One Size Fits All:
A Thermal Distribution Efficiency Standard

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

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,

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.

   

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.

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