Question: I have pleated air filters for my furnace that have a wire meshing on one side. Which way does the filter go in the furnace with regards to airflow direction. With the wire side opposite of airflow or on the same side as the direction of airflow?

Also, for replacement, what Merv value should I use, is it best to find the highest value or is some lower value actually better?

Many pleated air filters have wire mesh on one side of the filter. This wire mesh is used to improve the filters ability to stand up to the forces exerted on it as air passes through it. The longer the filter is in use the dirtier it becomes and more force is exerted on it. If there were no wire mesh the filter may collapse and be sucked out of the filter rack.

Wire mesh is usually found on filters that are made of cotton or some other filter material that is not sturdy enough to withstand the stresses they can encounter in a heating or air conditioning system.

The filter should be installed so that the wire mesh side is toward the fan allowing it to hold the filter material securely in place so that it does not collapse. For more information on this topic read which way do furnace filters go.

Pleated air filters with wire mesh on one side of the filter usually have lower Merv ratings than pleated filters that are sturdy enough to hold their shape without the extra support.

One of the benefits of these filters is that they have less impact on air flow than more efficient pleated filters but you also don’t get the filtering capabilities that the more efficient filters such as Nordic Pure will provide. It’s really a matter of preference. If you want cleaner air use filters with higher Merv ratings. If clean air is not a concern for you then you can use one of the less efficient pleated air filters.

Question: I have a Honeywell electronic furnace filter on my furnace. The power panel failed and the unit is very expensive to replace. However, I can install pleated air filters in place of the original electronic cells. The filter size is 20 X 25 X 4 inches thick. As a temporary measure, I have installed 3-1 inch thick MERV 8 air filters in parallel into the filter opening.  How does the MERV rating change when the filters are placed in parallel? Is what I have done a good solution? If not, what would you recommend that I do?

I would remove two of those 1 inch pleated air filters ASAP. Installing filters in this way has a very negative effect on air flow and reduces equipment efficiency greatly. Take a look at your air handler to see if there is a place to install a 1″ filter. Most air handlers come equipped with this filter rack.

Take a look at the pictures on this page “where is my furnace filter” if you are not sure where to look. If you can’t find a place to install a 1″ filter, you can get a 20 X25 X4 MERV 12 filter at NordicPure for a very reasonable price.

Question: I measured the metal frame of my furnace filter at 19 1/2 x 24 1/2 x 3/4 inches but after looking closer at it I saw a measurement of 20 x 25 x 1 on the frame.  Now I’m not sure what size filter to replace it with.

Shopping for a new air filter for residential use, usually only requires that you remove the old one and check to see what size is written on the frame, then purchase a new one online or pick one up at your local hardware store based on that information.

Sometimes, for many different reasons, you can’t determine furnace filter size by looking at it.  Trying to determine the proper size replacement for your HVAC (Heating, Ventilation and Air Conditioning) equipment can be a bit of a puzzle.

How Sizes Listed on the Filters Label Compare to Actual Filter Sizes

Typically, manufacturers label their air filters in inches.  For example 20 x 20 x 1 or 20 x 25 x 2 and so on.  What often causes confusion is that the sizes listed on the filter and the actual size vary by as much as 1/2 of an inch.  You measure the filter because there is no label on it telling you what size it is and you get odd sizes.  This can make it difficult to order the proper size with confidence.

So Here’s How it Usually Works for Standard Filter Sizes

If you measure your old filter and it is 15 1/2 x 19 1/2 x 3/4 inches, the size you want to order is 16 x 20 x 1.  The reason filters are actually smaller than the size listed on them is to allow for a margin of error.  They are actually sized to fit into a filter rack that is 16 x 20 x 1.  If you ordered a filter with dimensions the same size as the filter rack it would obviously not fit.

The same rule applies to the thickness of the filter.  A filter that is labelled 1 inch thick will actually measure closer to 3/4 of an inch thick.  A 2 inch filter will measure closer to 1 3/4 inches thick.

This rule applies to all filter types including Pleated, Media Pads, Electrostatic, HEPA, Fiberglas Panel filters and most other types available.

Something to Check Before Applying the Above Information

Before ordering a new filter based on the information above make sure the filter rack is not undersized.  This is easily done by removing the filter and measuring the filter rack size.

If the filter rack is a bit smaller than standard filter measurements simply contact the filter supplier and ask them for the exact measurements of the filter that you think you need.  This should eliminate and doubt when sizing a filter.

Special Order Filter Sizes Cost More

Often people order special sized filters online when they only require standard sizes.  Custom order filters are more expensive than standard size filters because they have to be custom made.  This takes more time and they cannot be mass produced.  Therefore manufacturing costs increase and they have to charge more so they can still make a profit.

When Should You Order a Custom Size Filter

For most people, standard filter sizes will do just fine.  If however, you are an allergy sufferer and require the absolute cleanest air possible in your home, a custom order filter is not a bad idea.

The tighter a filter fits into the filter rack the better job it is able to do.  This is because a loosely fitting filter allows more air to bypass the filter and return to the living space, making it less effective.  For example if you go the expense of installing a HEPA filter and it fits poorly, you are not getting the full benefit of the filters higher efficiency.

This doesn’t mean that the air conditioner filter should fit tightly, that would make it difficult to change.  It simply means that it should not be a poor fit either.