How to Balance a Variable Speed HVAC System

Balancing a variable speed fan poses a significant challenge because the fan speed and airflow may constantly change as the balancing hood is placed on each grille. Let’s take a closer look at how to accurately test and balance these systems.

Variable speed blowers were introduced to the residential HVAC market in the mid 1980’s. As efficiency ratings climbed, Electronically Communicated Motors (ECM) became a key factor in achieving higher efficiencies because of the lower watt consumption they can deliver.

Originally these motors were pitched as the solution to poor quality duct systems. However, it was soon discovered that ECM motors were suffering short life cycles as the demands that undersized duct systems placed on them was realized. Also field watt consumption numbers were up to four times higher in the field than reported in the laboratory as residential field static pressures were sometimes found to exceed 1.5-in. w.c.

As coil and filter pressure drops increased across the industry, more powerful and efficient ECM motors have become an essential component in a well operating residential HVAC system.

The Balancing Challenge

In some instances, when a balancing hood is placed over higher flow registers, the hood actually exerts a back pressure on the airflow moving through the register. When the fan senses this change in pressure, it does what it is designed to do and ramps up the fan RPM to maintain the programmed airflow. This is an excellent and valuable feature of an ECM fan, but presents a constantly moving target for an air balancer trying to balance a grille’s airflow.

The Balancing Solution

When using a commercial balancing hood with a throat area exceeding 1.2 sq.-ft., there is little, if any back pressure caused by the hood when reading registers below 200 CFM. In this case you will find little, if any fan speed change during testing.

When reading higher grille airflows from a variable speed fan when the hood is placed on the grille, the fan will sense the change in static pressure and respond by increasing RPM. This will result in an increase in fan airflow.

The solution is to simply leave the hood in place on the register or grille for 10 to 15 seconds and allow the airflow to stabilize. After the airflow has stabilized, take and record the airflow reading as normal. Then move on to the next grille.

Sounds too simple, doesn’t it? The time delay simply allows the fan to do its job and respond to the change in static pressure. Delaying taking the reading until the fan has done its job is the balancing solution.

Damper Adjustment Challenge

Adjusting balancing dampers when testing and adjusting a variable speed fan poses a similar set of conditions that must be considered to effectively balance a system.

First, let’s take a look at what happens when balancing a constant speed motor. Say we have a register that’s delivering 200 CFM, but only 110 was specified. As the damper to that register is closed down to reduce airflow, static pressure at the fan is increased and fan airflow decreases. Of the 90 CFM that was diverted by closing the damper, perhaps 30 was lost due to the increase in static pressure and only 60 CFM is rerouted to other registers.

When balancing a variable speed fan under the same conditions, the results are different. If the register is getting 200 CFM, and the damper is closed to 110, the static pressure changes, and the fan ramps up to deliver the same amount of total airflow as before the damper was closed. So the full 90 CFM moves down the duct to other registers. As the next damper is closed the fan will ramp up again.

Damper Adjustment Solution

To effectively set dampers on a variable speed fan, set the fan airflow as normal. Confirm the delivered airflow by a clean traverse of the supply or return duct near the fan.

With an ECM motor, as each damper is adjusted, allow 10 to 15 seconds for the fan to stabilize. Then place the hood on the register, and allow another 10 to 15 seconds for the airflow and reading to stabilize and take the reading. Adjust each damper to required airflow and do not expect or provide for a reduction in total airflow losses due to the normal increase in fan static pressure.

In other words, slow down and let the variable speed fan do its job. By understanding that function of the ECM motor, balancing will become easier. Those that fail to understand this function, or don’t give the fan time to respond will go nuts chasing an ever moving target as airflow and static pressure constantly changes. As always, there is more to the story, but this should help you begin to measure and balance variable speed fans.

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company with technical and business level membership organizations. If you're an HVAC contractor or technician interested in a Variable Speed Balancing 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.

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