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Testing Diagnostics


Duct system repair involves replacing or repairing any component part, including flexible metallic and non metallic “plastic flex” ducts, rigid metal ducts, sheet metal duct components, duct supports, boots and registers. Repair is necessary when the existing duct system is damaged or components are missing or disconnected.


Why is Duct Testing Important?

Savings and Comfort

savings and comfort

In residential central heating and cooling systems, the conditioned (heated or cooled) air is delivered to each room of the home through a “supply” duct system, the first half of the duct system. These ducts are used as pathways to deliver air to each room in the home and the “return” duct system or second half of the duct system, returns air to the heating and cooling system. The condition of both duct systems is vital to the overall efficiency of the heating and cooling system and the comfort levels in each room of the entire home.


Energy loss is not the only concern, duct systems can also affect the indoor air quality of a home. If the return duct system is leaky, it could be drawing stale or polluted air into the duct system and distributing it throughout the home. This air could come from an attic, combustion air from the gas furnace, stove, water heater, or the outside. If the supply duct system is leaky, the home can become depressurized and air from outside may be drawn into the home through infiltration. Either situation may contribute to breathing problems such as allergies and asthma.


Broken Duct - why duct cleaning is important

Now that you know that duct systems can affect both energy usage and indoor air quality, how do you know if your duct system is in good condition? The most reliable and cost effective way to determine this is to have a “duct test” performed by Miller Service Company using the proper test equipment. Duct testing is the process of using calibrated mechanical equipment to measure the amount of airflow that is lost through the duct system when it is pressurized to its normal operating pressure. While some joints or seams may have only small leaks, other sections may be completely disconnected.

Duct testing can indicate the relative leakiness of the ducts and help determine if the duct system should be sealed, repaired, or renovated. Duct testing is strongly recommended when a new heating and air conditioning unit is being installed. If the existing duct is leaky and inefficient before the new unit is installed, it will still be leaky and inefficient after the new unit is installed unless the ducts are tested and sealed. It does not make sense to install a new energy efficient heating and air conditioning unit unless the duct system is also energy efficient and provides the correct amount of heated or cooled air to each room.


Duct Testing Methods and Equipment

The duct blaster uses a calibrated fan that gently pressurizes the ducts and measures the flow through the ducts leaks to indicate total leakage. The duct blaster consists of a portable fan with calibrated digital pressure gauges that is connected at the blower compartment of the air handler or attached to the main return grille. All the duct registers and grilles are temporarily sealed and the duct blaster fan is adjusted to pressurize the system. The fan pressure is read from the gauges and converted to an equivalent duct leakage rate in cubic feet per minute (cfm).


What To Do With The Results

If the test indicates the leakage is greater than acceptable, the ducts should be sealed, repaired, or renovated (replaced). In new homes Miller Service Company has set 6% of total air flow as the goal below which a duct system must test. In older homes the goal is usually set at 12%. In either case the closer to 0% leakage the better.



Duct sealing involves using approved materials and procedures to seal air leaks. Locations where sealing is typically performed include the supply and return plenums (which are connected to the furnace cabinet), starting collars (where ducts are connected to the plenums), fittings in the duct runs (wyes – where ducts branch off – and elbows), splices within the duct run, and termination where the ducts connect to the supply registers. The most important leakage areas are in the supply and return plenums because these areas are closest to the air handler and under the highest pressures when the system is operating. All sealing should be performed using approved mastic type materials. Duct tape should never be used. Miller Service Company is familiar with all applicable building codes and standards that apply to duct sealing techniques.



Duct renovation should be performed if the duct system is in poor condition or improperly sized. Renovation involves replacing a significant portion of the duct system to restore it to proper operation. It is sometimes necessary to replace the entire duct system, but often only a portion of the duct work needs to be replaced.


Another commonly needed renovation is to increase the capacity of the return ducts which are too small, causing whistling grilles, noisy ducts, and reduced operating energy efficiency. If the entire duct system needs to be replaced, Miller Service Company can correctly size the ducts and registers to make sure it will provide the most comfort and efficiency.


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