Question: I have a Trion Air Bear media filter installed close to my furnace in my basement.  I also have two Newfoundland dogs who shed like fiends.

I have found the expensive Trion media filters clog quickly (well before a year, more like a month) with fur. I wonder if it would make sense to install return air grille filters with cheap fibreglass filters as a sort of pre-filter to extend the life of my Trion Air Bear media filters?  It seems like this could help prevent pet fur from building up in my ducts. Do you see any problems with this?

Your Trion Air Bear filter probably has a MERV 11 rating. Furnace filters and air conditioner filters with high MERV ratings already restrict air flow so adding another filter has an even greater impact.

I typically don’t recommend using fibreglass filters because they do such a poor job and I am always cautious about using two media air filters together.

However, every filter has its use and fibreglass panel filters are great for your problem. Be very careful not to let them get too dirty because having two filters in a system can cause a number of problems. I would recommend changing them monthly at least and maybe more.

I would also recommend that you check the MERV rating of your Trion Air Bear filter. The rating should be marked on it. If the MERV rating is higher than 8 I would recommend changing it to a filter with a MERV 8 rating. These have less impact on air flow than the higher MERV filters and will still do an adequate job.

Read the articles below for more information about some of the potential problems that can be created when restriction air flow.

When to change your air conditioner filter
How often do you need to change your air conditioner filter?

One of the problems when trying to compare the different types of furnace filers available is that there is quite a bit of information to sort through. When trying to compare efficiencies, brands, MERV ratings, price, characteristics and the types of filters available, you can suffer from information overload.

The first thing to do before sorting through all of the above information is to determine what type of air filter is best suited for your specific needs. After deciding on the type of filter you want to use then it is time to compare and review the filters of that type to make your decision.

I have compiled a chart to help compare air filters most commonly used in heating and air conditioning equipment. It lists some of the pros and cons of each of the common types of filters and also lists some important details of each type.

Question: I have a whole house electric AC with gas heat in my home, which is approximately 1800 sq ft.  There are 2 return air grills with filter in the house and a filter tray at the bottom of my furnace in the return air system.
The sizes are as follows:

Grill #1 is 20×20x1
Grill #2 is 20×24x1

The under furnace tray is 24×24x1

What filter combination should I run?  Filter in all 3? And what type.  I’m looking for the best airflow (low pressure drop) with the most filtration.

If you are looking for the best combination of air flow and filtration I would recommend a pleated filter with a relatively high MERV rating 10 - 12 and change it monthly.  Although filters with higher MERV ratings have more pressure drop than air filters with lower MERV ratings, they will provide the best filtration.  Change the filter monthly to prevent the air flow from being effected greatly.

Stacking filters is not a good idea because combined they may cause excessive pressure drop and can be far worse than having one pleated air filter with a high MERV rating.  I would remove the filters from the return air grills.

If you are interested in upgrading your system, you can install an electronic air cleaner and install a pleated air filter as a pre-filter.  Electronic air cleaners have almost no effect on air flow.  Remove the foam pre-filter that comes with the electronic air cleaner and use a pleated filter instead.  This combination will provide the best filtration and the least pressure drop.

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.