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

One Size Fits All:
A Thermal Distribution Efficiency Standard


By Mark P. Modera

Mark P. Modera is a staff scientist in the Indoor Environment Group at Lawrence Berkeley Laboratory.

When it comes to residential duct systems, even the experts can't agree on the proper definition for energy efficiency. For example, how could one possibly compare an hydronic system with an air system? Or an uninsulated basement duct system in Iowa with an attic duct system in Arizona?

A project, supported by the California Institute for Energy Efficiency and the U.S. Department of Energy, is underway to devise a universal yardstick--or Figure of Merit-- for thermal distribution efficiency. For the most part, forced-air distribution systems, ducts in particular, are what we have researched. The yardstick for efficiency, however, will apply to all thermal distribution types--forced air, hydronic, and refrigerant.

One of the key impediments to working out the efficiency characterization for thermal distribution systems has been the degree of interaction between these systems, the heating and cooling equipment to which they are connected, and the building envelope. In addition to these interactive effects, other interactions--system location, climate and fuel mix issues associated with thermal distribution efficiency--also need to be addressed (see Table 1).

One Possible Figure of Merit

We know a lot about typical thermal distribution systems in homes in the South and Southwest, and less about homes in the North. John Andrews at Brookhaven National Laboratory, Esher Kweller of the U.S. Department of Energy, and I have done research that attempted to incorporate the duct and envelope interactions in crawlspace and slab-on-grade houses. We extended the work of the ASHRAE SP 43 project, which focused on the interactions between forced-air systems and furnaces in basement houses (see The New Monster in The Basement ). We can now suggest a yardstick for duct efficiency.

The most basic definition for thermal distribution system efficiency is: the ratio of the energy that would be consumed by a house using a given piece of heating or cooling equipment, to the energy consumed by that house with the thermal distribution system connected to that same piece of equipment. In other words,

All space conditioning energy without Efficiency of distribution system _______________________ distribution = All space conditioning system energy with distribution system

This ratio assumes that all interactions are included, but some formula has to be devised for quantifying or isolating those interactions. The yardstick we came up with combines 1) the way the ducts affect the equipment and 2) the interactions between the ducts and the envelope. Each is incorporated into the basic formula by means of a multiplicative factor (see Figure 1).

The factor for the impacts of the ducts on the heating or cooling equipment takes into account

  • The temperature of the air (or water) entering the distribution system--rated and actual.

  • The heat exchange efficiency due the air (or water) flow rate--rated and actual.

  • The efficiency of the equipment due to fixing the duct.

    The factor for the interactions between the ducts and the envelope incorporates

  • The envelope infiltration rate due to the operation of the system.

  • Natural infiltration when the system is not operating.

  • Thermal exchange with the buffer zones (unconditioned spaces) due to the operation of the system.

  • Thermal exchange with buffer zones when the system is not in operation (for example, thermal siphon effects).

  • Any heating or cooling recovery of losses from the ducts to the conditioned space.

  • Changes in the required thermostat setting due to the distribution system.

  • Zoning.


Typical Ducts

The base case for the house with a 100% efficient distribution system is one without a distribution system. In the real world, studies of Sunbelt and Frostbelt houses have allowed us to categorize the duct systems of houses. Four prototypic houses emerge:

1) Attic supply and return ducts with R--4 insulation, a single return register, 12 in2 of supply leakage and 12 in2 of return leakage, connected to a furnace, a heat pump, and an air conditioner.

2) Uninsulated supply and return ducts with plenums in an unconditioned basement and half the ducts rising through exterior walls, including two return registers, 25 in2 of supply leakage and 25 in2 of return leakage, connected to a furnace, a heat pump, and an air conditioner.

3) Uninsulated supply and return ducts installed in the space between floors in a two-story house with two return registers, and specified air and thermal connections between the duct space and the attic or outside, including 25 in2 of supply leakage and 25 in2 of return leakage, connected to a furnace, a heat pump, and an air conditioner.

4) A hydronic, heating-only system installed in an unconditioned basement without pipe insulation.

These prototypes can be used as base cases that can be used to establish reference efficiencies.

The most common type of residential distribution system is a forced-air distribution system connected to a furnace and air conditioner combination. Taking this system--installed in a crawlspace and a basement (the first two baseline combinations)--as an example, we computed attic duct performance for houses in Sacramento, California, and basement duct performance for houses in Washington, D.C. (see Table 2).

Duct Labels Coming?

With a figure of merit for all thermal distribution systems, homeowners or builders could receive efficiency credits for reduced leakage. Getting credits would be a matter of meeting duct tightness levels after installation, or proving that the system, as designed, has less leakage than an established marker, perhaps the average of all duct leakages in the population. Efficiency penalties, on the other hand could be doled out for placing the return filter at the register (which significantly increases the pressures across the return leaks).

All the features of ducts of other distribution systems will someday be quantified and presented in a single figure for efficiency, and perhaps even presented in an EnergyGuide-type label for ducts. The American Society of Heating Refrigerating and Air-conditioning Engineers (ASHRAE) is developing just such A Standard Method of Test for Determining the Steady-State and Seasonal Efficiencies of Residential Thermal Distribution Systems (SPC 152P).


Figure 1. Duct efficiency, in concept.

Raw efficiency Factor for duct interactions Factor for duct interactions ratio with equipment with envelope _____________________________________________________________________________________________________________ Energy in (thermal energy Efficiency of the heating or delivered to the registers cooling equipment with the Building load without the Efficiency of or radiators) distribution system distribution system distribution = ------------------------------- X ------------------------------ X ----------------------------- system Energy out (thermal energy Efficiency of the heating or Building load with the delivered to the distribution cooling equipment without distribution system air or water) the distribution system
Table 1. Factors Influencing a Duct Efficiency Yardstick Interactions Influences and results _________________________________________________________________________________________ Ducts and equipment * Equipment type, sizing, and known ratings (heating and cooling) such as Annual Fuel Utilization Efficiency The effect of the equipment * System cycling on the distribution system * Heat exchanger * Variable capacity unit and fans * The temperature of the distribution medium (air in the case of ducts) * Equipment capacity and the thermostat The effect of ducts on * Heat transfer at the heat exchanger equipment efficiency * Effective capacity of the equipment after duct repairs _________________________________________________________________________________________ Building envelope and ducts During operation * Infiltration rates and locations * Unbalanced air flows * Temperature changes due to leakage and conduction losses to unconditioned spaces (basements, attics, crawlspaces, garages) * Envelope and internal wall design of the zone(s) * The mix of radiant and convective heating sources, in relation to thermostat setpoints When idle (off-cycle) * Increased natural infiltration rate due to duct leakage * Thermal siphon flows and heat exchange through poorly insulated ducts in unconditioned spaces * Hydronic system losses during off-cycle periods (in the case of radiators, for instance) _________________________________________________________________________________________ Other Interactions * Energy use by fans, in relation to thermal energy use * The location of the system in the house * Weather * Local climate
Table 2. A Duct Efficiency Yardstick For Two Locations Heating Cooling _____________________________________________________________ Attic Basement Attic Basement Duct ducts ducts ducts ducts efficiency (Sacramento) (Washington) (Sacramento) (Washington) ______________________________________________________________________________________ Overall efficiency-- 68% 88% 61% 79% with interactions of ducts, equipment, and envelope Raw efficiency ratio-- 63% 62% 51% 63% average energy in to energy out Raw efficiency ratio-- 49% 59% 29% 59% peak demand energy in to energy out
This label represents one possible implementation of a standardized rating procedure for residential duct systems.


Related Articles

Discovering Ducts: An Introduction
Duct Fixing in America (Penn)
Duke Power's Success (Vigil)
Guidelines for Designing and Installing Tight Duct Systems (Stum)
Integrated Heating and Ventilation: Double Duty for Ducts (Jackson)
Leak Detectors: Experts Explain the Techniques (Proctor, Blasnik, Davis, Downey, Modera, Nelson, and Tooley)
Managing Large-Scale Duct Programs (Downey)
Mobile Homes: Small Zones, Big Problems (Kinney)
New Group Hunts Bad Ducts (Obst)
The New Monster in the Basement (Treidler)
Stories from the Buffer Zone (Kinney and Stiles)
Two Favorite Test Methods, By the Book (Modera)
Will Duct Repairs Reduce Cooling Load? (Parker, Cummings, and Meier)
Chasing the Golden Carrot (Frantz)
Checking Out HUD's Proposed Mobile Home Performance Standards (Judkoff)
Hauling in the Culprits: Michigan's Bounty Pilot (Witte and Kushler)
How Accurate Are Yellow Labels (Meier)
Making Energy Mortgages Work (Luboff)
New Standards Begin, But Will Rebates Continue? (Morrill)
Telecommuting: An Alternative Route to Work (Quaid)
Weatherization Assistance: The Single-Family Study (Brown and Berry)
What's Wrong with Refrigerator Energy Ratings? (Proctor)

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