In high-performance homes, all HVAC equipment, including the furnace or heat pump air handler and any ducts, should be located within the thermal envelope of the home. When the air handler is located within the conditioned space, it is tempting to think that sealing the cabinet is not that important because conditioned air will leak into the home rather than being lost to an attic or crawlspace. However, sealing cabinet air leaks is still very important for maximizing the performance of the HVAC equipment because it helps to ensure maximum air flow to the ducts. In a central forced-air system, the highest air pressures are experienced at the air handler, with pressures increasing the closer one gets to the air-handler fan. It is common for air pressures in the supply and return plenum at the air handler to equal or exceed 0.5-inch water column (125 Pascals). Therefore, it is critical to seal up the knockouts, seams, and slots in the air-handler cabinet.
Gas- or oil-fired furnaces are typically equipped with an add-on refrigerant coil (called the evaporator coil) to provide cooling during the summer months (see Figure 1 below). The connection between the evaporator coil cabinet and the furnace cabinet is likely the highest point of pressure in the system and can be a large source of leakage if care is not taken to seal this juncture properly during installation. Refrigerant coil cabinets do not always fit directly on top of the furnace; many coil cabinets have a larger footprint than the furnace, so the seam between the two boxes is uneven. The coil cabinet should be sealed to the furnace using mastic or an Underwriters Laboratories (UL) 181-approved foil tape. For larger gaps (greater than 3/8-inch) mastic and fiberglass mesh tape should be used.
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.
The furnace or air handler and all associated ducting should be installed within the conditioned space of the home. An air handler should be selected that has a manufacturer’s designation showing that air leakage is no more than 2% of the design air flow rate when tested in accordance with ASHRAE 193 (per 2012 IECC R403.2.2.1).
A duct blower tester could be conducted to test the airtightness of the air-handler/furnace cabinet and ducts. The duct system could be verified to meet code or program airtightness requirements.