Figure 1. Air seal all holes and seams in the furnace cabinet with mastic, foil tape, or putty. Pay special attention to sealing the junction between the furnace cabinet and the evaporator coil cabinet.
Air handlers (see Figure 2 below), furnaces, and evaporator coil cabinets come from the factory with holes in the form of knockouts, penetrations, and slots for installing piping and wiring. These holes are there for ease of installation and service. However, when installation is completed, any unused holes should be sealed, along with any gaps around wiring and piping. Holes where the condensate line and refrigerant lines penetrate the evaporator coil cabinet will be the next highest pressure point, and depending on the model, these may be a point of negative pressure. Seal around these lines with non-hardening putty. Use non-hardening putty to seal around pipes, tubing, and conduit penetrations in the air-handler cabinet as well. This putty comes in strips, slugs, and cords (see Figure 3) and does not dry out but remains pliable so it can be removed and reapplied. Seal unused electrical and piping knockouts with mastic.
The third point of high (negative) pressure is the area of the cabinet that houses the indoor blower fan. With respect to indoor air quality, this may be considered the most concerning area for air leakage, especially if the furnace is located in a garage or any other area where chemicals are stored or where there is exposure to carbon monoxide. Any seams or unused holes should be sealed with mastic.
The connections with the supply and return plenums are additional areas that experience high pressures. These seams and other cabinet seams should be sealed with mastic, mastic and fiberglass mesh tape, or UL-approved tape.
There are some penetrations in a furnace cabinet that are not considered leakage points. The furnace in Figure 1 is a condensing gas furnace. The penetrations for the condensate line for the condensing gas furnace, vent pipes, gas line, and high voltage wiring (not shown) are not connected to the conditioned air stream and therefore are not areas of concern for leakage.
Regarding cabinet panels that must be periodically removed for routine maintenance of the HVAC equipment, some HVAC technicians suggest using cloth-backed duct tape to seal the panel seams because it is easy to remove or cut through.