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Millis, MA, USA (800) 553-3554

Common Ductwork Dos + Don’ts


Many job sites don't understand the importance of having an efficient ductwork design for a temporary industrial heater. Adequately designed ductwork can directly impact air temperature and quality, energy consumption, and the system's lifespan. Read on to learn more about the do’s and don’ts to follow when installing ductwork on a commercial job site.

Duct Work Do's:



Use the appropriate sized duct

One of the common mistakes we see on job sites is when the duct is reduced without consideration for what it does to the heater or leaving at least the same square area for the airflow to escape. Since the safeties in our heaters include the ability to prove airflow, if airflow can’t be determined, the main control of the heater will not open the gas valves or continue through the ignition process. The control will then lock out to protect the heater.  

We know what you’re thinking…it sounds like it is not a big deal to reduce duct sizing from 32” down to 24”, but it’s crucial to do the math and keep the same size duct that the cut sheet on the heater calls for. Sound complicated? Here’s an example to help you out: 


If you have to reduce due to window size or a similar situation, most people will look at a 32” to 24” change as a 25% reduction. However, the volume of a circle is what you need to base your calculations on, and the correct formula needs to be used. So, when you do the calculation for a 32” duct (16 x 16 x 3.14159 = 804.24 square inches) versus the 24” duct (12 x 12 x 3.14159 = 452.38), you’re actually reducing the ability of the airflow by 44% (much higher than the 25% you initially guessed).

In addition, if you do need to reduce duct for any reason, keep at least the first outlet at the same diameter as specified for use with the heater, and do not block it off at any time. Then you can reduce the size beyond that. That allows the airflow to escape and won’t risk tripping the airflow switch. 


Another option is to split the duct into two outlets that the square area matches or exceeds in terms of the original size. For example, using a Y fitting with a 32” duct coming into it and two 24” ducts coming out exceeds the area of the original circle. Keep a full trunk line at the same diameter but have multiple reduced outlets. An example of this is when you have a 32” diameter duct, but the openings in the building are smaller. In this scenario, you can have the reducer come off one of our lateral T fittings at the smaller size if you calculate the areas, and when combined, they are greater or equal to the 804 square inches.

So, why do most companies end up reducing the duct size? Most of the time, it comes down to cost and availability. It costs less to buy the smaller duct, and it is easier to work with, but what happens is you won’t be able to take full advantage of the heater you are paying for. Even if the heater starts initially, it now has the potential to lock out at the wrong time (typically at night when no one is around and then the customer comes into a cold site).

Another common problem is that the heater has another safety that can be affected. Because the heat is being held back due to the back pressure of the duct, the high limit can also be tripped. This can also cause a lock out and leave you in the cold.

Lastly, the combustion process will also not pull as much air through the burner, so the CO levels that heater discharges will be higher.


Use the correct fittings to split airflow


Our heaters use high velocity to discharge the airflow. Most tin knockers are accustomed to low airflow systems and use the same design for a duct with a squared-off duct and tee fittings that look like a capital T. Unless the T has turning vanes inside, the airflow slams into the “brick wall” at a high velocity, and the backpressure can shut down the unit. Turning the T fitting sideways will allow it to discharge on each floor on a duct riser. Again without a scoop or turning vane, the airflow velocity will push by the floor and keep rising. That usually means the top floor gets most of the heat, making it too warm while the lower floors are colder. 

Check with BABFAR on your duct design to make sure it will work properly with our heaters and, at the same time, distribute the airflow evenly on your project. When we split airflow, we use our Y fittings with two 45 degree elbows to do a sweeping split to divert the airflow. In addition, we use a lateral T fitting to discharge the duct to one side while continuing airflow to the next fitting.



Use Flex Duct

The use of incorrect flex duct is one of the biggest issues we face. It sounds convenient to use flex duct, but the problems start with the kind of flex often used, because most of the time it is not correct. Any flex used with our units has to be rated for the volume of air and the temperature of the heaters. The safeties on the heater are set for 310 degrees, and the duct has to be rated for 350 degrees. And with velocities ranging from 2000 to 21,000 cfm, the duct has to be designed to work with both.

The cost of that flex and availability often has people purchasing the wrong thing. They end up with insulated ducts that are not rated for high temperatures. Because of the high temperature, the insulated duct tends to bake and disintegrate from the inside, often causing restrictions that shut the heater down. And if placed too close to the discharge of the heater, it can catch fire. The ducts that are not rated for high temperatures often start to bake and melt and off-gas inside the area being heated. All of these issues can be avoided by either using rigid spiral or by ordering the correct duct. Due to today’s supply chain issues, the lead time is about 12 weeks and costs $15 per foot for a 24” duct and $15.60 per foot for a 32” duct.


Note: The yellow flex is rated for the temperature and airflow. The sagging insulated flex is not.



Use Dampers or block off discharge on floors


We recommend that you never use dampers with any BABFAR heaters. The use of a damper allows for a duct to be restricted and the heater to shut off. This often happens on multi-story projects with discharges on several floors. Inevitably someone on each floor shuts the damper, and like a domino effect, each floor gets too hot, closing the damper until the unit shuts itself down or overheats. 

 When a proper plan and duct design is created with our sales team, you can ensure even heating and that you will never need dampers.


BABFAR heaters are some of the most powerful on the market and put out highly reliable temperatures at a high air volume. When you follow proper duct suggestions and design with your BABFAR sales representative, you can capitalize on the heater you rented. Plus, you will have peace of mind that the heater operation will be reliable and not shut down due to something that could have been prevented.

For more information on ducts and materials, contact BABFAR today.