What Filter Should I Use In My Furnace?


The correct furnace filter size and type for your furnace is an air filter that allows for the proper amount of airflow through the system to maintain the proper temperature differences and static pressures designed by the manufacturer of your equipment. The right size is not always the filter that fits the slot provided by the people who installed your furnace.

The correct filter is one that filters the air enough to keep the blower motor, fan and coil clean and operates within the static pressures designed by your manufacturer.

Proper fitment of the filter is essential to not allow unfiltered air past the filter into the system.

Not enough can be said about proper fitment and correctly sealed duct work.

When Does Your Furnace Filter Need ...

When your filter is installed it should be sealed on both sides and the front and back.

Having the slightest leak around your filter gives the dust bunnies a short cut around the filter into your furnace and duct system.

When asking what filter should be used in your furnace their are two questions created…….

  • What is the correct filter size?
  • What is the correct filter type?

If you change the type of filter, let’s say from a fiberglass to a pleated filter it may be necessary to change the size because pleated filters can be more restrictive.

Having the wrong and possibly a more restrictive air filter can be just like having high blood pressure which can be an indication that there are restrictions in a persons blood vessels.

You measure blood pressure with a blood pressure monitor.

You can measure static pressure two ways.

  • Temperature rise method
  • Measure with a manometer

But if your furnace system has high static pressure it will probably take more than a pill or exercise to fix.

A manometer can be purchased for as little as $30.00 or you could have your technician perform that test. Keep in mind that cheap manometers will not be accurate enough for good diagnosis

It’s very likely that a static pressure test using a manometer has never been done on your system. It’s just very rarely done, it’s so easy to do and should typically take only about 5 minutes.

It’s easy to tell if a test has ever been done. Small, approximately 3/8 hole will be in about 4 strategically located positions around your indoor furnace or air handler.

These holes should be plugged to prevent unnecessary duct leakage.

If you see these plugged holes, you had a very savvy tech or installer and chances are your system static pressure is good.

What are the different types of home air filters?

What if the installers provided the wrong size filter compartment?

It may seem obvious to you that you would install the same size filter that you removed. But what type of filter should you choose? Pleated or fiberglass, that’s the complicated part.

Filters now come in what is referred to as MERV ratings. An acronym for Minimum Efficiency Reporting Value.  MERV has become a universal measurement of efficiency for air filters. The range for MERV ratings is 1 to 16. The higher the number the smaller the particle it will collect.

Manufacturers of furnaces would like you to use filters with fairly low MERV ratings.

To be ridiculous, filters with low MERV ratings will catch golf balls and toys.

Then the stuff it doesn’t catch will go into your blower wheel, motor and duct work causing all sorts of expensive maintenance like duct cleaning and new motors.

A dirty blower wheel can lower system performance more that you would suspect.

Those fiberglass air filters that you have probably seen or used are porous enough to pour salt through.

Home owners want to stop dust with high MERV rated filters that fit in that original filter slot so they buy a one inch thick pleated filter with a higher efficiency rating.

When home owners do that they have just inserted a possible problem into there system.

Does Furnace Filter Thickness Matter?

One inch thick pleated filters have more resistance to air flow than the one inch thick low efficiency fiberglass filters that your furnace manufacturer wants you to use to protect his elements or heat exchanger.

Filters with a high MERV rating usually come in a pleated form. These thicker pleated filters also allow for more air flow.
With pleats you gain surface area and with more surface area you can use a high MERV rating. Especially if you are able to use even a thicker 4″ filter.

But how do you know for sure that the size of that filter slot is large enough to properly filter the air in your home?

The size is normally stamped on the side of your existing filter along with an arrow that indicates the direction the filter should be inserted into that slot. Hopefully it points toward the furnace.

To illustrate this it helps to get very extreme. Suppose we use a furnace filter that is way too small. Let’s say for example the filter size was 10″ X 10″ X 1″ and your furnace is trying to pull all the air it needs to heat and cool your home through that tiny filter.

The velocity or speed of the air through that filter would be so great that most of dust would be sucked through into the blower and then the duct work.

Until that tiny filter plugs up like a board and your air flow goes to zip and your compressor in your cooling unit dies.

You may have noticed that when your 1″ filter gets pretty dirty it sort of collapses or caves in and is hard to remove.

In the heat mode the furnace would run so hot that the elements or heat exchanger would fail prematurely.

In the cooling mode the ac coil would ice up and eventually your ac or heat pump compressor would die.

The other extreme would be a very large filter, let’s say 40″ X 40″ X 2″. The air would move much slower over the entire surface area of the filter and dust would settle and collect much more efficiently onto and into the filter.

This filter pictured on the left is an Aprilaire 2200.

This Aprilaire measures 20 by 25 inches and it’s 6 inches thick.

It’s due to be changed, but you can see the light shining through so it still has a little life left.

This large filter would, what the industry calls “load” more efficiently and filter a much finer material.

Also with a extremely larger pleated filter the dust will collect and not tend to be sucked through into your duct work and home.

To the right is a furnace specification sheet which shows what Florida Heat Pump wants to have for there GS036 water source heat pump.

Look just below where it says “Blower Performance”. With blower speed normal follow the column to the right and you can read the CFM. Cubic Feet Per Minute that the blower should deliver.

Above that see the figure 0.60 which idicates the static pressure required to make this a long lasting and very efficient piece of equipment.

If you look between the Aprilaire filter and FLP labels you should see a black dot.

That is the static pressure port with a rubber plug stuck in it.

Taking the static pressure of your equipment, like taking your own blood pressure will determine the life of your heating and air conditioning system.

How Much Should A Service Call Cost To Measure My Static Pressure?

When it comes to taking measurements of things like static pressure it’s best to call a professional that can make the determination.

It shouldn’t be an expensive call if you consider the length of time we discussed that it takes to measure your static pressure which was about 5 minutes.

So it cost about a hundred bucks. You get piece of mind knowing you’ve done something that very few home owners do.

Hopefully your test will pass. But, if not you will come away with the right type and the right size of filter.

Chad Peterson

Chad Peterson is near 40 year veteran of the HVAC industry. "I like to explain heating and air conditioning problems in a way the average home owner can understand. "

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