Installing a New Blower Assembly to Increase Airflow

Installing a New Blower Assembly to Increase Airflow

This is the fourth in a series of HotMail articles by Doc Falke focusing on specific techniques designed to test, diagnose and improve the performance and efficiency of residential HVAC systems.

There are several high-capacity variable speed/variable torque replacement blower assemblies on the market today that may provide an invaluable solution when you need to increase system airflow. Let’s take a look at replacement blower assembly basics, how to do necessary testing and diagnostics, and how to select the right blower. We’ll also consider what additional changes may be needed in the system so you don’t create a whole new set of problems as you replace a system’s blower assembly.

Blower Assembly Basics

As the practice of measuring system airflow increases, more technicians and salespeople are discovering low airflow problems in the systems they service and sell. Best industry practices require an HVAC or energy professional to provide their customer’s options to solve this problem.

One solution is to replace the equipment’s blower assembly with an increased capacity blower that’ll move the needed airflow. This will allow the operating efficiency of the system to increase to a level that will delight your customer and make them loyal to you forever.

Most of the time an Electronically Commutated Motor (ECM), will be included with your new blower assembly and will be your best choice. There are two types of motors to consider, a constant torque motor and a variable speed motor. In most cases, select the constant torque motor. These motors are available with the wiring and instructions needed to enable any savvy tech to complete the job. Most of the manufacturer’s provide ample, well-versed tech support as well. You may need the help, especially on your first one.

The most common reason additional blower capacity is needed, is to overcome the restriction of the duct system, the coil or the filter, in order to move the airflow required to allow the system to operate as specified. More often than not, it’s a combination of the resistance of all three of these components that’s restricting airflow.

Your job is to inspect and test the system to determine if replacing the blower assembly is the best solution.

How to Know if You Need More Fan Capacity?

Unfortunately many contractors may purchase equipment containing inadequate blowers when installing replacement equipment. They fall into the race to have the lowest price and buy the cheapest equipment available. Many do not take the time to consider the sizing, installation quality and resistance to airflow of the existing duct system, coil and filter. You need to understand their impacts on the replacement equipment.

The way to determine the need for additional blower capacity is to measure airflow and static pressures under live operating conditions with the system calling for maximum airflow. Compare these numbers to the measured airflow and pressures of the equipment performance specifications. Although we don’t have the space to describe this testing, you can get procedures for this testing. See the end of this article for more details.

If airflow is less than 90% of the required airflow, it should be increased to improve system performance. If airflow is less than 80% of the designer’s required airflow, improving it is mandatory for any respectable technician.

How to Select a Replacement Blower Assembly

If testing and inspection reveals the need for increasing blower capacity, determining the right blower type and size is essential.

First, determine the type of blower needed to increase system airflow. A typical blower assembly with a Permanent Split Capacitor (PSC) motor is generally the multi speed blower used in many lower priced units. These motors simply react to increased resistance by lowering airflow as they react to the total external static pressure they operate under. A replacement PSC motor will only solve the problem if a larger blower motor is selected with significantly higher horsepower.

A blower that’s a constant torque ECM will typically be your best selection for several reasons. These motors are generally more efficient and also include a wider range of operation to assure the best results. Also, these motors may not ramp up excessively and should use less power if you follow a few rules referred to below.

At this point let me address one issue that often comes up. Is it acceptable to replace a blower assembly? Well, we replace blower motors every day, and other components too. So, are we voiding the equipment manufacturer’s warranty and intent? That’s up for you to decide. Many justify it by the measured increase in performance that the original equipment is now able to produce with the new blower assembly. Who can argue with that?

Second, size the new blower. Size a new blower based on the required system airflow and pressure that you can calculate under the new system operating conditions when the fan is replaced. Say you have a 3 ton system requiring 1200 CFM in cooling mode. Measure the system’s current operating airflow, and say it’s 1000 CFM. Next, measure the system’s current operating total external static pressure. Then using Fan Law Two, calculate what the static pressure will be once the system is operating at required airflow. Here’s what the math looks like:

When airflow is adjusted to 1200 CFM, the operating static pressure of the system will be .78-in.

Finally, select a replacement blower assembly that will move 1200 CFM at .78-in. of total external static pressure. Be certain the new fan will fit in the available space in the blower compartment and verify electrical and thermostat requirements.

Select a fan where this operating point is mid-way in the fan curve, and check the watt draw under these conditions to assure there won’t be an increase in electrical cost that may offset the savings by the improvement in the system operating efficiency. Some fans will ramp up until required airflow is achieved without regard for the cost of the excessive watts required to do the job. This problem is yours to prevent.

This Decision Must Consider the Duct System

Now that you know how to do necessary testing and diagnostics and select the right blower let’s consider what system changes you can make to avoid changing the blower assembly at all. You can see changing the blower assembly is often a last resort.

In most cases other changes can be made in the system that are easier, cost less money and will provide a better long term solution.

For example, you can add an additional return duct to the system. This will significantly reduce total external static pressure and increase airflow. You can inspect and test the duct system and remove installation defects such as kinked duct, loose duct liner, restrictive transitions and perhaps add a couple of badly needed supply ducts to those uncomfortable rooms lacking airflow.

Restrictive “high efficiency” filters and coils are often a cause of system inefficiencies. Measure the pressure drop over these components and verify if the fan can afford such restrictive coils and filters. (These subjects were each discussed earlier in this HotMail series.) If needed, the best solution may include replacing such a restrictive coil or modifying the filters in the system to reduce their resistance to airflow.

Hopefully, you are beginning to see that there’s a solution for every poorly performing system. In every situation, you’re the judge and the jury charged with the job of gathering the information, carefully weighing the evidence and prescribing the best solution possible to each poorly performing HVAC system. If you don’t diagnose the situation and solve the problem, who will?

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 free procedure describing how you can plot fan airflow 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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