What is pressure drop? How does it work?
In the world of HVAC, static pressure is defined as the resistance to airflow in your HVAC system created by the components of said systems. Generally, lower static pressure is better. High static pressure results in lower airflow, which allows your HVAC system to condition less air at a time. This forces it to work longer (not harder), which if left unchecked can result in higher power bills and potentially shortened lifespan. When selecting a filter, static pressure is an important factor, but it is far from the only factor to consider.
Pressure drop is defined as the change in pressure from one defined area to another. In HVAC, this is referring to the change in static pressure as air moves through your air filters. Put simpler, it's how much your air filter restricts airflow. The most common cause of a pressure drop that exceeds acceptable levels is your air filters being left unchanged for far too long. Many still believe that an air filter with a high MERV rating can instantly create a pressure drop that is above acceptable levels, but this is actually false.
The most common myth around pressure drop is that pleated air filters have too high of a pressure drop. We’ve already discussed the airflow myths around pleated air filters, but we’ll expound a little more with regards to pressure drop specifically.
The pressure drop of a fiberglass filter is about 0.1 inches of water. (inches of water is a measure of pressure differential. The name sounds counterintuitive when talking about air, but you can read more about it here if you want). These super cheap filters are the thin, flat, and usually green or blue ones that are typically less than a dollar. They are often erroneously recommended by technicians because of their extremely low pressure drop. Here’s what you’re not being told.
The pressure drop of a MERV 8 pleated air filter is around 0.12 inches of water, which is almost exactly the same as the 0.10 of the fiberglass. A MERV 13 pleated air filter, the highest quality offering from Second Nature and most other filter companies, has a pressure drop of around 0.25. That’s definitely more, but if you recall from our study referenced in this article, even with that pressure drop, the likelihood of it reducing the lifespan of your HVAC unit is slim to none.
You might be wondering how that is possible. It has to do with the construction of the filter. There is a relationship between the surface area of the filter and the pressure drop, which is the reason for the pleats. By folding the filter media accordion-style, the surface area increases dramatically while the filter frame remains the same size, allowing for greater airflow without having to increase the size of the actual filter. The filtering efficiency itself is also increased by changes in thread materials to materials with better electrostatic properties, which physically attract particles with static electricity. The result is a lower pressure drop with higher filtration efficiency. This is what makes pleated filters the best on the market. They're able to achieve high levels of filtration without high pressure drops, also known as too much airflow restriction.
There is not really much risk with a brand new filter anymore because of the technological advancement of the pleated air filter. Problems that your system will experience from high pressure drop are usually a result of a filter left unchanged for far too long. It’s commonly understood that an air filter will actually become more effective as it fills with particles. This is true. So why not just leave it in the return forever? Because the buildup results in an increased pressure drop. If left unchanged for too long, your filter could fill to the point where airflow is zero, meaning your system is running, but accomplishing absolutely nothing. This is not good.
Making sure you're using a good filter is important. Making sure you're changing it on time is equally important in order to avoid increased power bills and excessive wear on the components of your air conditioning system. According to the EPA, changing your filter on time can save you 5-15% on your heating and cooling bills. Our goal as a company is to make sure people remember to change their filter, which is why we offer an air filter subscription with a completely customizable schedule.
From sizes to types to qualities and more, here is everything you could ever need to know about air filters.