This article was originally published in the January/February 1996 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online January/February 1996
Ananda Hartzel seals the connection between a boot and a supply register in a manufactured house.
Although the thermal shell of the typical mobile home has changed radically over the last 30 years, the air distribution system has not. The walls have gone from 2 x 2 framing with an R-4 fiberglass batt to 2 x 4 or 2 x 6 walls with a high-density R-21 batt (codes vary from state to state). The attics are now R-38 and the floors R-30. And the windows have a U-value of 0.4. Left off of this evolutionary train toward greater comfort and lower energy bills was the ductwork.
Applying diagnostically based duct-sealing techniques to mobile homes and manufactured homes has produced some interesting findings and creative solutions to duct leakage problems. Researchers and practitioners throughout the country, including Rob DeSoto, Rick Hanger, John Krigger, Cal Steiner, and scientists at the National Renewable Energy Laboratory (NREL), have studied duct leakage problems and found ways to apply pressure diagnostic techniques in mobile homes.
|Table 1: The effect of duct leakage on Delivered Heat Efficiency|
|Houses Studied||Duct Leakage to Outside at 50 Pa (CFM)||Delivered Heat Efficiency|
|Eugene Water and Electric Board||267||68%|
|(21 older homes)|
|MAP* homes (154 new homes)||156||NA|
|MAP homes (9 new homes)||87||84%|
|(tested using coheat method)|
|*Manufactured Home Acquisition Program|
and Efficiency Delta-T and Ecotope measured rates of duct leakage to the outside in a wide variety of older mobile homes (inspected for the Eugene Water and Electric Board), as well as in newer manufactured homes built under the Manufactured Home Acquisition Program (MAP), run by Bonneville Power Administration and other Northwest utilities. We performed short-term system efficiency tests to measure the difference between heat delivered at the furnace and heat leaving the registers. We also calculated the contribution from heat recovered from off-cycle thermosiphoning, heat recovered from floor framing, and so forth. Nine MAP homes were tested using a more sophisticated coheat test. The system efficiency numbers listed in Table 1 were estimated with a mathematical model developed by Larry Palmiter of Ecotope.
The study found that duct leakage can lower Delivered Heat Efficiency by as much as 32%. Although MAP homes have tighter duct systems than older mobile homes, they were only about a year old when tested, and could be as leaky as the others once the tape fails. All of the MAP manufacturers are required to use adhesive-backed metal tape to seal ductwork (older mobile homes may have ducts sealed with cloth duct tape). However, the longevity of all tapes is questionable, especially when they are applied to the inside of the plenum and not to the outside. Delta-T duct sealing crews have repaired duct systems where the tape had failed after three heating seasons.
|Mobile Home Glossary|
crawlspace connector: a metal boot linking the air handler or furnace closet to the crawl space, whose purpose is to bring in outside air and distribute it through the duct system.
crossover duct: a duct connecting the plenums of the two sides of a double-wide mobile home.
plenum: the piece of duct work connecting the furnace to the supply ducts; it has a fairly constant pressure throughout its length.
road barrier: also called the rodent barrier; a rigid or flexible layer that protects the bottom of the home from road dirt during transport or from rodents at the homesite.
supply leakage fraction: the percentage of supply air that leaks through the ducts.
takeoff: a branch duct that joins a main duct in a T-shaped configuration.
tilt-out: a room added onto a mobile home which can be stowed inside the home when it is transported.
in Mobile Homes Most mobile home air distribution systems are manufactured from flat aluminum or sheet metal in the factory. The metal enters the manufacturing process as a flat sheet, and comes out as a rectangular box about 14 inches wide and 6 inches high. In the house, the assemblers attach the box to the subfloor and cut holes in the subfloor for the registers. A boot connects the register to the duct system.
Most mobile homes have a plenum running the length of the house; double-wides have one on each side. The work we have done on single-wides indicates that the pressure is more or less constant along the entire length of the plenum. For double-wide mobile homes, a crossover duct (generally an R-4 flex duct) is used to carry the conditioned air from one side to the other. There is usually a large discrepancy in air flow between the two sides.
From a duct repair perspective, the constant static pressure through the plenum is important. It means that, regardless of whether a hole is 2 ft or 25 ft from the air handler, sealing it will reduce the same amount of air leakage. (This is not the case in site-built homes, where the pressure 2 ft from the air handler may be 60 Pascals (Pa), and the pressure near the end of a duct run may be only 5 Pa.)
Figure 1. Cross-section view of the underbelly of a mobile home.
Unlike site-built dwellings, 95% of mobile homes have no return ducts. This provides one less opportunity for leakage. Typically the air handler sucks the air through a grille in the furnace closet door. However, having this single return will cause pressure imbalances that increase infiltration and duct losses when interior doors are shut.
The Furnace to Plenum Connection. Delta-T crews have found the sleeve connection between the furnace and the plenum to be the biggest hole in the system, especially in mobile homes where the joists run across the width of the home. This connection is often misaligned, crumpled, or very leaky. Any tape that may have been applied here is exposed to radiant gain directly from the heating elements, shortening its life. The repair can often be made by going in from the top via the air handler. If an air conditioner or heat pump coil prohibits access from the top of the home, the crew must go under the house and cut the bottom of the plenum open directly beneath the air handler to gain access to this joint.
Boot to Floor and Boot to Plenum Connections. If the house has been moved a few times, these connections are probably in bad shape. Angle drills and patches of sheet metal sealed with mastic help to make these repairs go faster.
Slip Joint Connections. In a slip joint connection, one piece of extruded duct work is slipped into or over another. Usually this joint is very poor, and major reconstructive surgery is needed: cut open the road barrier; cut a large hole in the bottom of the duct; seal the top and sides of the duct, adding pieces of sheet metal if necessary; close the hole in the duct and seal it; and seal the road barrier.
An entire assembled plenum duct system awaits installation into a mobile home.
The Ends of the Plenum. The plenum usually runs the entire length of the house regardless of where the registers are located, so it may extend 8-10 ft beyond the last register. In some cases, the method used to close off the ends of the plenum is temporary at best. Making a sheet metal dam at the downstream side of the last register and sealing with duct mastic is a good way to fix bad end caps. However, Delta-T crews have seen few mobile homes that actually need this treatment. Taking pressure pan readings and doing good visual inspections can stop a crew from sealing a virtual leak.
Crossover Connections. There are many stories of double-wide mobile homes with a $500 heating bill for January due to a crossover duct disconnect. Many crossover ducts are crushed, impeding air flow to the other side. The crossover duct itself may be in good shape, but the various flanges and takeoffs used to connect the duct to the plenums on each side may be extremely leaky by design. Another common problem occurs when a setup crew hacks a hole in the plenum and fastens the crossover duct to the plenum with 50 yards of duct tape. If the crossover is leaky, we have found it best to replace it completely. We pay extra attention to the takeoffs.
The Outside Air Make-Up Hole. Many mobile homes have a sheet metal connector that links the bottom of the furnace to the crawlspace. The purpose of this connector is to draw air from the crawlspace into the furnace (when it's operating) and distribute it through the duct system. Depending on its size and on its location within the furnace cabinet, the connector can draw a substantial amount of fresh crawlspace air into the mobile home. The decision to seal the connectors off is based upon the overall tightness of the home (if the house is really tight, don't fix it). Even if connectors function as makeup air inlets, the crawl space is probably not the best source of makeup air.
The Furnace of No Return. One brand of furnace has no cold air return cut in the top of the air handler and usually has no grille in the furnace door closet. The door may have two narrow slits to draw inside air. The furnace is equipped with a crawlspace connector that draws return air from under the house. The entire furnace closet develops negative pressure whenever the furnace comes on, drawing almost all the air for return through the crawlspace connector. To correct this problem, our crews seal off the connector, cut the largest hole possible in the top of the furnace, and place a return in the furnace closet door. A filter slot is added to the top of the furnace.
Pressure Pan versus Duct Blaster
In the first 50 mobile homes sealed in a project for the Eugene Water & Electric Board (EWEB) in Eugene, Oregon, both Duct Blaster tests and pressure pan tests were taken. A goal of the project was to correlate pressure pan readings with Duct Blaster readings to determine whether it was necessary to have a Duct Blaster at all. Several different approaches were tried to find a simple correlation between the two test methods. The most reliable approach was to add the pressure pan readings, and use these as an index of leakiness. In the 50 homes tested (100 total tests), when the sum of the pressure pan readings was less than 3 Pa, the Duct Blaster reading was less than 100 CFM50; when the sum was less than 6 Pa, the reading was less than 200 CFM50; and when the sum was less than 9 Pa, the reading was less than 300 CFM50 (see Figure below). Individual variance was large. (These readings are for mobile homes only and should not be used to infer duct leakage on site-built homes.)
Experienced technicians can learn a lot from the pressure pan test about where the holes are and how big they are likely to be. They may even want to leave the Duct Blaster home: pressure pan readings can provide equally accurate information, can be taken much faster because the duct registers need not be sealed, and are much easier to teach. But perhaps the most important tools are drop lights and mirrors. With these, a visual inspection of the entire duct system is possible from inside the mobile home. If only that could be done in a site-built house!
Many Unhappy Returns. The 5%-10% of homes that have ceiling or belly return systems are doubly cursed. Not only are there the usual supply-side problems to fix, but the return system is a catastrophically stupid design that must be fixed.
The two basic systems (each with its own variations) are roof and belly returns (the belly is the zone between the floor and the road barrier). In these systems the return registers are cut in the floor or ceiling and are not connected to any ductwork whatsoever. There is a large hole in the furnace closet, ceiling, or floor. The belly or the space between the ceiling and the roof is used as a nonducted return plenum. There are several problems with these nonducted systems. First, drawing return air from zones containing fiberglass insulation is never a good idea. Second, they are always very leaky. Fixing them involves sealing all the return grilles and the furnace closet return hole (we use sheet metal) and cutting a return grille into the furnace door closet. It is not unusual to achieve a house leakage reduction of 1,000 CFM50 with this repair.
Just Plain Dumb Holes. Humans have a limitless capacity to accidentally (or deliberately) punch, cut, puncture, rip, and otherwise mutilate the duct system of a mobile home. A good visual inspection and a protocol that retests tightness are critical if the crew is to find these holes. Among our favorite just plain dumb holes are
- Wire and pipe penetrations.
- Poorly cut or badly placed register holes.
- Vents in the duct designed to keep pipes from freezing.
- Uncapped takeoffs for tilt-outs that were never installed.
- Mystery holes cut for no apparent purpose.
To estimate energy savings, use exterior duct leakage measurements at 25 Pa as the basis, since normal operating pressures are often close to this value. Operating pressure is often higher in heat pump systems, so the exterior leakage at 50 Pa can be used, or an extrapolation from the 25 Pa exterior leakage measurement.
The next step is to express air leakage as a percentage of the HVAC system's air handler flow. This percentage is the supply leakage fraction, and describes the portion of conditioned air that doesn't make it to the supply registers. The most commonly found furnace blowers deliver remarkably consistent flow over the range of external static pressures encountered in normal operation. However, in most cases, the air handler does not deliver the flow listed in the operations manual. So it is best to measure the air handler flow to determine the supply leakage fraction.
|Table 2. Savings from duct sealing in two Oregon cities|
|Home Size||910 ft2|
|Home UA||682 Btu/Hr F|
|Home UO*||0.19 Btu/hr F/ft2|
|Pre-retrofit exterior duct leakage at 25 Pa||170 CFM|
|Pre-retrofit supply leakage fraction||20%|
|Post-retrofit exterior duct leakage at 25 Pa||35 CFM|
|Post-retrofit supply leakage fraction||4.1%|
|*UO = UA divided by the total exterior surface area of the home.|
|Portland, OR||Redmond, OR|
|Annual base heating load (no ducts)||16,300 kWh||24,700 kWh|
|Pre-retrofit load with supply leakage||20,375 kWh||30,875kWh|
|Post-retrofit load with supply leakage||17,000 kWh||25,760 kWh|
|Annual energy savings||3,375 kWh||5,115 kWh|
|Annual dollar savings (at 5/kWh)||$170||$255|
A lot of mobile home duct repair can be done from the inside with mirrors and mastic.
Measuring air handler flow is not straightforward. The best approach is to use a Duct Blaster in combination with a calibrated flow hood. The flow hood should be one that is most reliable within the customary flow range for residential supply registers (50-150 CFM). Select a register far from the air handler, since air from the house sometimes flows into the register near the air handler. Turn on the furnace in normal operating mode and measure the flow through the selected register. Also measure the temperature of the air stream (don't rely on the flow hood thermometer, if it has one) in order to account for the expansion of heated air. (This step can probably be skipped if the furnace is only on for a couple of minutes and the auditor is pressed for time.) Finally, measure the static pressure inside the flow hood with a Pitot tube or other static pressure tip. This value will be the reference pressure.
Set up the Duct Blaster and perform leakage tests. At the end of the leakage tests, adjust the Duct Blaster so that the reference register flow and pressure match the flow and pressure produced by the furnace under normal operation. Assuming that the Duct Blaster is set up properly to supply all flow to the home's duct system, the flow through the Duct Blaster is the air handler flow.
In retrofit situations, one measurement of the air handler flow should be sufficient. The pre- and post-retrofit measurements of exterior duct leakage at 25 Pa can be divided by the air handler flow to find the pre- and post-retrofit supply leakage fractions.
The supply leakage fraction is a direct contributor to overall heating system efficiency loss; that is, if there is a 10% supply leakage fraction, a home with no conductive losses from ducts and no mechanically induced infiltration will have an overall heating system efficiency of 90%. So if the retrofit produces a reduction in the supply leakage fraction from 10% to 5%, the retrofit should (all else being equal) provide an overall benefit to the homeowner of 5%.
Sealing ducts in homes with high heat loss rates (calculated by multiplying U-value by surface area by temperature difference) will save more energy than in homes with low heat loss rates. For example, we looked at an older, single-wide mobile home in two Northwest climates-Portland, Oregon, with long-term base 65F heating degree-days (HDD65) of 4,520, and Redmond, Oregon, with long-term HDD65 of 6,745 (see Table 2).
A thorough visual inspection can catch a range of obvious gaps and holes in the duct system of a mobile home.
The home contained R-4 walls, an R-7 ceiling, R-7 belly wrap, and 115 ft2 of windows (U-value 1.1). A blower door test found leakage of 2,000 CFM50, or 0.9 air changes per hour under normal conditions (ACHnat). The measured air handler flow was 850 CFM. We ran a daily simulation program (SUNDAY 3.0) to estimate the annual heating load. Ecotope routinely uses this program to estimate heating loads and the impact of energy efficiency improvements on heating requirements.
The estimated savings are significant. If the weatherization protocol included improving the conduction efficiency of the floor by replacing the crossover duct or adding belly insulation, more energy would be saved. These estimated savings are for homes heated with forced-air electric furnaces and no wood heat. If fossil fuel furnaces are used, the savings in input energy would be estimated by dividing the energy savings by the Annual Fuel Utilization Efficiency (AFUE) of the furnace.Seal Those Ducts We can make people more comfortable and save them money by fixing their ducts. By doing this intelligently, we are probably performing one of the most cost-effective and hassle-free fixes that can be done on manufactured homes. Further, with the invention of the pressure pan and the Duct Blaster, diagnostics and overall estimates of system efficiency improvement are within the reach of many weatherization crews.
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