Are Variable Speed Fans Becoming a Necessity?

The more HVAC performance test reports I review, the more I’m convinced that variable speed motors are becoming a necessity. As coil and filters are being built to more “efficient” specifications, their resistance to airflow has increased to the point where a typical constant speed fan can rarely deliver the required airflow to allow the system to operate properly.

Are constant speed .50-in. w.c. fans fading into the past? In the quest to improve our systems, the available system components have become so restrictive to airflow that the old standby .50-in. constant speed fans cannot move the required system airflow through the systems any longer. Without required airflow, system performance and efficiency are dropping steadily.

Residential Case Study

A contractor from Texas recently called me and was completely confused. He had installed a furnace, coil and filter package promoted by his local distributor. The super high efficiency equipment qualified for current federal tax credits, met the local utility rebate requirements and delivered “maximum” indoor air quality. Energy star stickers were plastered all over the equipment. The duct system passed the “tight duct pressurization testing” required by the local utility.

The equipment was installed and started up. Then the complaint calls began. First it was comfort issues then, the first utility bill arrived. The customer was totally irate and fit to be tied. His utility cost had doubled over the previous year.

The system total external static pressure measured .89-in., the coil pressure drop was .34-in., and the filter pressure drop was tested at .32-in. It turns out the fan included in the distributor package was only rated at .30-in. System airflow was only at 230 CFM per ton.

The solution; either the cooling coil and the filter have to go, or the furnace must be replaced with one having three times the fan capacity that will make the system operate as efficiently as the tax rebate and utility incentive promised.

The contractor had followed the advice of his distributor and complied with all the local utility requirements and had met the federal energy efficiency requirements to the tee. Who’s to blame?

Fan Selection
Good design practices require each of us to consider the fan capacity when selecting system components. Unfortunately the pressure drop or resistance to airflow of many of today’s filters and coils prohibits the use of fans with pressures less than .70-in. or .80-in. Very few constant speed fans are able to deliver the required airflow much above .50-in. of pressure.

Today’s average cooling coil has a rated pressure drop of around .30-in. If a .50-in. fan is selected, only .20-in. is left over for the filter and duct system. It’s not uncommon for a medium efficiency single filter to have a pressure drop of .20-in. This leaves no pressure available for the supply or return duct system. The result is often cooling airflow below 300 CFM per ton.

NCI pressure budgets call for 60% of the fan budget to be available for the coil and filter. 60% of a .50-in. fan is only .30-in. Most external cooling coils alone exceed that pressure drop. It seems the only choice is a well performing variable speed fan rated for at least .70-in or .80-in.

Duct System Sizing
A NCI study of well operating HVAC systems revealed another way of looking at fan capacity. The study evaluated duct system sizing and installation quality. We found well performing duct systems typically consume less than 40% of fan rated pressure. However, it was not uncommon to find poorly performing duct systems consuming up to 80% of fan rated pressure on .50-in. fans. Are there coil and filters out there with a combined pressure drop of less than 20% of a .50-in. fan or .10-in.? I don’t think so.

The net result was once again poor system performance. These are some of the primary reasons why the typical North American residential HVAC system delivers less than 60% of the equipment rated BTU into the building envelope.

In order to reduce the pressure drop of a duct system, additional duct capacity muse be added to the system. This is often done at the time of equipment change-out by installing an additional return duct to the system and by increasing the size of several supply ducts where needed.

This step is required even when utilizing the added capacity of a variable speed fan with greater pressure capacity.

Variable Speed Fan Limitations
The other warning is that although variable speed fans have increased pressure and airflow capacity, they are not a fix-all solution. Should a variable speed fan be installed on a system with a restrictive coil, filter and an undersized or poorly installed duct system, pressures will typically exceed the capacity of the fan.

When this happens, the fan ramp up to deliver the required airflow and basically operates at conditions beyond its capacity. Life expectancy is decreased significantly and the motor fails. Until it fails, the watt consumption of the fan is sometimes double of the watt rating the fan is rated for when the equipment efficiency is established.

This may turn your 18 SEER system into a 12 SEER system overnight. Check your equipment specifications. 300 Watts is more than the electrical consumption difference between these two systems. Understand the result of an improperly designed and installed system. What are we really doing out there?

There are many reasons why blower motors fail. The primary reason variable speed motors fail is that they ramp up to deliver the required system airflow under impossible field conditions.

How long will it be until manufacturers require evidence of operating static pressure to be below the fan rated pressure before they will warranty a cooked blower motor?

Is it time to take a closer look at the fans and system components we design and install? Sure it is. We are living with a series of slow changes over a long period of time that have brought us troubles or opportunity depending on how we meet the challenge.

Consider selecting and installing high capacity variable speed blower motors in your systems and then measure the installed operating conditions to verify that the actual pressure does not exceed the motor specifications.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company with technical and business level membership organizations. If you're an HVAC contractor or technician interested in a free report about static pressure budgets, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at for free information, articles and downloads.

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