Through-the-Wall AC Units and PTACs: What the Wall Sleeve Decides

Two products that look alike from the street

From outside, a wall unit is a metal grille in a wall. The Department of Energy (DOE) writes the rules for the two products in different places. Its definition of a room air conditioner reads: a "window-mounted or through-the-wall-mounted encased assembly, other than a 'packaged terminal air conditioner'" that cools air for an enclosed space. It also says the unit runs on single-phase electric current. Our reading: "encased" means one self-contained box.

A packaged terminal air conditioner, or PTAC, is built differently. DOE defines it as "a wall sleeve and a separate un-encased combination of heating and cooling assemblies specified by the builder and intended for mounting through the wall." Two terms matter here. The wall sleeve is the metal box set into the wall opening. The chassis is the heating and cooling assembly that slides into it from inside.

A packaged terminal heat pump, or PTHP, is a PTAC that uses reverse-cycle refrigeration as its prime heat source. Our reading: you can think of a through-the-wall room AC as a box that goes into a hole, and a PTAC as a sleeve with a swappable chassis.

Why the wall sleeve decides what a replacement fits

Because the sleeve stays put, the sleeve sets the size of any chassis that can follow it. Friedrich's PTAC manual tells the installer to "Ensure that the chassis is installed in a 16" high x 42" wide wall sleeve that is no deeper than 13 ¾"." It adds that a baffle kit is required if the sleeve is deeper. Its cut-out dimensions for the sleeve are 16-¼ inches by 42-¼ inches.

DOE uses nearly the same numbers to sort PTACs into two size classes. A standard-size unit has a sleeve opening of at least 16 inches high or at least 42 inches wide, with a cross-sectional area of at least 670 square inches. A non-standard-size unit is defined by "existing wall sleeve dimensions" smaller than that. Multiplying 16 by 42 gives 672 square inches, just above the line.

Each class has its own efficiency levels in the federal rule at 10 CFR 431.97(d). Our reading: the non-standard class exists because many buildings have older, smaller sleeves, and DOE chose to keep a place for units that fit them.

Sleeves are not interchangeable between brands or product types. Friedrich's instructions for its WallMaster WSEZ sleeve give a sleeve 27 inches wide and a wall opening of 17-¼ by 27-¼ inches. Our reading: a new unit's maker will name the sleeve sizes it fits, so ask for that list before you buy.

What a sleeve controls that a new chassis cannot

Two Friedrich documents show how much of a wall unit's performance lives in the sleeve. The maker's PTAC manual says the sleeve should sit "with a slight tilt towards the outside to facilitate condensate drainage." Condensate is the water that the cold coil pulls out of the air. The same manual warns "DO NOT allow any pitch toward the inside."

The WSEZ sleeve sheet makes the same point from the other side: "Sleeve projections and leveling precautions must be observed to prevent water entry into the room." It also calls for caulk around the whole opening "to seal all cracks against air and water infiltration."

The PTAC manual's troubleshooting table lists two water symptoms. For water dripping outdoors, it gives the cause as no drain pipe kit installed. For water dripping indoors, it gives the cause as "Wall sleeve is not installed correctly." Our reading: those are sleeve and drain problems, so a new chassis in the same sleeve would not remove them.

Support matters too. Friedrich's sleeve sheet says lintels, which are beams over an opening, must be used in brick veneer and masonry walls. Durastar's sleeve manual says the sleeve "must be fastened securely, before installing the air conditioner unit."

Electric-heat PTAC or heat-pump PTAC

PTACs come in two heating types. In Friedrich's model code, "E = Cooling with electric heat" and "H = Heat Pump with Auxiliary Heat." An electric-heat PTAC warms the room with a heating element, called a strip or coil. Our reading: it has no reverse-cycle mode.

A heat-pump PTAC cools like the other type but can run its refrigerant cycle backward to heat. DOE's definition says it has "a supplementary heat source available," which can be hot water, steam or electric resistance heat. Our reading: the auxiliary strip is the backup, as in the central systems covered on auxiliary heat.

Friedrich says its heat-pump models "will automatically run the electric heater" when first energized, then return to heat pump mode. It also says that when the outdoor coil falls below 26 °F for 3 minutes, the unit runs the electric strip instead of the heat pump. Our reading: those figures are one maker's settings, not a rule for every model.

Florida-specific heat-pump issues are in heat pumps in Florida. For the wider choice between heating and cooling-only units, see heating versus cooling-only units.

Where each type is used

DOE's definition says a PTAC is "specified by the builder." Our reading: that is why PTACs show up where a building was designed with a sleeve in every room.

In its 2023 notice on PTAC standards, DOE modeled energy use with its small hotel reference building and used only the guestroom loads. That is a modeling choice, not a count of where PTACs are installed. We found no federal or maker document that gives a split between hotels, condos and additions, so the rest of this section is our reading.

Our reading for a condo or apartment with a sleeve already in the wall: the decision is mostly about the sleeve. A unit that fits the existing opening is a swap. A different opening is a wall change, which can bring in structure, building rules and the association. Ask the building before anything else.

Our reading for an addition: a room being built has no sleeve yet. For a new room, a ductless system is one option; see ductless mini-split systems. How much cooling the room needs is a separate question, answered in what a BTU means.

Tampa Bay and Florida notes

Salt air is the local issue. Friedrich's manual says that if wall covers are not bought together with the sleeve, "the user must purchase a wall cover that meets the G90 standard or salt spray test." G90 is a grade of galvanized steel coating. Our reading: near the bay or the Gulf, ask what the outdoor grille and wall cover are made of. For the outdoor coil, see coastal coil protection.

Friedrich also says the wall must be "structurally adequate to support the unit including the sleeve, chassis and accessories." Permit needs for work on a wall opening depend on the local building department. See HVAC permits in Florida for background. Durastar says to consult a licensed contractor on wall framing.

What to ask before you replace a wall unit

1. Which sleeve is in the wall, and what is its brand, height, width and depth? Can I see the maker's list of units that fit it?
2. If the new unit does not match the sleeve, what changes: an adapter, a baffle kit, a new sleeve, or the wall opening?
3. Is the sleeve tilted slightly toward the outside, sealed on all sides and drained, as the maker's manual requires?
4. Is the wall around the sleeve supported, for example by a lintel in masonry?
5. Is the new unit electric-heat or heat-pump, and what does the maker say about its backup heat?
6. What are the outdoor grille and wall cover made of, given salt air?
7. Who pulls any permit, and does the building or association need to approve the work?

Refrigerant and electrical work on a PTAC is for licensed professionals. This page gives no steps for it.