Four Steps to Determine the Correct Mixed Air Temperature
How to predict what happens when outside air is added to the mix
Have you ever been scared to add outside air to a residential system’s return air duct for ventilation purposes? If you’re like me, I worried the outside air would affect the return temperature too much and create uncomfortable conditions.
Then I found out that you can predict what will happen when return air and outside air mix together. This knowledge gave me the confidence to look differently at adding outside air for ventilation.
With a few measurements or design numbers and some quick math, you can also estimate the influence outside air has on a system. Let’s look at four simple steps you can use to determine this impact.
HOW AN OUTSIDE AIR INTAKE WORKS
For purposes of this article, I’ll use a typical residential outside air intake installation as an example. If you’re unfamiliar with this system, it uses an outdoor intake duct attached to the return duct system before the air filter.
The intake duct has a manual damper for airflow adjustment and either a barometric or motorized damper to close the duct when the fan is off. The intake duct is also insulated to prevent condensation on duct surfaces during extreme weather conditions.
When the thermostat calls for fan operation, the blower energizes, and the outside air intake damper opens. Outside air is brought in through the intake duct, where it mixes with return air that’s traveling back to the air-handling equipment from the conditioned space.
Once the air-handling equipment shuts off, the barometric or motorized damper closes the duct to prevent uncontrolled leakage into or out of the building.
STEP ONE: MEASURE AIRFLOW
To determine mixed-air temperature, you need two airflow measurements. They are:
- Fan airflow – plotted, traversed, or design
- Outside air intake airflow – traversed or design.
For field calculations, you must measure and record airflow from the air-handling equipment and the outside air intake using skills I’ve written about in past articles.
If you use design airflow to predict mixed air temperature, find the system’s required fan and outside airflow, and use these values. Once you know both airflow amounts, use them in the next step to determine the percentage of outside air entering the return duct.
STEP TWO: DETERMINE AIRFLOW PERCENTAGES
Since there are two airstreams mixing, you need to know the percentage of air coming from the return duct and outside air duct. Fan airflow is the baseline — it is the total or 100 percent of both airstreams when mixed together.
Once you know fan airflow, you can subtract the outside air amount from it to determine return duct airflow. This is the amount of air moving through the return ducts before any outside air is added into the duct system.
Let’s say you have a system where the fan is moving 1,200 cfm (cubic feet per minute) and it has an outside air intake bringing in 60 cfm. Return duct airflow in this system before mixing with outside air would be 1,140 cfm (1,200 cfm – 60 cfm = 1,140 cfm).
Now that you know how much air is coming from each location, determine the percentage of outside airflow compared to fan airflow. The remaining airflow is the percentage of return air moving through the duct system. Here’s our airflow values from the example:
- Fan Airflow = 1,200 cfm
- Outside Airflow = 60 cfm
- Return Duct Airflow = 1,140 cfm
Divide outside airflow by fan airflow to determine the percentage of outside air brought into the return.
60 cfm ÷ 1,200 cfm = 5 percent of fan airflow
This means that 5 percent of fan airflow (60 cfm) is coming from the outside air duct, while the remaining 9 percent (1,140 cfm) is entering the air-handling equipment through the return duct. This assumes an awesome duct system that doesn’t leak like crazy. Once your airflow percentages are calculated and recorded, you’re ready to measure dry bulb temperatures in the next step.
STEP THREE: MEASURE DRY BULB TEMPERATURES
The final measurements you’ll need are the dry bulb temperatures from the outside air intake and return duct. If you can, measure your temperatures at the same time for best results.
First, measure the return air dry bulb temperature about 18 inches before it mixes with outside air. This is often taken in the return trunk duct before the outside air intake duct connection.
Next, measure the outside air duct dry bulb air temperature. An easy location to gather this measurement is within the outside air duct. Don’t use the temperature from the weather app on your smartphone. You should already have a test port installed in this duct for the traverse measurement. You can also use it to obtain your dry bulb temperature reading.
Once you have the temperature readings, record them along with where you took the measurement. Let’s say you measure and record the following dry bulb temperature readings during heating operation:
- Outside air intake dry bulb temperature = 30.2°
- Return duct dry bulb temperature = 72.6°
Now it’s time to plug the airflow percentages and temperature values into the mixed air formula.
STEP FOUR: USE THE MIXED AIR FORMULA
First, match the appropriate airflow percentage with the temperature measured at that location. In our example, outside airflow is 60 cfm at 30.2° and return duct airflow is 1,140 cfm at 72.6°. Next, complete the formula.
Mixed air temperature = (O/A temperature x percent O/A cfm) + (R/A temperature x percent R/A cfm)
- O/A temperature = The outside air temperature within the outside air intake duct.
- Percent O/A cfm = The percentage of airflow from the outside air intake duct.
- R/A temperature = The return air temperature within the return duct (before outside air).
- Percent R/A cfm = The percentage of return airflow from the return duct.
Anytime you see parentheses, it means to do this part of the formula first. Here are the values and how they would appear in the formula.
- O/A Temperature = 30.2°
- Percent O/A CFM = 5 percent (60 cfm)
- R/A Temperature = 72.6°
- Percent R/A CFM = 95 percent (1,140 cfm)
Mixed air temperature = (30.2 x 5 percent) + (72.6 x 95 percent)
Mixed air temperature = 1.51 + 68.97 = 70.5
At these conditions, 70.5° is the estimated mixed air temperature you would have entering the equipment. The return temperature entering the equipment drops 2.1° under these conditions. To see what would happen as outdoor air temperature or airflow changes, adjust the values in the formula and recalculate.
Note: In this article, I’ve only used dry bulb temperature for the example. You can also use wet bulb, dewpoint, humidity, grains of moisture, or other air properties in this formula to predict changes.
The biggest reason why so many fear introducing outside air into their systems is a lack of concrete information obtained through measurement. Fear also comes from not being able to estimate the impacts after mixing occurs. Accounting for the source of these fears takes the edge off and replaces guesses and assumptions with real test results you can verify.
The best place to start is by gathering your measurements. Begin with fan airflow and then move to testing outside air intakes. Next, gather your temperature readings and then plug them into the formula. It’s amazing what you can discover with these simple tests and this formula.
If you don’t have the test instruments, give the formula a try with different airflow percentages and temperatures. See how far you can take a system before you get uncomfortable — it’s a great way to gauge your boundaries. Then, dedicate yourself to purchasing the right test instruments so you can test for and solve issues others miss.
In many cases, return duct leakage affects mixed air temperatures far more than bringing it in from a known location, so pay attention to your installations. Do the math on what a return duct leak bringing 50 percent of the return air in from a 140° attic will do to a cooling system.
My life changed the day I learned to estimate the outdoor air impact on a system. I hope this skill helps you better understand outside air intakes and the confidence to use them as part of your solutions.
If you’re an HVAC contractor or technician interested in learning more about system temperatures, contact me at firstname.lastname@example.org or call 800-633-7058. NCI’s website, www.nationalcomfortinstitute.com, is full of free technical articles and downloads to help you improve your professionalism and strengthen your company.
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