Duct Dynasty: Taking the Guesswork Out of Fan Speed Settings
The arithmetic behind proper heating and cooling fan speeds
Contrary to popular belief, fan speed settings aren’t always correct from the factory. It’s been a long-standing assumption that you should use the high-speed tap for cooling and low-speed tap for heating when using a constant-speed fan. The same type of assumption leads users to believe their variable-speed dip switches are correctly preset from the factory.
This type of guesswork often leads to customer complaints, poor system performance, and frustrating callbacks that can be avoided with a little bit of upfront planning, preparation, and testing. Let’s look at how to set fan airflow based on the needs of the equipment in both cooling and heating mode.
INSPECT THE EQUIPMENT
Before you decide which fan speed(s) to use, you should inspect the system and gather some essential information from the equipment. Begin by determining the equipment’s rated capacity (in Btu) from the model numbers found on the nameplates of both the indoor and outdoor units.
From the outdoor unit nameplate data, determine how many nominal tons of cooling the condenser or heat pump is rated for. Be sure to determine the outdoor tonnage rating from the outdoor unit model number and not the indoor coil model number. Many indoor coils are rated to handle multiple sizes of outdoor units.
If the indoor equipment is a gas furnace, determine the Btu input capacity and efficiency of the furnace. There are different fan airflow requirements for natural-draft, induced-draft, 80 percent-plus, and 90 percent-plus condensing furnaces.
When you have a gas furnace with straight cooling, there is a high likelihood that you’ll use two different fan speeds for cooling and heating. If the system you’re setting up is a heat pump, you’ll use the same fan speed for cooling and heating mode operation.
Once you’ve documented the size of the outdoor and indoor equipment, you’ll need to determine the existing fan’s speed taps or dip switch settings and how they’re currently setup. Record the current fan speed settings and the mode of operation.
For constant-speed, permanent split capacitor (PSC) fans, this reading will be determined through fan speed taps. For variable-speed fans, this will be determined by the dip switch settings on the control board. You’ll need this information to determine if the existing fan speed settings are correct.
As you document the fan speed settings that are being used, inspect the cleanliness of the equipment. This includes the condition of the fan, filter, and coil. If these components are dirty, they should be corrected before you can expect to achieve the desired amount of airflow. Fix these obvious defects before attempting to set the fan speed. If fan speed is being set on a new install, it’s still a good idea to perform this inspection, just to be safe.
Another critical piece of information you’ll need is the manufacturer’s fan performance tables for the equipment you’re working on. Many equipment manufacturers make this information easy to access inside the installation instructions for the indoor equipment. Other manufacturer’s will require you to obtain this information directly through their websites.
Without the fan performance tables, you can only guess which fan speed to choose. Be aware that many installation instructions have multiple fan performance tables for the entire range of equipment sizes offered in that particular make of equipment.
Refer to the model number of the furnace to assure you’re using the correct fan table. Some manufacturers require additional information not included in the model number, such as fan motor horsepower and blower wheel dimensions.
Once you’ve completed the initial inspection, it’s time to figure out how much airflow you need moving through the equipment for each mode of operation. You won’t always be able to hit these target values exactly, and that’s OK. The trick is to make sure you’re close to the required airflow.
For cooling and heat pump airflow, let’s stick with the industry standard of 400 cfm per ton. While this is the most widely used airflow value for cooling requirements, there are many parts of the country that will need more or less airflow per ton to achieve comfortable conditions. These airflow ranges can vary from 350 cfm per ton up to 600 cfm per ton. Be sure you know which rules apply for your location.
From the equipment nameplate data, determine the nominal tons of cooling the system is rated for and multiply this value times the required cfm per ton. Let’s say you have a 3-ton rated condenser.
The equation is 3 ton x 400 cfm per ton = 1,200 cfm of required fan airflow for cooling or a heat pump.
This is the value you’ll use when figuring out which cooling speed is needed as you compare it with the fan performance table.
Gas furnace heating airflow is based per 10,000 Btu of input just as cooling is determined on a per ton basis.
Required gas heating airflows are as follows:
• Natural draft furnaces need 100 cfm per 10,000 Btu of input;
• 80 percent-plus induced draft furnaces need 130 cfm per 10,000 Btu of input; and
• 90 percent-plus condensing furnaces need 150 cfm per 10,000 Btu of input.
Remember, just as with cooling airflow requirements, this equation will get you within a target range on the fan performance table.
To calculate your needed fan airflow for the heating mode of operation, divide the furnace’s rated input by 10,000. This will give you the necessary multiplier to obtain the cfm per 10,000 Btu for the gas furnace type you’re setting up.
Let’s say you have a 60,000-Btu input, 90 percent-plus furnace. A 90 percent-plus furnace needs 150 cfm per 10,000 Btu of input. Divide the rated Btu input by 10,000. This value is then multiplied by 150 cfm to obtain the required airflow for the furnace in heating operation.
Example: Rated Btu Input = 60,000 / 10,000 = 6 x 150 = 900 cfm of required furnace airflow. Use this airflow value when determining your heating speed setting.
Note: If this example furnace is paired with a 2.5-ton condenser, there’s a pretty good chance you’ll be using the same fan speed for heating and cooling.
PLOT FAN AIRFLOW
Once you’ve determined how much airflow the fan needs to move for cooling and heating operation, it’s time to get the fan performance table and the needed test equipment to measure static pressure. The majority of fans you’ll encounter residentially are rated at a maximum total external static pressure of 0.5 inch of water column (wc). However, the average residential system with a 0.5 inch fan operates at more than 0.8 inch of pressure, which will result in far less airflow than intended.
Because of this, you’ll need to verify the amount of airflow the fan is moving instead of simply assuming it. To accomplish this, you’ll need to add the measurement of total external static pressure to the existing information you’ve gathered.
To measure total external static pressure, you’ll first need to install test ports (drill holes) for your measurements. Once obtained, this pressure measurement will be used to plot fan airflow on the manufacturer’s fan tables so you can see how much air the fan is really moving.
Be careful when measuring total external static pressure. This often means drilling into equipment and other areas that could have some bad consequences. Poor planning when drilling has resulted in many coil leaks and assorted misfortunes. Use a drill bit sheath so the drill bit penetrates the metal only one-eighth of an inch or so and doesn’t get pulled into the equipment to pierce a coil or heat exchanger.
Once the test ports are installed, measure actual total external static pressure. On the fan performance table for the equipment you’re working on, locate the fan speed currently being used and line it up to the measured system total external static pressure.
The point where these two values intersect is the amount of fan airflow that is being moved through the equipment. Hopefully, you can adjust the fan speed to obtain the right amount of fan airflow if you don’t already have it.
Perform this step for both the heating and cooling mode of operation to assure you’re using the correct fan speed settings. If the system pressure is too high and you can’t obtain adequate fan airflow, additional pressure testing will be needed to determine further system repairs that will be needed to optimize the performance of the system.
Publication date: 12/21/2015