Air of Importance: A Study of Air Distribution Systems in Manufactured Homes
Sponsored residential energy research has advanced the understanding of building systems in general, but this research is not always transferable from site-built to factory-built housing. The scarcity of articles in the literature dealing with new manufactured homes demonstrates the disproportionate level of research investment being spent in this burgeoning home construction approach. The small amount of manufactured housing research that does exist has recently focused mainly on ways to improve the thermal envelope. This led, in part, to an increase in the thermal requirements for manufactured homes in the recent changes to the HUD Standards (October, 1994).
In the last several years, building scientists across the country have been quantifying the contribution made by air distribution systems to building efficiency loss in site -built homes. Estimates vary, but the average duct system appears to reduce overall system efficiency by 20 to 40 percent. This efficiency loss can have a multiplying effect with air-flow sensitive, compressor-based space conditioning systems such as air conditioners and heat pumps. Meanwhile, air distribution systems in new manufactured homes have received very little attention. This study is a step forward in trying to better understand air distribution in manufactured homes and their affect on overall system performance.
An incentive for the manufactured housing industry to volunteer to improve the quality of their product is found within the home buying market. The industry has demonstrated its willingness to invest in change if such a change improves ability to sell homes or decreases problems that require on-site visits from the retailer or factory to remedy. An in-house survey of 50 manufacturers and 50 retailers showed that a callback cost of 10% was budgeted by the average retailer, and 12% of field visits had to do with a variety of air distribution related issues. A goal of this project was to suggest changes in the air distribution system that will significantly affect these costs. Four technical bulletins were developed as part of this project to help educate retailers, manufacturers, and contractors about how to optimize air distribution performance and at the same time reduce callback expenses for their housing product.
This study analyzed the performance of air distribution systems in 24 manufactured homes in Alabama, Florida, New York, and North Carolina, and compared them with the results of 9 homes built to the more stringent MAP specifications and tested previously in the state of Washington. One of the objectives in this study was to go beyond just describing the magnitude of air distribution system loss and to uncover the source.
Understanding the root cause of air distribution system efficiency loss will help to identify the appropriate remedies. The manufactured housing industry has shown a increasing level of proactivity. For example, by the time the new HUD regulations went into effect in October 1994, many manufacturers were already providing as a standard or, at least as an option, homes with an equivalent level of thermal insulation. The information in this report should help those innovators in the industry improve the quality of their products.