Question: I was recently told that I could put (4) 1” inch filters together and use as a replacement for the expensive 4 inch pleated filter in my Aprilaire Spaceguard furnace filter system.
It is never a good idea to stack filters in this way. It restricts air flow and can cause the air conditioner to ice up, among other things.
Any savings would be quickly lost due to decreased equipment efficiency and possible service calls.
Problems related to restricting air flow.
- Reduced operating efficiency
- Increased operating costs
- Evaporator coil icing up
- Reduced air flow to the home
- Longer run times
- Filters could be sucked into the fan causing damage
- Liquid refrigerant flooding back to the compressor
- Possible equipment failure
If the cost of the filter is a real concern, you could possibly install a 2 inch pleated filter into the Aprilaire Spaceguard furnace filter system. As long as the outside dimensions are the same as the Aprilaire filter it should work.
When installing the narrower filter make certain that it is installed against the down stream side (toward the air handler) of the Aprilaire unit. This way, air flow will help to keep the narrower pleated filter in place. This will only work if the filter is mounted in a vertical position. If it is mounted horizontally, this will not work and you will have to figure out a way to hold the filter in place.
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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.
