How to Install a Tankless Water Heater: A Complete Guide

The first time you feel endless hot water in the shower, you understand the appeal. But the decision to install a tankless water heater is more involved than swapping a tank, and the details that get glossed over in the marketing are exactly the ones that decide whether you love it or curse it. I have set dozens of these units, and the good installs all share the same handful of choices made correctly up front.

Below I will walk you through sizing, the gas and venting realities, real costs, and the parts of the job that genuinely belong to a licensed pro. If you take one thing away, let it be this: tankless rewards planning and punishes shortcuts.

How Tankless Actually Works

A tankless (or “on-demand”) unit heats water only when a tap opens. Cold water flows through a heat exchanger, a burner or electric element fires, and hot water comes out the other side within a couple of seconds. There is no 40 to 50-gallon reservoir losing heat all day, which is where the 25 to 35 percent energy savings comes from for most households.

The tradeoff is that the heater has to raise water temperature instantly, and that takes serious power. A whole-home gas unit needs 150,000 to 199,000 BTU. A whole-home electric unit can pull 120 to 150 amps. Those two numbers drive almost every other decision in the install.

Sizing: Flow Rate and Temperature Rise

Forget gallons of storage. Tankless is sized by flow rate in gallons per minute (GPM) at a given temperature rise. Temperature rise is your desired output (usually 120°F) minus your incoming groundwater temperature. In the northern states that groundwater can sit at 40°F in winter, meaning an 80°F rise; in the South it might be 65°F, an easy 55°F rise.

Add up the fixtures you want to run at once:

• Shower: 1.5 to 2.5 GPM
• Kitchen faucet: 1.5 GPM
• Bathroom faucet: 1.0 GPM
• Dishwasher/washing machine: 1.5 to 2.0 GPM

If you want two showers plus a sink in a cold climate, you are looking at roughly 5 GPM at an 70 to 80°F rise, which pushes you to the largest residential gas units (a Rinnai RUR199, Navien NPE-240, or Rheem equivalent). Undersize it and the unit throttles flow to hold temperature, which feels like a weak shower. When in doubt, size up one model.

Gas, Electric, or Whole-Home vs Point-of-Use

Gas is the standard for whole-home service because it delivers the BTUs cheaply. The catch is that a tankless typically needs a 3/4-inch gas line where your old tank ran on 1/2-inch, and your meter and regulator have to supply the higher demand. Verifying gas supply is the single most-skipped step, and it causes the classic “won’t stay hot” complaint.

Electric tankless avoids venting entirely but demands a heavy service. A 27 kW unit needs three 40-amp double-pole breakers and appropriately sized wire; many older 100-amp panels simply cannot carry it. Point-of-use electric units (under a sink, at a remote bathroom) are a great supplement but are not a whole-home solution in cold climates.

Venting and Combustion Air

Condensing gas tankless units, which are now the majority, vent through PVC or polypropylene because their exhaust is cool. Non-condensing units run hot and require stainless-steel Category III venting, which costs far more per foot. Either way you must follow the manufacturer’s maximum vent length and termination clearances, keeping the exhaust away from windows, soffits, and intakes.

Many units are direct-vent (concentric), pulling combustion air from outside through the same penetration. That keeps things simple and avoids starving the burner in a tight utility closet. Get the venting spec from the actual model’s manual before you cut a hole, because the numbers differ by brand.

What It Costs to Install a Tankless Water Heater

Here is where honesty helps. To install a tankless water heater is usually pricier than a tank swap, mostly because of the supporting upgrades:

• Condensing gas unit (equipment): $1,000 to $1,800
• Gas line upsize to 3/4-inch: $300 to $900
• Venting and termination kit: $150 to $500
• Electrical (120V for the fan/controls, or heavy service for electric units): $150 to $2,000
• Labor, permit, and inspection: $700 to $1,800

All in, a professional whole-home gas conversion commonly lands between $3,000 and $5,500. That number stings next to a $1,500 tank replacement, but the unit lasts 20+ years versus 10 to 12, so the lifecycle math is closer than it looks, especially with the federal energy-efficiency tax credit and any utility rebate.

The Installation Steps (and Which Are Yours)

Here is the sequence a pro follows. I have flagged the parts a handy homeowner can legitimately prep versus the parts that require a licensed plumber and a permit.

1. Pull the permit and confirm gas/electric capacity. Licensed pro. This is the make-or-break step.
2. Mount the unit. Wall-hung on a solid backing, level, with the specified clearances. A homeowner can help here.
3. Run gas and confirm supply pressure. Licensed plumber only. Includes a sediment trap and a proper shutoff.
4. Install venting. Cut the penetration, run pipe within the max length, seal the termination.
5. Plumb water with isolation (service) valves. These flush valves are non-negotiable; you will need them to descale.
6. Wire power and set the temperature to 120°F.
7. Pressure-test, purge air, and commission. Then pass inspection.

Because gas, venting, and code inspection are involved, I steer nearly everyone toward a licensed installer for the core work. A backdrafting flue or an undersized gas line is not a cosmetic mistake.

Hard Water and Ongoing Maintenance

The heat exchanger is the heart of the unit, and scale is its enemy. If your water hardness is above about 7 grains per gallon, plan on a whole-home softener or expect to descale more often. Either way:

• Descale (flush) the exchanger yearly using the isolation valves and a few gallons of white vinegar or a descaling solution circulated for 45 minutes.
• Clean the inlet water filter every few months.
• Check the venting and condensate drain (condensing units produce acidic condensate that needs a neutralizer and a clear drain path).

Skip the flush and you will watch the flow rate and temperature slowly degrade, then get an error code two winters in.

The Cold-Water Sandwich and How to Beat It

One quirk trips up new tankless owners: the “cold-water sandwich.” If someone shuts off a tap and another person opens it a minute later, a slug of cooled water sitting in the pipe reaches you before the heater catches up, so you feel hot, then cold, then hot again. It is normal physics, not a defect.

The fix that homeowners love is a built-in recirculation feature, which several Rinnai, Navien, and Rheem models offer. A small internal pump and, ideally, a dedicated return line or a crossover valve under the farthest sink keeps hot water primed so it arrives quickly and steadily. Recirculation adds convenience and cuts the wait at distant fixtures, though it uses a little energy, so put it on a timer or an on-demand button rather than running it around the clock.

Choosing a Location That Makes Life Easier

Where you hang the unit affects both performance and cost. Mount it as close as practical to the highest-demand fixtures, usually the primary bathroom and kitchen, to shorten the hot-water travel time. Keep it near an exterior wall so the venting run stays short and within the manufacturer’s maximum length, which lowers material cost and improves draft.

Indoor units need a spot that will not freeze, or you rely on the heater’s built-in freeze protection, which requires uninterrupted power. In mild climates you can hang an outdoor-rated model on the exterior wall and skip venting entirely, which is a tidy solution for a home with no good interior spot. Whatever you choose, leave the service clearances the manual specifies, usually a few inches on the sides and generous room in front, so descaling and repairs are not a contortionist act later.

Is It the Right Call for You?

Choose tankless if you have adequate gas supply (or heavy electric service), you value endless hot water and floor space, and you plan to stay in the home long enough to earn back the premium. Stick with a tank if your gas line and panel would need expensive upgrades, or if your usage is modest and predictable. Either way, size honestly for your coldest month, budget for the supporting upgrades, and let a licensed plumber handle the gas, venting, and permit. Done right, it is the last water heater you think about for two decades.