Tankless Install in a Slab Home (No Basement)
Slab-on-grade construction is the default in much of the South and Southwest and a growing share of new construction nationwide. It is also one of the harder retrofits for tankless installation. There is no basement or crawl space to route plumbing, gas, and venting through. Every line either runs inside finished walls, through the attic, or along an exterior wall. This page walks the practical install paths and the 2026 cost.
Why slab construction changes the install
A typical basement or crawl-space home gives the plumber three feet of clearance beneath the living space to run new gas and water lines, to install a recirculation return loop, and to route condensate drains to a floor drain or sump. Most of the install labor happens in that clearance space, out of sight of the finished living areas. Drywall stays intact. The homeowner sees the unit on the wall and not much else.
A slab home removes that clearance entirely. The first-floor framing sits on a poured concrete slab that is the foundation, the floor, and the limit of routing access. New gas and water lines have three options. They can run through the attic, drop through walls at each appliance and fixture, and back up to the tankless. They can fish horizontally through interior partition walls, which requires opening and patching drywall. They can run along exterior walls inside decorative chases or behind cabinetry. Each path adds labor cost compared to simply going under the floor.
The condensate drain is the most often awkward routing in a slab home. A condensing tankless produces 1 to 2 gallons per day of acidic water that needs to reach an approved drain. In a basement home, that drain is the basement floor drain or sump. In a slab home, there is no floor drain. The condensate has to be pumped or gravity-fed to a sink, a laundry standpipe, or an exterior drain, none of which are conveniently located near the tankless.
Garage install (the easiest slab option)
In a slab home with an attached garage, mounting the tankless on the garage wall is usually the cheapest install. The garage typically shares a wall with the kitchen, laundry, or a bathroom, which puts the tankless within 10 to 20 feet of the existing hot water main. Gas is often already routed to the garage for a gas dryer or workshop. Venting through the garage exterior wall is straightforward with no setback complications. The garage floor itself is sloped to a drain in most slab homes, so condensate routing is simple.
Three considerations specific to garage installs. First, in any climate that sees freezing temperatures, the unit needs freeze protection. Most modern tankless units have built-in low-temperature shutoff and small heat coils, but the condensate drain and water lines outside the unit still need protection. Second, the unit cannot be installed within 18 inches of the floor (IRC 305 for gas appliances in a garage) to keep the burner above potential gasoline-vapor zones. Third, the gas shutoff and water shutoffs must remain accessible from the garage, which is generally fine.
Total install cost premium for the garage location: typically $0 to $300 over a generic slab install quote, sometimes a savings of $200 to $400 compared to an interior closet location with longer plumbing runs.
Interior closet retrofit
When the existing tank heater is in an interior closet (common in 1960s to 1990s slab construction), the tankless can sometimes replace it in the same closet. The water connections are right there. Gas line is usually right there. The challenges are venting and condensate.
Venting from an interior closet. The old B-vent ran straight up through ceiling and roof. The new tankless cannot reuse that flue (different material, different temperature). The new vent has to run either through the ceiling to attic and out through roof (longest run, most labor, $600 to $1,400 in vent labor and material), or horizontally through finished walls to the nearest exterior wall (shorter run but requires opening and patching drywall, $400 to $1,000 with patch labor included).
Condensate from an interior closet. A gravity drain to anywhere is rare in this scenario. The condensate pump is almost always the answer, plus a 1/4-inch tubing run to the nearest drain (laundry standpipe, bathroom sink tailpiece). Total condensate work runs $200 to $400 in this scenario.
Exterior wall cabinet (warm-climate special)
In Florida, Texas, Arizona, Louisiana, and parts of California and Hawaii where freeze risk is minimal, outdoor-rated tankless units mount directly on an exterior wall. The unit's own enclosure provides the weather protection. No separate venting is needed because the burner exhausts directly to atmosphere through louvers in the cabinet. Common models include the Rinnai V94eN, the Noritz NRC83-OD, and the Rheem RTGH series with outdoor enclosure.
Advantages are real. No interior space consumed. No vent pipe routing required. No combustion air calculation. Gas and water connections come straight through the wall. The condensate drips off the unit directly to a splash block or french drain. Install labor is straightforward.
Trade-offs are weather and aesthetic. The unit lives outside, so the homeowner sees it from the yard. Some HOAs restrict visible mechanical equipment. In any climate that occasionally freezes, the unit must have working freeze protection and reliable electrical power. A power outage during a Texas or Louisiana freeze can damage the heat exchanger if the freeze protection cannot run. Outdoor units in marginal climates need to be paired with a battery backup or generator for the controls circuit, adding $150 to $400.
Long-run recirculation in slab homes
Many slab homes are long and shallow, with bedrooms and bathrooms strung along one side and the kitchen and laundry on the other end. A tankless mounted on the garage wall might be 40 to 60 feet of plumbing run from the master bath. Without recirculation, hot water at the master bath takes 45 to 90 seconds to arrive. The user runs the tap, walks away, and comes back to a warm shower.
Two recirculation approaches work for slab homes. The first is a dedicated return line, which means running a third pipe (the return) alongside the hot water main from the farthest fixture back to the tankless. In a basement home this is easy. In a slab home it means either through the attic (extra labor and material) or inside walls (drywall work). Cost $400 to $1,200 for the dedicated return.
The second is a crossover valve at the farthest fixture, which uses the cold water line as the return path. The tankless circulates hot water out the hot main, the crossover bleeds it into the cold line, and the now-cool water returns through the cold main. No third pipe needed. Cost is $200 to $500 for the pump and crossover valve. Trade-off is small temperature mixing at the cold tap during the first few seconds of cold-water use, a minor inconvenience most households tolerate.