By David Richardson
Low airflow through a furnace forces it to operate outside of manufacturer specifications. The heat exchanger overheats and may lead to premature equipment failure. Let’s look at some the issues that may be killing airflow and damaging your customer’s systems.
Restrictive Return Fittings
Undersized return drops and sharp transitions restrict airflow and shorten equipment life. One solution to this problem is to add an additional return air inlet to the equipment when the capacity exceeds 80,000 Btu. This is done by adding a second return drop to the opposite side of the equipment or into the bottom of upflow equipment.
Contractors often assume a blower will pull airflow regardless how return fittings are installed. If the duct is large enough to move the proper amount of air, it should work as intended. This is a dangerous assumption to make. Depending on the number of turns built into fittings, serious airflow reduction can occur.
Sharp turns in a duct system effectively reduce the size of an opening when air attempts a 90-degree turn. As air tries to go around a sharp turn, a tremendous amount of turbulence is created and air speed decreases.
Make sure your return fittings are sized correctly and installed properly for the application. They work best when you use long sweeping bends that allow airflow an easy path to travel back to the furnace.
Restrictive Air Filters
Installing the filter at the inlet of a furnace is another issue to consider. A filter installed in a poor location reduces its effectiveness because the full area of the filter isn’t used. Factory filter racks are often undersized and allow unfiltered air to bypass the filter due to their design.
Incorrectly sized and restrictive filters also reduce airflow across the heat exchanger and lead to unnecessary overheating. Filters suffering from this problem create issues even when clean. They are too restrictive for the right amount of airflow that must move through them.
Consider increasing your filter sizes or add an extra filter when this is a concern. Adding a second return drop as described above allows space to add a second filter into the system. Using filter grilles is an easy way to increase filter surface area where conditions permit. Think twice about installing 100,000 Btu input furnaces with a single 16 x 25 x 1 filter.
Installing an undersized coil on a larger furnace is common in the north where cooling loads are low and heating loads are high. Typically an extremely small evaporator coil, matching the cooling load, is placed on top of a larger furnace.
The smaller evaporator coil restricts airflow causing a high pressure drop creating excessive supply-side static pressure. This installation kills heat exchangers, as the fan can’t move proper airflow through the coil. The furnace continually cycles on high limit causing undue heat exchanger stress and eventual failure.
Correct this condition by using a larger evaporator coil matching the size of the furnace, which will decrease static pressure and increase airflow.
Deadhead plenums or bullhead tees leave air with no direction as it exits a furnace. Air ends up “splatting” into the end of the duct creating a tremendous amount of turbulent airflow.
Airflow performs best with at least three duct diameters out of the fan before it is forced to turn. If you ever experienced temperatures much warmer on one side of the duct system than the other, you’re seeing the results of this installation technique.
A better option is to use a properly sized plenum with take offs that have an opening sized approximately 50 percent larger than the duct. This gives air coming out of the furnace a low-pressure path to merge smoothly into the duct system.
Coil Panning and a Round Duct Plenum
The practice of panning off the top of an evaporator coil and tapping a round duct directly into it has never made sense. This chokes off the coil opening and creates an extreme amount of turbulence.
Correct this issue by installing a long centerline square-to-round transition coming off the coil to the round duct size. Air will flow much smoother out of the furnace with this style duct fitting.
The chances of premature heat exchanger failure and poor performance are greatly increased when any of these common installation mistakes are made. Who is to blame when the heat exchanger fails under conditions like this? It probably isn’t the equipment manufacturer’s fault.
The answers are out there and together we will find them. If you need any additional information on this material or have any questions, feel free to e-mail me at firstname.lastname@example.org.