Do Air Register Filters Make A Difference?

Every single year a new heating and air conditioning scam is invented.

This is one of them. Nowhere in any scientific study are you going to find that adding register filters in each room of your home is going to increase the quality of the air in your home. These simply cannot work the way they are advertised to and they certainly can damage your heating and air conditioning system.

Stay Away…Far Away From Vent Filters

Why They Don’t Work

Quality filters are rated as to their loading ability at the speed of the air that goes through them. The higher quality filters are rated on the MERV scale with 16 being the highest limit. Air cleaners can exceed the MERV scale, but I know of only 2 that do so, the American Standard AccuClean and the Trane CleanEffects.

After purchasing, examining and researching filter media and filter media I have found that not single one of the vent filter companies has a product that I have seen that meets any scientific testing on the MERV scale and even if they did, it wouldn’t be accurate.

That Is Pretty Harsh Criticism

In order to understand why they will not work you have to understand the dynamics of an air duct conveyance system. At the beginning of the air conveyance system we have a very large return air duct or return air chamber. The size of this chamber is to ensure adequate air deliver to the entire system and to ensure that the airflow across the air cleaner or filter is slow. Slower airflow across a filter allows that filter to work far better and have better dirt loading characteristics.

The ducts are large and they take off from the main trunk line at the furnace or air handler. They then progressively get smaller as branch lines are taken off to serve various rooms. This progressive size decrease keeps a constant steady static pressure in the ducting to ensure that long runs still get an adequate amount of the airflow.

At the end of the runs just before the registers we narrow the focus of the airflow to increase velocity. The reason for this velocity increase is to increase the throw of the air so that is circulates throughout the entire room creating and convection current. This convection current allows for a more uniform temperature throughout the room. If you have ever been in larger rooms in home and found the air to be somewhat stagnant it is usually from inadequate air throw from the register.
So at you furnace the air is moving slower allowing the filter to be more efficient catching smaller particles of dirt o stick to the filter. This clean air is then forced into the ducting and sped up through the register where those vent filters are supposed to be located. Faster air equals less cleaning. This means that if your ducting is sealed properly the filters will do nothing or very close to nothing to clean the air.

Testimonials Say they Work

You can get testimonials to say just about anything. Everyone in the trade knows what the very worst brand name of furnace there is on the market, yet that same furnace is a top seller. Some of those testimonials also claim the filters get dirty proving that they work. Yes, they can work if your ducting is open and sucking in dirt from the attic or you filter is bypassing. Both these situations need to addressed and should never be addressed with secondary filters.

They Can’t Make My Air Dirtier, Can They?

Yes they can. When you install a filter at the supply register you increase the static pressure of the ducting system. If the ducting system has holes, gaps, tears or poor connections (they all do) this will force more out of the ducting into the unconditioned space (think between floors, attic and crawl spaces). This increase in lost air creates a vacuum in the home and the vacuum is then satisfied by pulling in unconditioned and unfiltered air from cracks and crevices throughout the home as well as in and around doors, windows, light fixtures, electrical penetrations and plumbing penetrations making the air dirtier.

These Supply Register Filters Are Not Dangerous Are They?

They can be very dangerous. A standard furnace contains PSC (permanent split capacitor motor) that is rated at 0.5 inches of water column. A variable speed motor is rated at 0.8 inches of water column. These are approximate and they do vary a little bit. What this means is that these furnaces are designed to operate at a static pressure that is equivalent to pushing water ½ inch to 8/10’s of an inch up a tube of water. That is not very pressure. Most of the systems that we test are at or exceed those ratings by a slight amount without the added pressure of the secondary filters at the registers.

If you increase the static pressure at the registers by adding a vent filter you reduce the amount of airflow going over the heat exchanger when the furnace is operating. If you decrease the amount of air across the heat exchanger you increase the operating temperature of the furnace. Increasing the operating temperature of the furnace can cause high temperature limit problems and a premature cracking of the firebox or heat exchanger. It is dangerous…don’t do it.

What Can You Do then?

Give us a call and we will come to your home, print the free indoor evaluation coupon, and we will find out what is going on with your system and give you a written solution the problem that you having. It is usually not a big deal…just some simple maintenance.

Forget About Cleaning Your Ducts Cleaned.

Have them inspected and tested for leaks. We perform State Certified Duct Testing. Give us a call and we can help.

If you are having significant allergy problems when the furnace is running you can read a little more about it here.