April is finally here, and the changeover from heating to cooling has begun in many parts of the country. Many of your customers will find themselves in need of your services when cold air doesn’t blow from the registers. Often, the customer will need new equipment.
Depending on how you handle a replacement scenario, you may unintentionally inherit bad installation conditions that prevent new equipment from operating to its full potential, forcing it to operate outside of manufacturer specifications.
Before you quote equipment replacement, look out for these four duct connections that kill airflow at a gas furnace or air handler. You can easily address many of these before selling the job and repair them during replacement.
Restrictive Return Fittings
Undersized return drops and sharp transitions restrict airflow and shorten equipment life. On gas furnaces, one solution is to add a return air inlet to the equipment when furnace capacity exceeds 80,000 Btuh. Many HVAC professionals will add a second return drop to the opposite side of the equipment or into the bottom of upflow equipment.
It’s easy to assume a blower will pull airflow regardless of the return fittings' installation. If the duct is large enough to move the proper amount of air, it should work as intended. However, this is a dangerous assumption. Depending on the number of turns built into the return fittings, severe airflow reduction can occur.
Sharp turns in a duct system reduce the effective size of an opening. When air attempts a 90-degree turn, it creates a tremendous amount of turbulence, and the air velocity decreases. To prevent this problem, make sure you correctly size and install your return fittings for each application. They work best when you use long sweeping bends that allow airflow a smooth path to travel back to the blower. When in doubt, bigger return ducts are better return ducts.
Restrictive Air Filters
Installing the filter at the inlet of a furnace or air handler is another issue. A filter installed in a poor location reduces its effectiveness because of the unused filter area. Factory filter racks are often undersized and allow unfiltered air to bypass the filter media.
Incorrectly sized and/or restrictive filters also increase the pressure drop across them and the filter face velocity. 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 filter size when this is a concern. Adding a second return drop as described above allows space to add another filter into the system. Using return air filter grilles is another way to increase the filter surface area where conditions permit. Think twice about installing 100,000 Btu input furnaces with a single 16 x 25 x 1 filter.
Restrictive Evaporator Coil
It’s common in northern climates to install an undersized coil on a larger furnace where cooling loads are low and heating loads are high. Typically, a tiny evaporator coil, matching the cooling load, sits on an oversized furnace. As indoor coils have become more restrictive, this problem has moved south, even if the coil dimensions match the furnace cabinet.
Restrictive evaporator coils have a high pressure drop and create excessive supply static pressure. If you select this component improperly, it can prematurely harm the replacement equipment. Unless you look at the manufacturer’s coil performance data, you’re guessing whether the replacement coil will work correctly.
There are a lot of side effects of restrictive coils. On a cooling system, you may end up with thermostatic expansion valve (TXV) hunting, which leads to premature compressor failure. On a gas furnace, the heat exchanger may fail because the blower can’t move proper airflow through the coil. The furnace continually cycles on high limit, causing undue heat exchanger stress and eventual failure. Inverter-based equipment simply won’t tolerate these conditions. This equipment will send you a friendly “fan-failure” code on the wall controller and then shut down.
Correct this condition by using a larger replacement evaporator coil with a rated pressure drop of .30 inches of water column (in.-w.c.) or less. This minor change will decrease static pressure and increase airflow.
Poor Supply Plenum Construction
Deadhead plenums or bullhead tees leave air with no direction as it exits a furnace or air handler. 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 making a turn. If you ever experienced temperatures much warmer on one side of the duct system than the other, you’ve seen the results of this installation technique.
A better option is to use a properly sized plenum that has take-offs with an opening sized approximately 50% larger than the duct. The larger inlet gives air coming out of the furnace a low-pressure path to merge smoothly into the duct system. Alternatively, you can use a splitter tee with radius throats and heels to direct air into the duct system.
Another example of poor supply plenum construction is panning off the top of an evaporator coil and tapping a round duct directly into it. This practice has never made sense to me. The fitting chokes off the coil opening and creates extreme turbulence.
Correct this issue by installing an extended centerline square-to-round transition from the coil to the round duct size. Air will flow much more smoothly from the furnace with this style of duct fitting.
Don’t Inherit These Conditions
Poor performance and premature equipment failure increase when any of these common installation mistakes exist. Who is to blame when the equipment fails under these conditions? It isn’t the equipment manufacturer’s fault.
To ensure you don’t inherit any of these four common equipment duct connection defects, you need to make the results visible to the customer before you quote a replacement. It’s much easier to address these defects upfront than after you’ve sold and installed the job, only to find it doesn’t work as promised.
If you’re unsure where to start, check out a proven approach to “Air Upgrade” your installations and the series of articles that follow. It will point you in the right direction on how to handle these pesky duct problems and convert them into profitable solutions your customers will gladly invest in.