This article was originally published in the September/October 1993 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online September/October 1993
A World Unto Themselves
The New Monster in the Basement
by Burke Treidler
Burke Treidler is a researcher in the Indoor Environment Group at Lawrence Berkeley Laboratory in Berkeley, California.
When coal-burning gravity furnaces were common, people spoke of feeding the monster in the basement. The monster seemed defanged when gas and oil replaced coal, but still appeared fearsome. With the introduction of forced-air units, the monster shrank in size but gained a few new teeth. Those teeth were additional energy conservation and safety issues that arose because of the introduction of the air handler fan.
Basements differ from houses where the equipment is located in attics or crawlspaces in several significant ways:
Basements are usually not well-vented to the outside, so much of the energy lost by the ducts will be recovered through a reduction in the house conditioning load. Computer simulations and field studies have shown how this load reduction occurs for heating--the basement temperature goes up by 5deg.F-30deg.F and the floor of the living space gets warm!
At Lawrence Berkeley Laboratory (LBL), we recently developed a computer simulation program for ducts and linked it to an existing building energy simulation system, DOE-2. The ducts were modelled to account for leakage, conduction losses, radiative losses, and the change in temperature of the ducts during equipment cycling. We found that 65% of the energy lost by ducts is recovered. The fraction of energy recovered will obviously depend upon the insulation levels of the basement walls and ceilings. Insulating the ceiling will reduce the fraction of energy recovered but decrease energy use. Insulating the basement walls will increase both quantities.
At Home in The Field
The complexity of basement houses has piqued the interest of building scientists. Field studies have shown that most basement houses are leaky, with most duct leaks in the basement. Three sets of measurements were recently made on real houses:
The ducts in Washington, D.C., leaked by about 1,300 cfm at 50 pascals (Pa) of pressure (1,300 cfm50) while the Minnesota ducts averaged 2,217 cfm50. For the two Washington, D.C., houses for which supply and return leaks were distinguished, 55% of the leakage was in the return ducts and a significant fraction was to the outside, in this case the attic.
The duct leakage values for the basement houses were higher than those found in attic and crawlspace houses. This is because the basement houses use metal ducts which tend to be leaky, while flex duct is typically used in other types of houses.
The Washington and Minnesota field studies revealed another problem due to different construction practices for basement houses. In some two-story houses, ducts were sent to the second floor via the stud spaces of exterior walls. The efficiencies of these ducts are lower than for ducts inside interior walls during both the heating and cooling seasons because they are in more extreme environments. This is an extra conduction leak for ducts in basement houses.
Basements generally contain the clothes dryer and water heater, as well as the air handler fan. The flows through the vents for these appliances or leaks in the return ducts may depressurize the basement if sufficient combustion air inlets are not provided. Depressurizing any zone has the potential for backdrafting natural gas appliances in that zone. In addition, return leaks will distribute any spilled combustion gases throughout a house. These effects can become stronger as non-duct air leaks in a basement are sealed, potentially producing a dangerous situation. A similar situation can exist when the furnace is in a closet inside the house, but it will not occur when furnaces are located in the attic or garage. Attics and garages tend to be well-ventilated to the outdoors.
Computer simulations show there is significant energy to be saved by improving a basement's ducts. ASHRAE Special Project 43 (SP 43) was launched in 1982 to determine HVAC system performance in basement houses. The results are summarized in Chapter 29, Furnaces, of the ASHRAE Handbook HVAC Systems ans Equipment . The simulations predicted that R-5 duct insulation decreased energy use by 4%. Decreasing duct leakage from 10% of fan flow to 0% would decrease energy use by 2.5%.
The effect of the basement configuration was also considered. Adding insulation to basement walls, and then to the basement ceiling, and then insulating and sealing the ducts would decrease energy use by 35%, 35%, and 42%, respectively. Insulating only the ceiling decreased energy use by 27%.
The LBL simulations predict that sealing 80% of the duct leaks in the basement and insulating the basement ducts to R-5 will produce a 10% savings in energy use. Since 10% of fan flow leakage for ducts is lower than field measurements, the SP 43 estimate for energy savings from duct system sealing and insulation are conservative estimates. The LBL model house has a larger amount of duct leakage, 20% in return and 10% in supply.
The SP 43 results suggest that insulating basement walls or ceilings will save more energy, but there are significant reductions in energy use when ducts are sealed and insulated. The LBL results give a less conservative estimate of the potential energy savings, but they still show less energy savings than from insulating walls and ceilings.
Tame the Monster?
Which conservation measures are worth doing in basements with ducts? Unfortunately, this question hasn't been directly addressed yet. We don't know the full story of the economics of conservation measures in basement houses because no comprehensive field studies have investigated their economic viability.
Figure 1. Zones and basement homes.
ENERGY LOSERS IN THE BASEMENT
The energy conservation and safety questions surrounding basement forced air systems are as complex as the systems themselves. This Maryland basement is festooned with testing cables used to determine duct efficiency. Note that all of the visible ducts are uninsulated.
Ducts lose energy to the basement both when the air handler fan is on and when it is off. Hypothetically consider an uninsulated sheet metal duct system at 95deg.F with 430 ft2 of area (80 ft of duct) in a basement where the temperature is 50deg.F at waist level and 70deg.F at the joists. The fan is pushing 1,500 cfm and 10% of the supply duct air flow leaks to the basement. The energy will flow from this duct system at rates of 12,000 Btu per hour by convection and radiation and 7,000 Btu per hour because of leakage. For an 80% efficient, 100,000 Btu per hour furnace this represents 15% and 9% of its total energy output, respectively. Twenty-four percent of the furnace's energy will flow to the basement. Insulating the ducts to R-5 will reduce the convection and radiation to approximately 2,300 Btu per hour. However, 12% of the furnace output is still going to the basement.
Another energy flow is the heating and cooling of the ducts. A sheet metal duct system typically weighs 400 pounds and will require 900 Btu to heat up 20deg.F, (for instance, from 68deg.F to 88deg.F). When the fan switches off, much of this heat goes to the basement. This energy may represent 10%-15% of the furnace output when the furnace is on 50% of the time and cycles six times per hour. Fan overrun and free convection flow in the ductwork when the fan is off reduce the energy conducted to the basement by a factor of four, but this still represents 3% of the furnace output going to the basement.
Note that these losses are only to the basement, and do not include the losses caused by increased infiltration in the house with fan operation. Also some of this energy lost above is recovered. The best estimates are that 50%-60% of it is recovered through reduced heating and cooling loads for the house.
TOP 10 REASONS TO SEAL DUCTS
Reason #10 Because they're there. Reason #9 Retrofitters get to play with neat-o equipment and say things like: Ma'am, it's your pressure differential that's causing the problem. Reason #8 It's getting expensive for homeowners to feed all those stray cats. Reason #7 Leftover mastic makes a tasty dip. Reason #6 Scientists say we should. Reason #5 It's the closest you can get to the thrill of being entombed, without actually dying. Reason #4 Raggedy Ann was getting lonely in there. Reason #3 You get a free pair of dice. Reason #2 Because your HVAC system is most effective at heating and cooling your backyard. REASON #1 Customers keep complaining about neighbors being sucked in their windows every time they turn on the bathroom fan.
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