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Duct Fitting Fundamentals

Survey after survey has shown that a large percentage of HVAC systems are not operating at peak efficiency due to insufficient indoor air flow. And, the chief reason for that condition is a problem with the supply air duct system. In many cases the heart of the problem is a leaky duct system that needs sealing, or, in some cases, the problem is a combination of a duct system problem along with negative factors regarding the building envelope.

In Figure One, we’re showing an elementary indoor air handling system and a simplified living space that should be able to handle air properly and maintain the desired comfort level in the building while operating efficiently.

Figure One

It’s a simple process to understand the fundamentals of an air handling system as we’re showing it here, with the trunk and branch duct, along with the register boxes and diffusers delivering air on the supply side of the system while the air intake, duct and plenum on the return side allow for air to get back to the air handler with a little resistance as possible. Taking a simple approach to understanding the process of moving air throughout the structure also requires a more refined understanding of a supply duct system. For example, consider the two velocity bonnets shown in Figure Two.

Figure Two

In both cases, the design shown here is considered to be sufficient to allow air to be propelled from the air handler into the duct system in the proper volume and velocity, the first step in ensuring proper air flow throughout the building. To put it simply, either of these two installations would be considered ‘good’. In Figure Three, though, our two examples illustrate a different point.

Figure Three

In this case, one of these two static take-offs would ensure that the duct system would be off to a good start, while the other may not. The illustration on the left is considered a ‘good’ design, while the one on the right is only ‘acceptable’.

The point to consider here is that an overall evaluation of the performance of this system may show a temperature drop across the indoor coil that may be slightly higher than it should be, or ‘acceptable’ (indicating that the volume of air is not a sufficient as it could be) while it should be ‘good’ in order to ensure an efficient operation of the system. And, the follow-up point to understanding this situation is that it may not take a major modification to the duct system in order to get to ‘good’. Consider the two illustrations shown below.

Figure Four

In Figure Four we’re showing a supply duct system that is in trouble due to the fact that the design called for a static take-off very close to the elbow. And, as you can see, there is a problem with turbulence in this elbow, resulting in an air flow problem. In Figure Five, the problem is solved with the addition of turning vanes that will promote proper air flow, not only in the first branch of the duct system, but also throughout the entire supply and return air duct system.

Figure Five


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