Danny Parker and Jim Cummings are senior scientists at the Florida Solar Energy Center in Cape Canaveral, Florida. Alan Meier is executive editor of Home Energy.

The results of duct repairs are also important to consumers in certain situations. If the consumer is paying a time-of-use rate, then reduction in peak electricity use saves when electricity is most expensive. Thus, duct repairs may be especially cost-effective for customers with this kind of rate structure.

Also, improved duct performance is important when the consumer expects to replace the old air conditioner with a new unit. Air conditioners are sized (in theory) such that they are just large enough to provide sufficient cooling capacity during peak cooling conditions. If duct repairs cut cooling requirements, then a new air conditioner with a lower capacity can be purchased. "Smaller" often translates into "cheaper," so the consumer can save here, too.

There's no simple rule of thumb for predicting savings because it depends so much on the amount of duct leakage sealed, the duct leakage location in the specific house, as well as the air conditioner characteristics and local climate. Still, here is a general guide.

Sizing Is the Key

The amount of peak savings from duct retrofits is influenced by the size--or capacity--of the air conditioner (see Table 1). If the air conditioner is oversized (that is, not constantly operating during the hottest conditions), then two things happen after a retrofit. First, the air conditioner operates even less during the peak. That means peak power savings. (Mission accomplished!) Second, the indoor temperature may change. In the case of oversizing, there's no reason to expect any changes because the occupants were already comfortable before the retrofit (see Figure 1).

Another common situation is where the air conditioner is undersized. During the hottest times, it is operating 100% of the time. More than likely, the inside temperature climbs a few degrees because the air conditioner cannot keep up with the heat gain. After the duct retrofit, more usable cooling capacity gets from the air conditioner to the living space. Two results can happen. First, the air conditioner may continue to operate 100% of the time during the peak, but the house is cooler. It is possible that no peak savings will result. (Mission failed!) Alternatively, the occupants tolerate the warmer temperatures during peak periods, allowing the air conditioner to begin cycling. That cycling translates into some peak savings. (Mission accomplished!) There can also be combinations of the lower temperatures and some cycling (see Figure 2).

Duct repairs can effect interior humidity levels. Air conditioners have different dehumidification capabilities. However, brief operating cycles generally remove less water than long cycles. A properly sized unit will probably be a more effective dehumidifier than an oversized unit. (A greatly oversized unit can cause an unpleasant cold, clammy indoor environment. ) It is difficult to predict how humidity will change after duct retrofits, because it will depend on the system's characteristics, the location of the duct leaks, and the climate. But don't be surprised if humidity does change.

Is This Air Conditioner Oversized?

Unfortunately it is difficult to determine if the air conditioner is properly sized without visiting the house during the peak periods. This is inconvenient, but careful interrogation of the occupants may yield information nearly as useful. The occupants will probably recall what happens during a very hot summer afternoon, that is, if the house temperature rises, if the air conditioner operates continuously, and other details. In Florida, we found that occupants who are unable to answer these questions are most often in homes with adequately sized units. Alternatively, those who complain about other aspects, such as high humidity, may have oversized units. Those who complain of inadequate cooling often indicate an undersized air conditioner, catastrophic duct leaks, or both.

Recommendations

Peak power savings from duct retrofits are not a sure thing. It may be possible to determine if the potential for savings exists, but the uncertainties of occupant behavior and system characteristics make it difficult to predict those savings with any confidence for an individual home. In field studies conducted so far in Florida, we have seen coincident peak demand savings vary from none at all to over 30% of pre-repair levels. Fortunately the peak savings are only one reason to undertake duct retrofits: The energy savings and improved comfort may be sufficient justification. n

Table 1. Comfort and Savings

        Initial air                 Inside                  Peak 
        conditioner               temperature           power savings
         capacity               (after retrofit)       (after retrofit)
_______________________________________________________________________
        Oversized                    same               considerable        
      Correctly sized     same, or slightly cooler        some        
        Undersized                  cooler             little or none

Figure 1. Electricity load for oversized air conditioner before and after duct retrofit.

Figure 2. Electricity load for undersized air conditioner before and after duct retrofit.

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