Manufactured Housing Duct Systems: Guide to Best Practices
Best Practices for Duct Systems
A manufactured home can have top quality carpeting, a Jacuzzi, and Palladian windows. But if the air distribution system performs poorly, the customer will be uncomfortable — and dissatisfied. "Customers don't think about it because it's unseen," says Frank Kern, national sales manager for Heatilator. "But it's the artery system of the home."
Unfortunately, poorly designed or installed duct systems are one of the more common problems in the industry. The good news is that most things that go wrong can be avoided by following the "best practices" in a guide recently published by the Manufactured Housing Research Alliance. The audience includes everyone involved in the design, engineering, fabrication and installation of new manufactured homes.
"This is not a design or a comprehensive technical manual," says Kern, who served as project chair for publication of Manufactured Housing Duct Systems: Guide To Best Practices. "Instead, it is an easy-to-read guide that brings together design methods, installation practices and material recommendations gathered from the cumulative experience of leading industry experts. It helps industry professionals choose the duct system best suited to the climate, as well as the design and layout of the home, based on currently available technologies. The guide has to do with selecting the type of system, whether you're motivated by optimum performance or optimum economy, and making sure the system is properly designed and installed," says Kern.
The project grew out of an earlier MHRA initiative that provided guidelines for sizing cooling equipment. One of the assumptions made in developing recommendations for properly sized cooling equipment is efficient distribution of the conditioned air. Committee members were not at all confident that current practices for designing and installing duct systems were performing near optimum, a basic underlying assumption in selecting equipment capacity. As Kern notes, "Leaky ducts can double a homeowner's energy costs while leaving rooms drafty and uncomfortable. Ducts and supply registers that are sized or located incorrectly can leave cold spots and cause drafts. Oversized equipment is installed to compensate, further raising costs and reducing efficiency. It's something that we in the industry have grown used to," says Kern. "It made us come to grips with the need to improve duct system design and installation in the plant and in the field."
This single-volume source has chapters on design decisions, selecting the system type, planning the system, making the connections, and evaluating the system. Each point is clearly explained and supported with diagrams showing best practices and, in some cases, practices to avoid. Charts make it easy to compare the advantages and disadvantages of different systems: underfloor vs. overfloor; in-line air supplies vs. perimeter air supplies; straight duct vs. graduated duct, and all the various combinations. "For the most part, a graduated perimeter system is the optimum system for any home, but we're not saying if you install an in-line system you're wrong, says Kern. "The economics must be taken into account when selecting a duct system type and design. The guide offers strategies for optimizing performance regardless of the system selected."
Connections are Critical
From Kern's perspective, the most important section in the book is entitled "Making the Connections." Most experts agree that quality and performance are easily compromised by poor workmanship and inferior joinery materials in making the system connections, such as joining the boot to the truck duct intersection. "Even the best system designs will underperform if they're not properly connected," says Kern. As the guide suggests, following three simple rules aimed at tightening duct connections can dramatically improve system performance: (1) cut an accurate hole using a template; (2) fasten components mechanically; and, (3) seal leaks using a high grade tape or, preferably mastic.
Perhaps number two on the list of ways to optimize duct system performance is the proper placement of the supply registers, which is covered in the "Planning the System" section of the guide. "You need to make sure the heat is going to the space where it's needed, not where it's convenient to place a register," notes Kern.
Register Placement is the Key to Comfort
The Guide to Best Practices is filled with the collective wisdom and experience of many people directly and indirectly involved in the manufactured housing industry. Among the many issues covered by the guide are best practices for locating supply registers. The following excerpt offers a quick look at guidelines for locating registers for in-line and perimeter systems.
An in-line system's supply registers are located in the floor directly above the trunk duct near the home's center line (or the center of each section in a multi-section home). For in-line systems, the only layout decisions are where to place the registers along the trunk and how far to extend the trunk duct.
Perimeter systems allow more flexibility for placing registers. Ideally, perimeter registers should be placed under windows so that they direct air upwards and spread it along the wall. Wider registers blanket the wall better, and fan-shaped perimeter diffusers provide the most effective distribution. Avoid placing registers where they are likely to be blocked by furniture or window coverings. Recommended practices for perimeter register layout are illustrated in the figures below.
Homeowner comfort is closely linked to register location. When properly located, registers provide even temperatures throughout the home without blowing air directly on occupants. Register location is most important in extreme climates and in homes with a large heating and/or cooling demand. Large window areas, for example, can create hot or cold spots within a room that can be balanced, to some extent, by carefully locating registers near windows and exterior walls. Register location is less critical in well insulated, highly energy efficient homes.