What Is the Ideal Filter Size for a 3-Ton AC?

man changing large AC filter

Sizing and installing your AC will go a long way in ensuring that you enjoy smooth, uninterrupted service. One of the key factors to keep in mind when installing the AC is the size of the air filters.

You also want to compare the available filters to the capacity of supply vents and return vents. Most importantly, the duct installation process should ensure that you have a smooth supply of air in the house at all times.

In this post, we look at a DIY question in regard to the necessity to increase the supply duct for a 3-ton AC. The DIYer wishes to find out whether a 20x20 filter and a 14'' return are sufficient for a 3-ton AC and what can be done if not.

To answer this question, we first have to look at the principles that guide supply and return vents in-home use ACs. For a 3-ton AC, the user should be a homeowner with a living space not larger than 2200 square feet. If you have a much larger space than this, you may want to consider a different type of AC.

Airflow Principles of ACs

The air that enters your AC goes through the return vent and the air filter. In theory, the air filter should be sized according to the CMF( cubic feet per minute) of the system. Different systems have different CMF ratings.

If a filter is too small compared to the AC CMF, it will cause problems. In most cases, the filter will clog more regularly than expected. The other issues you may have to deal with include critical air velocity and the chances of air dirty going through are high.

As a homeowner, you want to avoid a situation where you experience inconsistencies in the running of your system. As a result, make sure you get the right calculations for the vents and filters from the start.

If you use a smaller air filter, the dirt may eventually get to the blower and reduce the surface area on the evaporator coil. If you notice that the filter and grill vibrating, chances are that your air filter is too dirty. Such issues will consequently affect the overall running of your ACV which may overheat in some instances.

Calculating the CFM of your AC and Determining the Right Vents

If you have a 3-ton AC working with a 20x20 filter, you must start by understanding the CFM of your system. For the 3-ton, the CFM is 1200, given that 1 ton is equal to 400 CFMs. If this is the system you have, you should use a return and supply vent that matches the size of the AC in terms of the CFM it supports.

If you use a small air filter than the system requires, you may also have a problem due to system cracks. No matter how airtight a system is, it will have cracks that allow the inflow of dirty air. These cracks may increase the pressure on your system. Having a large enough air filter negates the effect of the cracks and ensures lower static pressure.

The minimum recommended filter area is 2.5 cubic feet per minute (CMF) per square inch of the filter area. In other words, to determine the ideal filter grill area, you will have to calculate by dividing your AC CFM by the 2.5.

For example, if you have an AC whose CFM is 2,000, you can determine the ideal grill area by dividing this figure by 2.5. This would be 2000/2.5= 800 square inches. The square inches could also be expressed in terms of their dimension, which could be 20x40 or the other way round.

The other aspect of the AC you want to consider is the velocity. The maximum allowed velocity is 300 feet per minute (fpm), For instance, in the case above, the system could yield over 350 fpm, which is recommended.

Bottom Line

For the case of our DIYer, the question is whether increasing the 20x20 is necessary. First, it is necessary to get the right airflow vents for the system and then calculate the right grill surface. Simply divide the 1200 CFM by 2.5 and get the right size of filter that works for your system.