#1 · MERCER ISLAND
Tankless water heaters heat on demand — endless hot water, 20+ year lifespans, and closet-sized footprints. Wet-side installs must be sized for our cold 45–50°F groundwater, which catches out units (and installers) from milder climates.
Wet-side market range — get itemized quotes before work starts
Companies whose focus matches this work are listed first, then Mercer Island's other top-rated plumbers. Same no-pay-to-play ranking as everywhere on Wetside Plumbing — the formula.
Includes pros based nearby in Seattle who serve Mercer Island.
Google Business data, 2026-08-07. See the full Mercer Island ranking →
Also searched as: tankless water heater installs, tankless water heaters.
Long-run economics favor tankless (lifespan, efficiency); upfront cost favors tanks. If you're staying put 10+ years or chronically run out of hot water, tankless usually pencils. Our tank-vs-tankless guide runs the numbers.
A tankless unit's output depends on how much it must raise incoming water temperature. Wet-side groundwater is cold most of the year, so a unit that serves a three-bath house in California may struggle here. Insist on sizing math for 45°F inlet water.
Ask for their Washington L&I contractor number and verify it at secure.lni.wa.gov/verify — active license, bond, and insurance. Listings above with a green badge are ones we've already matched to an active license in state records.