The following will help you to conduct a modified duct blower test (leakage to outside only).
If some of the ductwork is within or otherwise in communication with the conditioned space, a more useful value of the duct leakage will be obtained if any leakage to the inside is "zeroed out" using the blower door. The procedure is similar to the above in most respects. Differences are shown in italics.
Test objective: measure leakage rate to/from outside of whole duct system (registers sealed) at 25 Pa pressure.
Method: A benchmark leakage rate at 25 Pa pressure is determined using a duct blower in combination with a blower door. A probe (typically in the supply plenum) is used to measure the static pressure in the duct.
1. Turn off the air handler fan of the duct system.
2. Select a place of attachment for the duct blower. The fan access opening of the furnace or heat pump is usually a good choice. Another good choice is a single large return register. If neither of these is accessible, choose the largest register closest to the air handler. Attach the duct blower at this location. It will usually be necessary to prepare a temporary baffle of cardboard or other stiff material as an interface between the duct blower and the attachment opening.
3. Assuming that the duct blower is attached to the duct system on the return side of the system fan, insert a static pressure probe into the supply plenum. (If the duct blower is on the supply side, insert the probe into the return plenum.) Run a plastic tube from the probe to the input port of a digital manometer. Locate the manometer within the conditioned space or run a plastic tube from the conditioned space to the reference port of the manometer.
4. Seal all other supply and return registers.
5. Make sure that any spaces with ducts are vented to the outside, if necessary by opening a door or window. The conditioned space should be closed off from the outside.
6. Set up a blower door in an outside doorway leading into the living space. Adjust the blower door so that the house is pressurized to 25 pascals.
7. Adjust the duct blower so that the reading from the pressure probe is zero. Record the flow through the duct blower.
Assessing duct system upgradesFor the contractor, time is money. It is therefore essential that he have, at the top of his duct-repair tool kit, some quick methods of assessing whether any particular duct system is likely to need upgrading. This section discusses some things you can look for that don't need any special measuring equipment.
General appearance. The quickest thing one can do to evaluate a duct system is¿ look at it! The first thing to note is the general appearance of the duct system. A good-looking system might or might not have unacceptable air leakage, but if a system is badly damaged in places or generally decrepit, it is almost certain to have problems. You may find that the system was installed properly but was damaged later on, either by the homeowner (by storing heavy items on top of ducts that are not designed to carry weight) or by other trades trying to get past the ducts on their to a plumbing or electrical repair job. (This is especially likely - and understandable - where the various services to the house are installed close together in tight spaces.)
Even if the duct sections themselves are in good repair, they may be disconnected in places. Occasionally sections are left disconnected during the construction process. More often the joints become separated later on.
One building practice that is somewhat controversial is the use of the building itself as part of the duct system. Most common is the "panned joist," in which two joists and the overlying subfloor are used as three sides of a duct, with the fourth side made up from a flat piece of sheet metal nailed to the bottom edges of the joists. Although there is no reason why such a duct can't be made virtually airtight, in practice they seldom are. On the return side, use of closets and chases as pathways for airflow from the living space back to the heating/cooling equipment is even more likely to give rise to large amounts of air leakage.
Location. In addition to general appearance, the location of the duct system within the house is also important. Location means what kind of space the ducts are in - attic, crawl space, slab, basement, etc. Of all these possible locations, the one that is most prone to high energy losses is the attic, particularly in hot, humid climates. Studies in Florida and California have shown that peak cooling loads can easily double because of poorly performing attic ductwork. Leaky return ducts cause hot, moist attic air to be drawn into the system, placing an extra load on the air conditioner. Leaky and poorly insulated supply ducts lose chilled air and cause whatever cool air is left in the duct to be warmed up. Cases have been reported in which the air coming from the supply registers was actually warmer than the air in the house, because the air conditioner couldn't keep up with the duct losses!
A vented crawl space is a somewhat better location for ducts than an attic, because peak-load efficiency in the cooling mode tends to be higher. As in an attic, the ducts are in an environment that is similar to that outside the house, but at least they are not super-warmed by the hot summer sun during the peak cooling season.
Basements, even if not intentionally conditioned, are usually better locations for ducts than either attics or crawl spaces, because they are not exposed to the harshest outdoor temperatures. Also, some of the duct losses are effectively regained. Heat lost into a basement tends to warm it somewhat. This somewhat retards further heat conduction losses from the ducts, and it also reduces the rate of heat loss from the house through the basement ceiling. However, there is some evidence that ducts actually installed in basements tend to leak more than ducts installed in crawlspaces and attics, and basement ducts are more often left uninsulated.
One question that has to be considered before repairing basement ducts is the probability that the basement temperature will be lower in winter as a result. One study reported an average basement temperature drop of 58F as a result of duct retrofits. Make sure that this will not pose a problem for the homeowners in terms of their use of the space. If fear of freezing pipes is the issue, consider whether this can be addressed by insulating any vulnerable pipe runs.
Energy savingsThe best location for ductwork is within the conditioned space. Any heat or cooling that leaks to the living space may be regarded as useful, and not lost at all, unless air leakage is so great that it becomes impossible to balance the system. Energy savings of up to 40% have been demonstrated by moving ductwork into the conditioned space.
People often ask whether a basement should be considered part of the conditioned space. If the basement is used for human activities and not just storage, and if there are registers in the basement, then it is part of the living space. In such cases, the basement walls should be insulated. Duct insulation is needed only if required to keep the ducts from "sweating" in the cooling mode.
Besides the general appearance and location of the duct system, the third important factor that would influence a decision to look at the duct system is if equipment replacement is needed. Fixing leaky and poorly insulated ducts is especially critical if new heating or cooling equipment is installed, because the sizing of this equipment depends on the load, and the load can usually be greatly reduced through duct repairs. Savings in first cost available through equipment downsizing can help to pay for the cost of repairing the ducts. In some cases these savings may completely offset the duct-repair costs, shrinking the payback time to zero.
In summary, the following situations represent opportunities to show the homeowner why his or her duct system's efficiency and performance should be investigated further:
- The duct system is badly damaged in places or has the general appearance of being "on its last legs."
- The duct runs themselves appear to be sound, but one or more disconnected sections are found.
- The system includes panned joists or other strategies employing the building itself as part of the ductwork.
- The house is in a hot, humid climate and the ducts are located in the attic.
- Regardless of climate, there are uninsulated ducts located outside the conditioned space.