The simplest and most common spot to install a ductless minisplit heat pump head is on an exterior wall. Once the head’s location has been established, a hole is drilled behind the unit for refrigerant pipes, communication wire, and drain hose. Typically, this hole is 2-1/2–3 in. in diameter and slopes slightly downward to the outside to ensure condensate will drain well. The communication wire is fed through the hole and attached to the unit. Then, as the ductless head is secured to the wall, the excess wire, refrigerant pipe stubs, and condensate hose are fed out through the hole. The remaining connections are made outside the building.
The final step is to seal the wall penetrations, which is important for several reasons:
Energy savings. An unsealed hole will allow cold air to enter in the winter, and hot-humid air to enter in the summer, thereby driving up energy costs year-round.
Humidity and condensation control. Summer air leaking into the wall assembly behind the ductless head may cause condensation on refrigerant lines and other cold surfaces. Left unchecked, mold can grow. The outside air also adds extra latent load, making it harder to dehumidify the living space.
Vermin, dust, and pollutants. An unsealed hole is a superhighway for vermin—from spiders and insects to mice and small snakes. It also gives outdoor dust, pollen, and insulation fibers a path into the ductless head, where they can build up on the fan, coil, and filter and make their way into the living space.
Temperature control. Most ductless heads use a thermistor mounted inside the head itself, rather than a remote thermostat, to monitor room temperature. A poorly sealed hole behind the head can allow outdoor air to leak more or less directly onto the thermistor, wreaking havoc on the head’s ability to sense and control temperature.
Sealing the wall penetration is straightforward in a wall filled with rigid material, whether it’s poured concrete, a structural insulated panel (SIP), or a frame wall insulated with polyurethane foam. Once all of the piping connections are made and tested, a generous squirt of expanding foam sealant does the job. The challenge comes with empty wall cavities and frame walls that are insulated with fibrous materials like fiberglass and cellulose. With nothing to contain the expanding foam, it rarely forms a tight seal. Even if the expanding foam does a good job of stopping outdoor air at the sheathing, the interior hole remains open to the wall cavity.
This is where the wall sleeve comes in. It’s a plastic tube that lines the passage through the wall cavity. I’m aware of three models from manufacturers Rectorseal, Diversitech, and Pioneer. They look slightly different but perform the same function. The Rectorseal model, which is the one I’ve used, telescopes to accommodate different wall thicknesses. The others are fixed tubes that can be cut to length with a utility knife. They all provide a rigid liner that can be filled with expanding foam to form a durable seal. Where rodents are a concern, I recommend stuffing the sleeve with copper wool before applying the foam. Although not necessary for air-sealing penetrations in a poured concrete wall, wall sleeves can be used in this application to protect line sets, insulation, and wiring from contact with sharp and corrosive materials in the concrete.
Detailing wall penetrations in this way takes little effort and comes with significant payback, which, in my opinion, means it should be standard practice.
Jon Harrod is founder of Snug Planet, a contracting company in Ithaca, N.Y., whose mission is to reduce building energy use in ways that make sense for people and the planet. Jon holds multiple certifications from the Building Performance Institute and has published numerous articles on energy efficiency and green building.