Installing a tankless water heater is usually a professional-level plumbing, gas, venting, and electrical job, so the safest route is to hire a licensed installer for gas units, and even many electric units still need code-compliant wiring and permits. A typical installation involves removing the old heater, mounting the new unit, running or upgrading gas and electrical service, adding proper venting and condensate drainage, connecting water lines, then testing for leaks and startup faults.

What the job involves

A standard gas tankless install usually includes:

  • Shutting off water, gas, and power.
  • Removing the old tank or previous unit.
  • Mounting the tankless heater on a wall with the correct clearances.
  • Upgrading the gas line if the new unit needs more capacity.
  • Installing intake and exhaust venting through an exterior wall or roof.
  • Connecting cold-water in and hot-water out lines.
  • Adding condensate drainage and, on some units, a neutralizer.
  • Powering the control system and performing a full leak and function test.

Electric tankless models are different, but they still require dedicated electrical circuits and proper wire sizing, and the manufacturer instructions must be followed exactly.

Safety first

This is not a casual DIY project because mistakes can create gas leaks, carbon monoxide hazards, flooding, or electrical shock risks. Home Depot explicitly recommends professional installation because the process is difficult and sometimes dangerous, and This Old House says gas piping installations are best left to a licensed plumber.

Basic planning steps

Before installation, check:

  • The unit size needed for your household’s hot-water demand.
  • Whether your gas meter and line can support the heater.
  • Whether the vent path is short and code-compliant.
  • Whether you need a condensate drain.
  • Whether the electrical supply is adequate.

Sizing matters a lot: undersizing leads to weak performance, while oversizing wastes money and space.

Simple outline

  1. Choose the right model for gas or electric use.
  2. Confirm local code, permits, and clearances.
  3. Shut off utilities and remove the old heater.
  4. Mount the unit securely.
  5. Install venting and condensate handling if required.
  6. Connect water, gas, and electrical service.
  7. Purge lines, check leaks, and test startup.

Good sources

For practical install overviews, these guides are useful:

TL;DR: tankless water heater installation is usually best handled by a licensed pro, especially for gas models, because venting, gas sizing, electrical work, and leak testing all need to be done correctly.