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.

Question: Can I use a Boair 5 stage air filter with a geothermal system?  The unit is 8 1/2 years old.  I have a metal filter that you hose off just like the Boair 5 stage, but they said it would last 8 to 10 years.  My thermostat is showing it needs changed.  I did clean it and replaced it in the unit but it is still saying change.  Thank You.

Changing to a Boair 5 Stage Electrostatic Filter

If you are currently using some type of electrostatic air filter in your geothermal heat pump I see no reason not to use the Boair 5 Stage electrostatic filter instead.  They are one of the most efficient electrostatic furnace filters available.  However, without seeing your old filter, I can’t say if you need to replace it or not.

I looked at a friends heat pump the other day and he was using an electrostatic air filter with a metal frame and the filter was still in very good condition after 10 years of use.

There are Two Types of Filter Change Indicaters

Thermostat Filter Change Timers

1) This type has an electronic timer in the thermostat that is used only as a reminder to change your air filter at pre determined intervals.  Depending on the type of thermostat you are using the intervals can be based on hours of operation or days of operation.  These times are usually programmed into the thermostat when they are installed and can be adjusted later if necessary.

Every time the filter change indicator illuminates on the thermostat it must be reset so that the timing sequence begins again from zero.  Resetting these timers is typically done by pressing a button or a combination of buttons.  Sometimes they can be a bit tricky to figure out.  Every manufacturer has there own way of doing things.

These filter change indicators do no reset automatically, so you will have to reset it yourself.  If you are unsure how to reset it contact the manufacturer or a local contractor.  You may also be able to find this information by doing a search online for the manufacturer and model number of the thermostat.

If the thermostat will not reset and you are sure you are using the proper procedure, you will have to change the thermostat.  These filter change indicators are only reminders and do not indicate if the filter is actually dirty or not.  If the thermostat is working properly otherwise I would not worry about a faulty filter change indicator.  Simply replace or clean the filter every one to three months and you will be fine.

Filter Clog Switch

2) The second type of filter change indicator is the filter clog switch.  These are much better devices and are activated when the furnace filter or air conditioner filter becomes dirty.

Unfortunately these types of filter change indicators are not often used in residential heating and air conditioning equipment.  There is a higher cost associated with these filter change indicators and of course to be competitive manufacturers have to offer similar products.

These filter change indicators sense the air flow through the air handler and react to changes in air pressure within the air handler.  There are a couple of different ways that this can be done, but that is beyond the scope of this article.

Basically what happens is, as the filter becomes dirty it causes a restriction to air flow through the filter.  This restriction to air flow causes a pressure change that the filter change indicator (air flow switch) reacts to, telling you it is time to change your air filter because it is getting dirty.

There are Two Things That Can Cause This Type of Filter Change Indicator to Indicate a Dirty Filter Incorrectly

1) If the air flow switch is not calibrated properly it may indicate that the filter needs to be changed when it doesn’t or it may not indicate the filter is dirty at all, when it fact, it is time for it to be changed or cleaned.

2) The other thing that can cause this type of filter change indicator to give false readings and is relatively common in heat pumps and air conditioners that are have been in service for many years and systems that use inefficient filters, is a dirty evaporator coil.

Before making any changes to the settings of the air flow switch, inspect the evaporator coil and make sure it is not dirty and restricting air flow.  A dirty air filter and a dirty evaporator coil show the same symptoms, so if everything indicates a dirty air filter but you know it is clean, check the evaporator coil.