Beware of Filter Pressure Drop

BEWARE OF FILTER PRESSURE DROP

Knowing the pressure drop across a filter and warning your customers about problems dirty filters cause will improve the air quality in their homes and help avoid other serious problems..

• Duct Leakage
• Bad Air
• Loading Up
• Clean and Change
• Measure the Flow

Indoor air quality (IAQ) is now a common buzz phrase in our industry. Even residential consumers are well aware of it. And, one of the easiest ways to improve the air quality in their homes is by installing high efficiency air filters. Or is it?

Usually service technicians are simply replacing a standard fiber glass filter, which is only there to protect the equipment from dirt and dust build-up. In tests weÕve performed at our shop, we found the average 20-in. x 20-in. x 1-in. fiber glass filter has a pressure drop of 0.08 in. wg at 800 CFM. WeÕve tested other, higher efficiency filters as well.

The results are listed in Table 1.

When you add a filter that increases the overall pressure drop to an existing system, you also decrease the overall airflow. If the air conditioner had sufficient airflow to begin with, the system may still operate properly. However, in most of the systems weÕve tested the airflow was slightly low to begin with, so even lower airflow will make the system operate less efficiently -- with some unforeseen side effects.

Most existing air conditioners use fixed restrictors to control refrigerant flow. Lowering the airflow will cause suction superheat to decrease. In extreme conditions, this can cause liquid to return to the compressor. Under low load conditions, the indoor coil can freeze.

The efficiency and capacity of the system can also be reduced due to low airflow because less refrigerant is boiled in the evaporator, so less heat is removed from the house. In systems with expansion valves on the indoor coil, the superheat will remain the same, but the total refrigerant flow in the system will be reduced, possibly overheating the compressor motor. In heat pump systems, air flow restrictions can cause high pressure in the heating season, tripping the high pressure switch and causing an unneeded service call.

Duct Leakage

If the more efficienct filter isnÕt installed in the air handler, the higher pressure drop across it will increase leakage in the return duct. ItÕll also increase leakage through the cabinet of the air handler or furnace. This can be a serious air quality problem if the air handler is located in an attic or crawl space because the leaked-in air will carry contaminants from the unconditioned space. In addition, the air wonÕt be at return temperatures, therefore the cooling or heating load will be increased, effectively further reducing the efficiency and capacity of the system.

When installing high efficiency filters, be careful where you locate them. If the filter housing is installed in an unconditioned space, and if the filter access door isnÕt tightly sealed, untreated air can be drawn in, carrying dirt, pollutants, and moisture with it. So instead of improving it, an improperly located high efficiency filter can actually reduce IAQ.

Bad Air

One of our customers had electronic air cleaners installed in his crawl space. When we conducted a duct leakage test, we found the homeÕs air cleaner housing, and its connections to the duct system, were the biggest source of duct leakage from the crawlspace. In addition, because the air cleaners were a long way from the access hatch, the electronic air cleaners hadnÕt been cleaned for 1.5 years! Obviously, air cleaners should be accessible for service and cleaning.

Higher pressure drop increases filter bypass, unless the filter fits tightly in its holder. Any filter bypass reduces the effectiveness of the filter. If the filter is installed in a filter grille, or anywhere near the living space, air noise will be increased as well.

Air filter pressure drop isnÕt linear with air velocity. Look atTable 1. The 1 in. thick 20-in. x 20-in. pleated filter (Brand C) has a pressure drop of 0.2 in. wg at 800 CFM, the normal air flow for that filter. If you reduce the airflow to 400 CFM the pressure drop is reduced to only .06 in. wg. This is less than the pressure drop for a standard fiberglass 20-in. x 20-in. filter at 800 CFM.

If the velocity of air through a filter is doubled, the pressure drop can be increased much more. This works the same in reverse. If you double the filter surface area, using the same type of air filters, the velocity through the filters is cut in half, reducing the pressure drop to less than half.

When using 1-in. thick, pleated filters, adding another filter grille will reduce pressure drop considerably. You can sometimes install larger filters, space permitting. Filter performance will usually increase for pleated filters because their efficiency usually improves with lower face velocities, catching more dirt.

Electronic air cleaners have similar properties but not as pronounced as media filters. If you increase the size of the filter holder and filter, you can reduce long-term media costs as well, because often a larger, standard-size pleated filter will have about the same price as a small standard filter. Note that this performance improvement wonÕt occur with electrostatic filters because their filtration performance can drop off with a face velocity of less than 300 FPM. This is because the filter doesnÕt develop a full electrostatic charge and its effectiveness is reduced.

Loading Up

As media filters become loaded with dirt, the pressure drop across them increases. Since pleated filters catch more dirt than standard fiber glass filters, filter loading is a bigger problem. Also, since media filters are more effective as they become loaded, filter manufacturers may tell you that the filters can be used several months before being replaced. Under these conditions, final filter pressure drop can be as much as two to three times the initial pressure drop. (See Table 1). Media filters should be replaced more often. Using 4-in. to 6-in. thick pleated filters will reduce this requirement, because the filter has more surface area.

Electrostatic filters should be cleaned as often as possible. Because thereÕs no new media used, when theyÕre cleaned theyÕre easier on the wallet to keep at maximum efficiency.

ItÕs more important to keep electrostatic filters clean, because as pressure drop increases, dirt is drawn deeper into the filter where itÕs more difficult to remove.

Electronic air filters donÕt suffer from pressure drop problems as they become dirty, but they do become less effective. Since, similar to electrostatic filters, thereÕs no media cost associated with cleaning them, they should be cleaned as often as possible. Combination media/electrostatic filters have the same problem with increased pressure drop as other media filters, so they should have their media replaced as it gets dirty to keep air flow penalties to a minimum.

The important thing to remember here is that since high efficiency filters catch more dirt, they should be cleaned or replaced MORE often than regular filters of the same size.

Clean and Change

If air filters are a high resistance type, make sure the condensate trap on your evaporator coil is deep enough to withstand the negative pressure produced by the filter. If the water is drawn out of the trap, the coil pan wonÕt drain. When the coil pan overflows, the insulation in the evaporator housing will get wet, possibly providing a place for mold and mildew to grow, thereby destroying air quality every time the indoor blower runs. It may also make a mess of a drywall ceiling, if the unit is in the attic.

Note that with duct-mounted media filters, itÕs not uncommon to use standard fiber glass filters in a filter grille as pre-filters. While this will make the expensive media filter last longer, it will add extra pressure drop to the filtration system. ItÕs a good idea to advise customers to change pre-filters often so the pressure drop doesnÕt get too high.

IÕve also seen air handlers installed in condominium mechanical closets. These usually have filters installed in the bottom of the air handler. If the mechanical closet has a solid door, thereÕs a filter grille installed in the wall of the closet. Often the filter in the wall is the one replaced, without anyone being aware of the filter in the air handler, which eventually gets very dirty and restrictive. Make sure homeowners are aware of ALL filters in the system.

If the filter is installed in the duct, be careful when installing it near a fitting such as an elbow. The air will not make the turn at the throat as well as at the outside of the elbow. Therefore the velocity through the filter wonÕt be uniform and the filter will experience uneven loading with a high pressure drop. This can also occur where the duct suddenly increases or decreases in size.

In some cases, newer electrostatic filters can be installed in place of older electrostatic filters. IÕve seen newer models advertised with pressure drops at rated air flow (usually 300 FPM) of 0. 1 in wg or less. This is a tremendous improvement over some of the filters listed in Table 1.

According to the fan laws, in order to increase air flow in an existing duct system, without replacing the blower, the blower RPM varies directly with the increase in air flow. The increase of static pressure varies according to the square of the air flow increase. The blower horsepower varies according to the cube of the air flow increase. In other words, if you increase air flow 10%, the RPM of the blower must be increased 10%, static pressure must be increased 21%, and the blower horsepower must be increased 33%. Sometimes itÕs necessary to improve the duct system or install a more efficient and powerful blower to compensate for increased pressure drop across the air filter.

Electrostatic and pleated filters are tremendous improvements over standard fiber glass air filters for particulate removal. They can be a real lifesaver for people with allergies. I have customers who used to hate to run their air conditioners because of the dust they stirred up. Now they hate to turn them off, because they keep the air in their houses so much cleaner. The downside of these filters is that they can seriously reduce airflow, reducing system capacity and efficiency. In extreme cases, they can also cause compressor failure due to liquid slugging.

Measure the Flow

By measuring airflow before installing high efficiency filters, you can determine if itÕs appropriate to install them at all. In some instances, you can even justify a new fumace or indoor blower or duct modifications if the need for better IAQ is great enough. You may also need to seal the return ducts and test them to verify reduced duct leakage to provide true IAQ improvement. Properly used, these filters can improve the health and comfort of your customers.

For the most part, the standard filters we tested were new and unused. The pleated filters were the same. The electrostatic filters were either new or cleaned thoroughly and dried just prior to testing. I am including a picture of our testing apparatus. You may want to do your own testing of the filters available in your area since the pressure drops of these filters varies so widely.

We used an old heat pump indoor unit with a sealed sheet metal box on the return. We installed an egg crate filter grille in the back of the box to hold the filter. An egg crate-type filter grille was used to minimize pressure drop in the test setup. We used an electronic balometer to measure air flow. We installed Magnahelic gauges with two different full-scale readings on the box to measure pressure drop. We installed a "blast gate" slide damper in the discharge duct of the blower to adjust air flows. With the air filter installed in the filter grille, we mounted the flow hood on the grille and adjusted the slide damper for 800 CFM. We read the total pressure drop on the Magnahelic gauges and deducted .03-in. wg for the hood and filter grille. This gave us the pressure drop for the filter.

Note that we tested some dirty filters as well. The standard fiber glass filter we tested was so dirty that we could probably have started seedlings in it. When we took it out of a customerÕs system, it had been sucked out of the filter grille into the return duct. The other dirty filters were from our shop. We have two return filter grilles in our system. We intentionally let one filter get very dirty so we could measure the effects in our filter tester.

We started to test air filter pressure drop after I read an article on duct leakage and its effect on IAQ. WeÕve used flow hoods for years to measure air flow in houses where we were going to do change outs. We also measure the air flow after the system changeout is complete, since itÕs so important in high humidity areas such as ours.

The 6-in. thick, duct-mounted pleated filter was tested at various air flows to show the relationship between air velocity and pressure drop. We fastened it to the back of the filter grille, then adjusted the air flow with the slide damper. Our test data closely followed the manufacturerÕs published data. When the air flow doubled from 600 CFM to 1,200 CFM, the pressure drop more than doubled from 0.6 in. wg to 0.145 wg. Increasing air flow to 1,400 CFM further increased the pressure drop flow to 0.2 in., a 33% increase in pressure drop for a 17% increase in velocity and air flow.

Kevin OÕNeill, CM,is the HVAC service manager at Carolina Cooling and Plumbing, Surfside Beach, SC. He is a certificate member of RSES and has 22 years of HVAC industry experience. OÕNeill can be reached by calling 843/238-5805.