This is the fifth in a series of HotMail articles by Doc Falke focusing on specific techniques designed to test, diagnose and improve the performance and efficiency of residential HVAC systems.

When a duct sealing job has a limited budget, there are a few areas of the duct system where sealing can deliver the greatest impact. Let’s take a look at a where some well-placed “pookey” (an NCI term for mastic) can yield the greatest energy savings and net our customers the most increase in comfort.

The three places in a system where duct sealing will normally have the greatest impact are: Near the equipment where pressures are the highest, at the boots, where unconditioned air can be brought into the building, And third, at ducts located in unconditioned spaces.

Safety First

One word of caution; always be certain to test and verify the effects of duct sealing on combustion appliances and venting to assure safety from carbon monoxide poisoning before and after you do this type of work.

Basic Duct Sealing

Ideally, a duct system should be like a plumbing system, it shouldn’t leak. The difference is that if plumbing leaks, the roof falls in or mold takes over the home. With ducts, your customer never knows, they just pay more to the utility company every month and are never as comfortable as you promised they would be.

Basic duct sealing requires the installer to create make a permanent airtight duct connection — permanent being a key word here. This can be done with sheet metal fittings, a few screws, and an additional coating of mastic. This forms a continuous airtight layer to prevent leakage.

Duct board requires careful manufacturing practices and the application of approved fasteners or tape per the manufacturer’s recommendations.

Flex duct requires inner and outer straps or draw bands secured behind a sheet metal bead on the fitting they are connected to. Be sure to meet and exceed your local code requirements or utility program recommendations.

The bottom line is that ducts should not leak air, just like plumbing should not leak the fluid or gas they are designed to distribute.

Traditionally duct improvements are completed at the time of equipment replacement. This is when customers are most aware and when money gets budgeted for this type of repair. But with the increase of service testing, more and more duct renovations are being done independent of equipment replacement.

Duct Leakage Inspection and Testing

When you measure duct leakage before and after you seal the ducts, you know how much leakage you had before sealing, and you know how much you have improved the leakage when the job is completed. This way you can quantify the improvement you made for your customers.

The most effective way to measure duct leakage is under live operating conditions, otherwise you’re guessing where improvements were made in the system. First, measure total external static pressure and verify fan speed and plot fan airflow on the manufacturer’s tables. (If you’re not sure how to do this accurately, we’ll send you a procedure; see the contact information at the end of the article.) Second, measure airflow with a balancing hood at the supply registers and add them together to determine the delivered airflow into the building. Third, subtract the total register CFM from the plotted fan CFM. The difference is supply duct leakage. Then do the same thing on the return side.

Seal Near the Equipment First

Leakage is created by a hole in the duct system and a pressure difference on both sides of the hole. The fan provides that pressure. The way airflow and pressure operate in a forced air system is that pressure is highest near the fan; both on the discharge side and the suction side of the fan. So a hole of equal size in the duct system will leak far more near the fan than it will at the end of the duct where pressure is lowest. Seal ducts near the equipment first.

Start by sealing the equipment being certain not to permanently seal any service access doors. Be very aware of leakage where the coil is attached to the furnace, and also upgrade any filter access. Once again, pressure is highest near the equipment and this is where the repairs will be the most valuable in terms of energy savings and comfort improvements.

Generously seal where the duct system connects to the equipment. Also seal any fittings to the trunk duct near the equipment. One benefit of sealing near the equipment is that normally this area is easily accessible and not difficult to get to.

Seal at the Boots

Measuring for leakage in boots near the registers is done by temperature measurement. It may seem strange to measure for leakage with temperature. But once you understand what happens at this point in the duct system, it will make more sense.

When there are gaps between the registers and sheet metal boot, and the supply air velocity from the duct is high, the action of the air blasting past these leaks will actually pull unconditioned air from the attic or crawlspace air into the home. This can be detected by measuring the temperature between the main airstream near the center of the register and the air temperature at the edge of the register. In winter time, the air near the edge will be significantly colder.

This gap can be sealed by nailing the sheet metal boot to the wood in the floor or ceiling framing, then by sealing the gap with caulking or thin tape to assure that no air leakage occurs. (Use thin tape or the register will not fit into the boot after the repair is made.)

Seal Return Ducts in Unconditioned Space

While supply duct leakage loses some conditioned air when temperatures in unconditioned spaces are extreme and when the heating or cooling is needed the most, return duct leakage can reduce the system’s cooling capacity by 30% or more.

Here’s a scenario: A three ton, system has a 225 CFM leak in the return duct located in an attic. The return air temperature in the return duct is 75F degrees and the attic temperature is 125F degrees. That’s a 50F degree difference between the air in the return duct and the attic air being pulled into the system. You can calculate the BTUs being pulled into the system by multiplying the 50F degree temperature difference by the 225 CFM of duct leakage times the sensible BTU multiplier of 1.08 to reveal 12,150 BTUs of sensible BTU gain, or over 1 ton of lost capacity through duct leakage!

Return duct leaks can be found utilizing the testing described above and can also be found by inspecting the duct system for disconnects. These can easily be identified by looking for darkened insulation.

Remove the duct insulation, make an airtight repair using appropriate materials and seal the fitting with an approved mastic or tape.

Next time you provide duct sealing with a limited budget, or a generous budget, pay special attention to these three areas of duct tightening to assure you maximize the impact of your duct sealing efforts.

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company and membership organization. If you're an HVAC contractor or technician interested in a free procedure showing how to plot fan airflow, contact Doc at robf@ncihvac.com or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.