A subject I have rabbited on about for ages is the folly of using general rules to size air conditioning units — the regular singleton of air-con capacity for every 400-600 sq. Ft of conditioned floor space simply does not work. We realize that load calculations focused on how specific houses will perform, in all likelihood, are far superior to general rules. Another field where general rules are prevalent is the installment of new filters on air conditioners (or furnaces or heat pumps). Conventional wisdom is to change them every month, correct?

Is One Month the Solution?

After you purchase regular, single inch, fiberglass HVAC unit filters, the enclosed instructions normally recommend changing them every month. This mantra is repeated religiously by many well-meaning organizations and people. And, in reality, you won’t create any issues by installing a new filter every month, and you may even avoid some. Thinking back several years, I attended service appointments in Atlanta for a day with Phil Mutz, from Moncrief Air Conditioning and Heating. Perhaps unsurprisingly, many of the issues our heating contractor encountered were insufficient airflow, because of filthy filters.

Installing a new air filter every month is a drag, however. Typically, during fall and spring, the filter will not become so filthy because there is less need for cooling and heating – so it is fine to change it less often. Moreover, in a property with little duct leakage or infiltration, and minimal dust levels overall, a filter will remain clean for longer – even if it is used frequently. Of course, this doesn’t apply if you have floor return vents.

Next, you have to select which filter to use. If you have read this blog during the previous twelve months, you will know that I like thick (two inches minimum) high MERV filters, because they are great at keeping the air clean. If you desire excellent IAQ (indoor air quality), your filtration must improve – and a MERV 13 filter is the best way to do this. However, a high MERV filter is fairly costly, so getting a new one every month will quickly burn a hole in your bank account.

Happily, most high MERV filters are able to store extra dirt. There is additional surface space, with pleated, thick filters. This means that, when they accumulate the same quantity of muck as normal single inch filters, the pressure change is not so pronounced. Therefore, how can you judge when to install a new one? Well, once your filter resembles the photograph at the start of this post, you have delayed for too long. However, there is a method that works better than visual assessment.

What about using the drop in pressure throughout the filter?

Lately, I covered ways of getting low pressure to drop in high MERV filters, and the problem of when to install a new filter was mentioned in the article comments. Here are the thoughts of John Semmelhack:

Generally speaking, when the drop in pressure throughout the filter is double the original reading, it is time to fit a new one. With deep, large filters, this will be time-consuming. In addition, the filters will appear revolting, long before the drop in pressure indicates that a change is needed.

The drop in pressure throughout our MERV 13 filter is just 0.06 WCI (water column inches).

The photograph above indicates how to carry out the pressure drop measurement. A hole is required on both sides of the filter inside the return ductwork. In this situation, I utilized a couple of static pressure implements; one pushed into each hole. Both implements have part of a hose linking them to the digital manometer’s reference pressure taps and input. The photograph above shows that the drop in pressure is 0.06 inches of IWC throughout this MERV 13 filter.

Using Magnehelic Gauge to Measure Pressure Drops

If you are not a home professional or HVAC professional and do not wish to purchase a digital manometer, it is possible to use a magnehelic gauge to set your system up, as the photograph above shows. Dwyer sells a good Magnehelic gauge for under $50. Ensure that you buy the lowest available range: 0-0.5 IWC. If any of your filters have a drop in pressure close to the top limit of this gauge, there is something wrong.

How about the pressure of the whole system?

Something that should be highlighted here: Waiting for a 100 percent increase in the drop of pressure throughout the filter will only work for filters that are correctly designed, and that have low drops in pressure with clean filters. Furthermore, you require a duct system that is built correctly, without excessive airflow resistance.

If you install a MERV 13 filter and the drop in pressure throughout a clean, new filter is 0.25 IWC (a fairly common occurrence), you can not allow that filter to load to a level where the drop in pressure is 0.5 IWC. This would mean that the drop in pressure throughout the filthy filter on its’ own equals what is permitted for the total static external pressure. In other words, the filter has consumed the total pressure that is meant to be consumed in the passage through the entire duct system (including the coil on the air conditioner with furnaces).

If you wish to opt for a performance-dependent strategy for determining when to install a new filter, you have to ensure that your filter design and duct system are suitable. If they are unsuitable, then you ought to rectify those issues, so you can reach a point where a 100 percent increase in the drop of pressure is your cue to install a new filter.