Using Variable Speed Motors and Static Pressure

Jan. 1, 2008
A number of years ago when variable speed fans began to become popular, their primary benefit was promoted to be their ability to overcome airflow issues in poorly performing duct systems.

A number of years ago when variable speed fans began to become popular, their primary benefit was promoted to be their ability to overcome airflow issues in poorly performing duct systems.

Fortunately, I learned from the first one I installed that they weren’t built to withstand total external static pressures over 1.2-in. — we received three replacement motors in the first year. Finally we fixed the duct system and saved our customer inconvenience, and our friendly manufacturer additional warranty expense.

Most residential variable speed motors have the capacity to deliver 400 CFM per ton at static pressure well over 1-in. of pressure — but not for long. That magic smoke that makes the motors work leaks out prematurely.

Typically, each manufacturer determines the operating range of their variable speed motors by the circuitry they choose to connect with the motors they add to their equipment. Different fan performance tables are then created and published by each manufacturer to reflect what their variable speed motor package will deliver in the field.

Years ago we collected and averaged the fan performance characteristics of variable speed fans. Then we published, as a service tool that could be used in a pinch, a generic 1.0-in. variable speed fan performance table to interpret fan CFM. We recently polled a number of manufactures and detected a significant shift in the published engineering data, and an even greater shift in how we’re being instructed to design, install and apply these fans in the field.

The New Recommended Maximum
While many variable speed fans can deliver 400 at total external static pressure exceeding 1-in., allowing them to operate under those conditions may void your motor warranty. Check the engineering data from your manufacturer and then measure operating static pressure to be certain you won’t toast your motors. Many manufacturers are now recommending their variable speed motors be operated at .50-in. maximum. Some may permit pressures of .80-in. or more.

Although we can get 400 CFM per ton during an air balance test while setting up the system, we now have been instructed and have learned by sad experience that running most variable speed fan near or above 1-in. of total external static pressure will terminate in excessive warranty expense for all parties involved and in most cases it is us, the installing contractor, that will end up with egg on our face.

The second result of what we learned is that hopefully each of us will try to design and install systems that will keep these fans operating under conditions that will allow them to survive. Hopefully we can sell thousands of duct renovations at handsome profit margins rater than turning in more warranty claims. Duct renovations are the best options for all parties involved.

In most cases, duct renovation work should include the addition of large return ducts designed to relieve fan pressure. Also the addition of more filter surface area to relieve filter restrictions would be greatly beneficial to variable speed motor performance. Both of these issues should be evaluated in each duct renovation.

We’re anxious to hear of the equipment manufacturers connecting the dots to open communication with the filter manufacturers to solve the filter pressure issue as a contributor to poor system performance as well as contributing to the short lives of many variable speed motors.

Several manufactures are recommending selecting system components and designing and installing duct systems aimed at a maximum total external static pressure of .50”-in. and reserving the other .30-in. of available pressure as a reserve. This will reduce the operating costs of the motor significantly and deliver a better performing system.

Unfortunately ARI still only requires a static pressure of .20-in. during their testing on many units in order to reduce the blower motor watts to increase apparent equipment efficiency. This never ending pressure to produce "higher efficiency equipment" will continue to deliver a stream of equipment with super high ratings that will never deliver adequate airflow in the field. There ought to be a law.

So select your equipment of choice carefully and be sure to renovate those duct systems to provide nice low operating pressures that will ensure your customers the high performing, no worry systems that you’ve promised them.

Pay more attention to your design and installations and check out your manufacturer’s bulletins and website information about their variable speed fan packages.

Rob "Doc" Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in a free generic .80-in. Variable Speed Fan Table, contact Doc at [email protected] or call him at 800/633-7058. Go to NCI's website at for free information, technical articles and downloads.

About the Author

Rob 'Doc' Falke | President

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