Tankless water heater installation: what to know

What the job involves, what changes the price, and what to ask a pro.

What tankless water heater installation is

Tankless water heater installation replaces a tank heater with an on-demand unit that heats water as it flows. It often means upgrading the gas line or electrical supply, adding venting, and mounting a compact heater on the wall. People choose it for steady hot water and to free up floor space, but the install is more involved than a tank swap.

Signs you need it

  • Your household runs out of hot water when two or three fixtures are on at once, and you want steady supply without a large tank
  • You have limited space in a closet, garage or utility room and the tank takes up floor area you need
  • The old tank is due for replacement and you want to move the heater to a wall
  • The gas meter, gas line or electrical service may not support a larger heater, which the pro needs to check
  • The old tank has heavy scale or sediment, and you want a unit with service valves so it can be flushed on a schedule

What the pro does

  • Check the gas line size, the gas meter capacity and the gas pressure, since on-demand units need a large gas flow
  • Plan the venting, which is usually plastic pipe for condensing units and stainless steel for non-condensing units, as the manufacturer specifies, with the correct slope and termination
  • Check the electrical service for electric units, since they can draw a large current and need a dedicated circuit
  • Mount the unit on a wall and install isolation valves, a water filter screen and a service port for flushing
  • Set the unit's flow and temperature settings, and check the flow at the fixtures that need the most hot water
  • Add a condensate drain for condensing units, then test the gas, water and venting connections for leaks and proper operation

What changes the price

Every job is different, so we do not publish prices. These are the things that move the quote up or down.

  • Fuel type, and whether the gas line or electrical service must be upgraded
  • Condensing or non-condensing unit, which changes the venting material and route
  • Number of fixtures and the peak flow the household needs
  • Venting route length, wall and roof penetrations, and any need for a new termination
  • Whether a recirculation loop or a service valve set is added
  • Permit, inspection and any gas meter or utility work needed for the upgrade

What a written quote should spell out

  • Unit brand, model, rated flow and fuel type
  • Gas line size and any upgrade, with the work done by a licensed gas fitter or the plumber
  • Venting material, route and termination, and whether the condensate drain is included
  • Isolation valves, water filter, service port and any recirculation components
  • Who handles the permit, the inspection and the warranty registration

Questions to ask before you hire

  • Does my gas meter and line support the heater's rated input, and will you check the gas pressure with the unit running?
  • Is the unit condensing or non-condensing, and what venting does that need?
  • How will you check the water flow at my highest-demand fixture?
  • Do you install a service port and isolation valves so the unit can be flushed?
  • Is the warranty dependent on annual descaling, and who does it?
  • Who pulls the permit, and will the gas work be done by a licensed gas fitter?

Red flags

  • Pro who installs a tankless heater without checking the gas line size and the meter capacity
  • Pro who uses venting material the manufacturer does not list for the unit, such as metal pipe on a condensing model
  • Pro who does not install isolation valves or a service port, which makes descaling hard later
  • Pro who sizes the unit only from the fixture count and does not check the flow the household really uses

What you can do yourself

A homeowner can plan the location, clear the wall space, and note the model and fuel type before the pro visits. Do not run the gas line, change the vent or wire the unit yourself, since a wrong gas connection or vent can leak carbon monoxide and a wrong electrical hookup can overheat.

Licenses and permits

Tankless water heater installation is commonly licensed plumbing work, and gas and venting work often requires a licensed gas fitter. Many areas require a permit and inspection, so check with your state or local licensing agency before work starts.

Tankless water heater installation questions

Will a tankless heater give me unlimited hot water?

No. It gives a steady flow of hot water up to its rated flow at a set temperature rise. Running several high-demand fixtures at once can exceed that flow, so the pro should size the unit from your real use.

Why does a tankless heater need descaling?

Minerals in hard water build up inside the heat exchanger and reduce flow and efficiency. Regular descaling through the isolation valves and service port keeps the unit working as designed.

Do I need a bigger gas line?

Often, yes. A tankless unit needs more gas flow than a typical tank heater, and the gas line and meter must be checked before the unit is chosen.

Can I put a tankless heater in a closet?

Possibly, depending on the clearance, the venting route and the combustion air. The pro should check the space and the vent before the unit is ordered.

About this guide

This national guide is written and checked by infinibell. Rules and conditions differ by place, so local pros are credited on guides for the cities they serve, not on national ones.

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