Home Run vs Trunk & Branch Duct Layouts: Which is Better?
Ductwork is a key component to a home ventilation system. While it is out of sight and out of mind once the building is finished, homeowner comfort and indoor air quality are dependent on it being properly designed, sized, and installed.
Efficient and effective home ventilation systems require a significant amount of planning and designing, particularly in high-performance new construction. Good ductwork design and layout, whether it’s ‘trunk and branch’ or ‘home run’, includes an understanding of balanced air pressure, the roles that friction and turbulence play in affecting airflow through the ductwork system, and the impact they have on the performance of a ventilation system.
Pressure, Friction, and Turbulence — Pushing Against the Tide
Air and water have similar characteristics when it comes to movement. You can see the way that water reacts to obstacles as it flows across and around them, and how it behaves when a large flow is contained in a narrow opening, or when the tide changes and there is an opposing flow. Air reacts to obstacles and restrictions in very similar patterns, as can be seen when there is a source of the smoke.
When air is being pushed by a fan into a ductwork system, it is being restricted. Any time the flow of air is restricted in a duct, it encounters what’s known as static pressure.
Static pressure, the resistance to airflow, is one of the most important factors in ductwork design and layout. To ensure the ventilation equipment is supplying or exhausting the correct amount of air from different areas of the house in an effective and quiet manner, the push of the air must be greater than the static pressure or no air will circulate through the ducts.
In a well-designed duct layout, air is moved through the ductwork and to its intended location by maintaining the correct static pressure. An effective duct design will keep pressure drops to a minimum so that there is adequate air flow at the end of the duct. If the ductwork is too big, the pressure of the air will drop and never arrive at its intended location. On the other hand, the more the air is restricted (through reducer fittings, and grilles for example), the lower the airflow through the duct, and not enough air will be moved.
Keeping friction and turbulence to a minimum are the designer’s key goals to maintain the correct static pressure for the system.
Friction is caused when the air flows across surfaces. When the inside surfaces of ducts are smooth there is very little friction but rough surfaces, or dirt, increase friction. Friction is also increased by how fast the air is moving through the duct. The more friction, the more loss of airspeed as it moves through the duct.
Turbulence is the result of ‘turning’ the direction the air is flowing, the most drastic example being when the duct direction takes a sharp turn in an elbow. Turbulence is the friction of air pushing against air and results in less air moving towards its final destination.
The result of friction and turbulence is a drop in pressure. In a ventilation system, the air is at it’s highest pressure when it is closest to the fan. The further down the duct it travels, the more it encounters friction and turbulence which impact the air flow.
Four common ductwork design mistakes that increase pressure, friction, and/or turbulence:
- Undersized ducts
- Runs that are too long
- Sharp Bends
- Poor air sealing
Undersized ducts cause friction. They restrict the flow of air, and, perversely, cause higher air velocity, which is a major cause of excessive noise in a ductwork system.
Duct runs that are too long require a higher power blower to get to the furthest reaches of a house. Not only does the actual length of each duct run to and from the ventilation unit need to be addressed, but each type of fitting comes with its own additional ‘equivalent’ length. Equivalent length is a way of calculating the amount of friction and turbulence that a fitting causes in any duct run. For example, each 90° elbow adds 50 feet of equivalent length to a duct run. The sum of actual and equivalent lengths impacts the air flow and the pressure needed for each supply or exhaust outlet, and so, determines the size of the ducts.
Sharp bends in ductwork cause turbulence, which slows air flow. A 30° or 45° turn causes less turbulence than a 90° elbow. An elbow with a gentle inside radius causes less turbulence than one that has a right angle. Straighter, shorter ductwork is always the best way to optimize the efficiency of an air distribution system.
Making sure the ducts are well-sealed reduces avoidable pressure losses in the system, improving the ventilation performance and occupant comfort. Poorly sealed ducts are one of the easiest fixes for improving home performance. Further, keeping ductwork inside the conditioned space improves the efficiency and effectiveness of a ventilation system.
What Layout Suits Your Needs?
There are a number of ways that ductwork for whole-house ventilation systems is laid out. The most common is the ‘branch and duct’ layout, but another option is the ‘home run’ layout.
A trunk and branch system has a large main supply and a large main exhaust duct (the trunks) that are connected to the ventilation unit itself. Smaller ducts (branches and runouts) are connected to the trunk and either feed fresh supply air from the trunk to the living space or pull moist, stale air from the living space and feed it into the exhaust trunk.
Trunk and branch, with a few variations, is a fairly typical way of laying out ducts for whole-home ventilation systems as well as for space heating/cooling systems.
A home run system has small ducts that run to the supply or exhaust grilles from a divided mixing box attached to the ventilation unit. Similar to a ‘home run’ plumbing system, this layout brings more precision and efficiency to the final distribution location. As there is no trunk duct, branch ducts or run outs, the short, direct duct runs maximize air flow.
While home run systems may use the more ducting material for installation there are many advantages. Home run systems can bring significant improvement to the efficiency and effectiveness of a ventilation system, as there are fewer fittings to cause turbulence, and a smaller number of possible air leakage locations.
You can design either type of layout for low-pressure drops, by using a bigger trunk duct right off the ventilation unit, or by splitting the airflow among several small home run ducts. One of the benefits of using smaller, smooth flexible ducts, like Zehnder’s, in a home run layout is that they are easier to fit into small spaces than branch ducts. This can result in faster installation times, with minimal chases and soffits required, and a better likelihood that the majority of the ducting can be contained within the conditioned space.
Zehnder provides high-efficiency ventilation units with a home-run duct system to provide high indoor air quality to the whole house. Installation is easy and quick with components that fit together easily.