Beware Of High Pressure Drops

By doing better diagnosis, you can uncover problems that are not immediately obvious and fix them on your first visit. This reduces callbacks, reduces the percentage of time traveling between calls, and may increase the amount of parts sold, which will increase the average amount of the service invoices. This will make you more profitable.

Where I have a problem is with his statement "If you ask any experienced technician, they'll tell you most breakdowns in HVAC systems occur because of the filth created by improper maintenance."

I have been an HVAC service technician since 1976, and I can tell you that most equipment failures are electrical in nature. While some electrical failures are caused by filth, many more are caused by simple old age. The biggest problem caused by dirt is a reduction in airflow, which reduces heat transfer. I have been measuring airflow in HVAC systems using an airflow hood since 1989. What I have found is that half of the systems that I measure have airflows of 200 cfm per ton or less!

In his article, Terry proclaims that by installing a particular high-efficiency air filter, you are doing your customer a favor while increasing your bottom line. I have measured the pressure drop across many high-efficiency air filters, and I find that they normally have pressure drops two to three times that of standard cheap fiberglass filters.

While I acknowledge that fiberglass filters are not very efficient at catching dirt, they are normally there to protect the equipment, not fix air quality problems that may or may not exist in any individual home.

I went to that filter's Web site. I noticed that on the first page of their filter section, they mention that pressure drop can be a problem with some other high-efficiency air filters.

Nowhere on the Web site do they say what the pressure drop across their filter is or how it will affect airflow. My experience is that high-efficiency filters reduce airflow in any residential system with a standard blower by 10 to 20 percent.

In addition, they recommend going three months between filter media changes. I have tested media and combination media/electronic air filters such as theirs after three months.

It is common for such filters to have a pressure drop of one and one-half to two and one-half times of their already high initial pressure drop. They are better at filtering the air than standard filters, but be-cause they catch more dirt, they get dirty faster than standard filters.

I feel it is irresponsible to recommend such filters without first measuring airflow. Then after the filter is installed, system airflow should be measured again. This is the only way you can be sure that you are really doing your customer a favor and not harming their comfort system.

The way I see it, you can let dirt (filth) degrade the performance of the HVAC system over a period of 10 to 15 years, or you can install a high-efficiency filter, and have it degrade immediately.

One exception is if a home has a variable-speed indoor blower. These blowers will normally overcome the high resistance of a properly sized high-efficiency media-type filter or combination media-electronic filter. These blowers will even speed up as the filter media loads up, delivering a fairly constant amount of air over the entire service life of the filter. Note, however, that most residential HVAC systems do not have variable-speed blowers.

The second article that I noticed was by Joanna Turpin, about decorative residential grilles ["More Customers Demand Stylish Grilles"]. If you examine many of the grilles shown in the pictures that accompany the article, you will notice the openings are smaller and fewer than most standard grilles. The openings are also less streamlined, potentially leading to more turbulence and a higher pressure drop. Again, this may lead to airflow problems.

Over the years, I have replaced many such grilles with standard grilles to improve airflow in individual rooms. I have found that changing to a more efficient grille with a lower pressure drop can increase the airflow 10 percent at any supply. The same can be true of the return grilles. Low-resistance grilles are also usually quieter than decorative grilles as well.

I am not saying that you shouldn't use decorative grilles, only that you should measure the airflow when you do so, and possibly run an oversized duct to an oversized boot if you decide to install a decorative grille.

Remember that air conditioning begins with air. Be careful that any change that you make to a customer's air conditioning system does not reduce the airflow and thus degrade their comfort system's performance!

Kevin V. O'Neill
Partner
O'Neill-Bagwell Cooling & Heating
Myrtle Beach, S.C.

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Publication date: 04/04/2005