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TRENDS

Home Energy Magazine Online July/August 1997

TRENDS

Ventilation Facts and Fallacies in Manufactured Homes

Window inlet vents are an alternate way to provide fresh air using an exhaust fan. When the fan is on, it slightly depressurizes the house, causing makeup air to enter through the window vents with minimal heat loss. However, n high winds, pressure changes can suck heat out of the vent on the downwind side of the home.

Manufactured housing has grown in size, features, popularity, and market; it currently comprises about 30% of all single-family housing in the United States. In some rural areas, these homes account for over 50% of single-family residences, with nearly 300,000 units being built each year.

In the past, manufactured homes have been known to be poor energy performers. But producers have recently improved the design of new homes, significantly reducing energy consumption.

Furnace-Based Systems

The 1976 U.S. Department of Housing and Urban Development (HUD) standards required that manufactured homes have either the equivalent of 4% of their floor area in operable windows or a mechanical ventilation system that would provide whole house ventilation. These requirements led many manufacturers to build homes with furnace-based ventilation systems, using a fresh-air duct from the outside to the furnace.

The 1994 revised HUD standards eliminated the option of operable windows or passive vents. They also required that the occupant be able to control the systems with an on/off switch. In spite of these changes, many manufacturers continue to rely on either electric or gas furnace-based systems to ventilate their homes. Many simply installed a timer on the furnace fan to meet the occupant-control requirement.

Fanning in the Northwest

Over the past seven years, electric utilities in the Pacific Northwest have been working to improve the performance of manufactured homes. One outcome of their efforts is that many new manufactured homes in the Northwest are today equipped with dedicated whole-house exhaust fans. Generally located in a hallway, these fans operate continuously, are relatively quiet (rated at 1.0 sone or less) and include a switch to turn the fan off during long periods of vacancy.

The continuous-exhaust fans use as little as 15 watts to run the fan motor. This is considerably less than the 60-75 watts typically used by the 1.5 sone combination bathroom/whole-house exhaust fans. The annual fan energy cost of the quieter continuous-exhaust system is less than $10 in most Pacific Northwest locations. This system relies on inlet vents in the window frames, which help to introduce outside air to living areas at a rate of about 5 CFM per vent with no noticeable cooling effects.

Which Consumes Less?

To better understand how furnace-based and continuous-exhaust fans contribute to energy performance in manufactured homes, the Bonneville Power Administration asked Ecotope in Seattle, Washington to compare annual operating costs of the two systems. Using a computer model, Ecotope evaluated the systems on the basis of their ability to ventilate an identical 1,500 ft², double-section home to an effective ventilation rate of 0.35 air changes per hour (ACH).

Table 1 Cost Comparison of Whole-House versus Furnace-Based Ventilation Systems in Manufactured Homes *
System	Annual Energy to Run System (kWh)	Annual Heating Energy for Introduced Ventilation Air (kWh)	Total Annual Energy Cost (Assuming $0.08/kWh)
Continuous exhaust fan	112	1,213	$106
Furnace-based system	2,848	835	$295
* Consumption estimates were calculated using Ecotope's SUNDAY 3.1 simulation software. Assumptions included: electric heating, 50% heat recovery from the furnace motor fan, 70°F setpoint, base infiltration at 0.2 ACH, and typical Seattle weather.

The modeling results show that even in an area with a mild climate and relatively low electric rates, the continuous-exhaust system costs less to operate than the furnace system. With an initial cost of under $100, the continuous exhaust system has a payback period of less than six months. In more extreme climates or in areas with higher energy costs, the payback is even quicker. In general, systems that depend on the furnace fan will cost $175-$300 more per year to operate than a continuous exhaust system.

Another drawback of the furnace-based system is that it requires a pressure relief damper to keep from pressurizing the home. Without this valve, furnace systems have been known to pressurize the home and drive moisture into the attic, causing condensation in cold and cool climates. HUD has recently allowed manufacturers to use the exhaust fan ducts to provide pressure relief. The irony of HUD's requirement is that furnace systems with supply ducts and no return ducts often create negative pressures inside the home due to duct leakage, which has caused some water heaters to backdraft.

The results clearly show that a continuous-exhaust ventilation system is more cost-effective than a furnace-based system. It offers homeowners efficient mechanical ventilation systems, maintains comfort, and reduces operating costs. These systems can thus allow manufactured homes to be made tighter without creating indoor air quality problems.

--Don Stevens and Mike Lubliner

Don Stevens is a ventilation consultant in Keyport, Washington. Mike Lubliner is an energy specialist with Washington State University Energy Extension.

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