and trapping small particles, dust, and hair. In other words, they are more efficient than the other two categories, which makes them the perfect choice for houses with small children and pets. They are also suited for inside smokers. As a safety measure, check your furnace’s manual to make sure you can use an electrostatic filter with ease.

Filter sizes

Air filters are always sized according to their thickness or depth, height, and length. As far as thickness goes, the most popular one is 1’’, but 4’’ is also a good choice is you have a larger system. The combination of height and length varies between 10’’X10’’ and 30’’X30’’. Here are the most common furnace filter sizes: 14’’X25’’, 16”x20”, 16”x25”, 20”x25”, and 25”x25”.

To find out what size you need for your home furnace, you have to remove and check your old filter. Its size is generally inscribed on its frame. The other option you have is to check its manual. However, if you know your furnace uses a custom-made filter, you need to refer to the furnace manufacturer. Otherwise, find out where you can buy such a replacement filter from.

How to replace your furnace filter in three easy steps

As stated above, for your furnace and your filter, for that matter, to work safely and appropriately, you need to replace the latter approximately every three months. That’s when it reaches its maximum and, seeing as it’s so important it runs at its peak efficiency, think about replacing it. Fortunately for all of us, it’s a much easier task than it’s believed to be.

However, even if it’s so easy, you can still basically botch this job. For example, you can buy the wrong filter or put it backward in the furnace. This means it will block the entire air flow instead of allowing free passage. It will also not clean the air properly. Therefore, here is how you perform this task in three easy steps.

Step 1

Find out which type of filter you have. As we have already pointed out, this is, probably, a crucial step. The mere misunderstanding of the kind of filter you need can lead to your furnace breaking down.

The first thing you need to do is to turn off your furnace altogether. This will prevent any accidents from happening because the furnace might start on its own accord. Remove the filter already in place. You will most likely find it inside the furnace or the air vent. Most furnaces have an arrow painted or inscribed somewhere. It will tell you the direction in which the air flows. With a permanent marker, draw a line indicating that direction on the outside of the furnace, so that you always know how to properly install the filter. Next, you need to write down the furnace filter’s size. You can find this piece of information on the cardboard box.

Please note that, upon opening the furnace, you discover the filter has a plastic frame, this means it’s reusable. Therefore, simply clean it from time to time using your vacuum and some water. You will want to do this outside, as it will have a lot of gunk coming out of it. Also, make sure it dries out completely before you reinstall it.

Step 2

Buy the replacement. You can find furnace air filters at hardware stores, home centers, and, of course, online. You can shop in places such as Builders Square, Home Depot or Amazon, but you might also find furnace filters at your local Walmart for example if that is better suited for you.

Step 3

Install the brand new filter. Normally, there should be some markings. They will tell you which side your filter should face the furnace. Slide it back into its original slot and replace the cover if it had one to begin with.

It’s a good idea to keep a record of all the times you replaced the furnace filter, so you’ll know when it’s due to be changed again.

Which are the best furnace filters?

Seeing as a high-efficiency furnace filter can actually slow down the air currents that need to waft freely through your house and that a low-efficiency one will do a poor job of cleaning the air, it can sometimes be difficult to decide which one to choose. Therefore, based on user experience and reviews we have put together a list of the best furnace air filters available on the market.

#1. The PUR5000 Whole House Furnace Filter

It has an electrically charged particle collection system which makes it perfect for trapping all unwanted particles from the air. This includes dust, hair, pollen, mold, bacteria, and viruses. It’s so efficient that is actually removes 99.97% of all the gunk floating about. It retails for $700, and you need to change it every month.

#2. The Blueair Central Furnace Filter System with MX2 Particle Charger

It’s considered to be a real breakthrough as far as furnace filter design is concerned because it crossed the barrier to air purifier. It has an ionizer that forces unwanted particles to stick to the furnace filter a lot more than ordinary filters do. It retails for $199.95, and the package it comes in includes two filters for a 6-month filtration period.

#3. The Envirosept Electronic Furnace Filter Air Cleaner System

This one is a permanent filter that is absolutely state-of-the-art. It has an enhanced media filter doubled by dielectric polarization that leaves the air extremely clean. The reason is that it not only removes particles from the air but sub-micron particles as well. It retails for $319.95 and up and is very low maintenance. You can also find some discount filters belonging to this range online.

#4. The Guardian Clean Air Furnace Filters by Aerus

This particular air filter was designed with the idea of maximum air flow in mind. This means its concept was based on it allowing as much air to pass through it as possible. It captures some 95% of all particles, including the ones as small as 0.3 microns. It has a MERV 12 rating, and it retails for $59.95.

#5. The Aircleen 1500

This one is an improvement of the older Airscreen 1000 air filter. It removes 97.7% of all particles up to 0.3 microns, which means it can also trap pollen, mites, pet dander, and mold spores. It retails for $259.95, and you need to replace it every month to ensure optimum air quality. Its pads are very easy to remove and replace and they come in 16 sizes for you to choose from.

As stated in the beginning, furnace filters might not seem so exciting at first glance. But once you start to understand their importance, you discover a whole new multi-layered world behind these white screens. One thing all furnace filter users say is how much they have improved their lives. And it’s true. Imagine not having to inhale dust, mites, dander, and pollen grains every day!


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