Green Building Library
Space Heating and Cooling

Ducts Increase Air Leakage and Energy Use

Furnaces and heat pumps are getting more efficient all the time. But, a sizable portion of that extra efficiency is lost due to the forced air delivery system. Some efficiency escapes directly through leaky ductwork. And, structural air leakage increases when the furnace blower puts the home under pressure.

According to experts, these losses reduce the efficiency of all forced air heating systems by 20 percent or more. For example, an electric furnace may be 100 percent efficient in theory. But in practice, the efficiency is 80 percent or less. This is also true for heat pumps and gas furnaces.

They point to recent research (see "Research Findings") that looked closely at homes with ducted heating systems. Several of these studies compared homes with forced-air heating and cooling systems to those with electric wall heaters or baseboards. The conclusion: homes with ducts have greater air leakage and they use more energy for space heating. Here's what seems to be happening.

Leaks in the structure

In a house with forced air heat, every room has at least one hole in it for a duct boot. Whenever that duct comes from an unheated area, any gap around the boot will be a source of air leakage, just like the hole around a pipe or a wire.

Leaks in the ducts

Most ducts are very leaky. And, returns leak even more than supply ducts—about twice as much. Common leaks include:

• joints between sections of duct
  
• joints between branches and trunks
  
• seams in ducts
  
• seams in air handlers
  
• holes in air handlers for pipes and wires
  
• access slots or filters

The actual amount of "leakage area" in the ducts may be a small percentage of the total house leakage area. However, duct openings can lose far more energy than other air leaks. The reason is air pressure.

Pressure in the ducts

The stack effect (warm air rising inside a house) typically exerts about 4 Pascals of pressure on the shell of a two-story house. The force of a 20-mile-per-hour wind is about 50 Pascals. When a furnace blower operates, the ducts come under air pressure ranging from 25 to 75 Pascals (O.1 to 0.3 in. of water). This tremendous air pressure in ducts pushes a lot of air through small cracks. So, openings in the ducts have a greater impact on the air leakage of the house than openings in other parts of the house.

And, fan-driven leakage couldn't happen at a worse time. Fans operate more during extremes in the weather. So, the greatest losses occur when there is the most to lose.

Pressure in the house

Most homes have only one or two air returns, not enough to properly balance the interior pressure. So, supplies blow air into some rooms, while returns pull air out of others. This increases air leakage in the house substantially. Two studies determined that the furnace blower doubled the air leakage of the house.

Forced-air heating systems tend to create an overall positive pressure in the house which increases air leakage by driving air out through openings in the structure. Water vapor could also be pushed into wall cavities, where it might promote decay.

Closed doors compound the problem by blocking the flow of air between rooms. Positive pressure builds around supplies and blows more room air outside. Meanwhile, negative pressure builds around returns, which increases the amount of outside air pulled in.

Return ducts running through crawlspaces and attics are the most likely cause of unbalanced pressure inside a house. Leaky returns pick up "extra" air and blow it into the house, generating the overall positive pressure.

Return ducts in a basement can cause a similar situation. Air may not flow from upper levels of the home well enough to make up for the air being pulled into unsealed return ducts. So, negative pressure develops in the basement pulling outside air through the rim joist, around basement windows and through other openings.

In addition to reducing energy efficiency, leaky returns in basements may create two other problems. If combustion appliances are located in the basement, the fan-driven negative pressure could prevent toxic fumes from venting properly. Negative pressure in the basement can also draw soil gas inside. In some areas, this soil gas contains radon.

Duct system leaks and fan-driven interior pressure create air leakage that increases energy use.

 

Research Findings Ducted homes had 70 percent more air leakage and used 17 to 22 percent more energy for space heating. These findings were based on the monitored energy consumption of 820 houses monitored under the Residential Standards Demonstration Program (RSDP) in Idaho, Montana, Oregon and Washington. "Evidence of Increased Levels of Space Heat Consumption and Air Leakage Associated with Forced Air Heating System in Houses in the Pacific Northwest." D.S. Parker, ASHRAE Transactions 1989, V. 95, Pt. 2. Homes with forced air heating systems used 25 percent more energy for space heating than homes with non-ducted heaters. This study focused on 16 RSDP homes built in Washington. "Thermabilt Performance Monitoring: Analysis of Energy Use, Ventilation Rates And Indoor Air Quality Testing," Richard Byers, Washington State Energy Office, March 1988 Homes with ducts had 35 to 45 percent more air leakage than homes with baseboard or wall heaters. The researchers attributed half of this leakage to the ducts. These findings were based on 134 randomly sampled homes. "Northwest Residential Infiltration Survey," Larry Palmiter and Ian Brown, Ecotope, August 1989 Duct leakage varied considerably from house to house. Overall, ducts added about 10 percent to house "leakiness". On average, the furnace blower doubled the air leakage rate of the houses. Duct leaks cause a 12 percent reduction in heating system efficiency. And the good news: sealing the ducts cut air leakage by 20 percent, even though many leaks were inaccessible and couldn't be fixed. "Field Investigation of Residential Infiltration and Heating Duct Leakage," D.H. Robison and L.A. Lambert, ASHRAE Transactions 1989, V. 95, Pt. 2 Air infiltration rates typically double when the distribution fan was turned on. "Residential Duct System Leakage: Magnitude, Impacts and Potential for Reduction," M.P. Modera, ASHRAE Transactions 1989, V.95, Pt.2 Ducts accounted for an average of 19 percent of the house leakage area for the three houses studied. These researchers noted an interesting relationship between the leakage area in ducts and the resulting infiltration in the house. By repairing 43 percent of the duct leaks, they cut infiltration by 63 percent. "Infiltration and Pressure Differences Induced by Forced Air Systems in Florida Residences," J.B. Cummings and J.J. Tooley, Jr., ASHRAE Transactions 1989, V. 95, Pt.

This article appeared in Energy Source Builder Vol. II No. 1, February 1990,
©Copyright 2008 Iris Communications, Inc.