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A plumbing manifold with multiple pipe runs leaving it

Tankless Water Heaters

Endless hot water, provided the gas line or the electrical service can actually feed it. That caveat is the whole story.

By Brijon M., Owner and writer, MadGrey LLC·Last updated October 6, 2026·How we pick →

A tankless water heater heats water as it flows, so it never runs out and has no standby loss. The constraint is supply: a whole-house gas unit needs up to 199,900 BTU/h of gas, and a whole-house electric unit can draw 100 A or more. Size it by GPM at your temperature rise, never by the headline GPM.

Tankless is the most oversold and most misunderstood category in water heating. The pitch is true: you genuinely do not run out of hot water, the unit genuinely lasts longer than a tank, and it genuinely takes up no floor space. What the pitch leaves out is that the appliance is the easy part. The gas line, the venting, the condensate drain and, for electric, the service panel are where the project actually lives.

So this hub is organized around the decision in the order it gets made: can my house feed one, how big does it need to be, gas or electric, and what does it need alongside it.

Can your house feed one?

Before anything else, two checks. They take ten minutes and they decide the project.

  • For gas: look at the gas meter and the pipe feeding your current water heater. A whole-house tankless unit wants up to 199,900 BTU/h. A 1/2-inch branch that was adequate for a 40,000 BTU tank is almost certainly not adequate for five times that. Upsizing the line, and sometimes the meter, is frequently the largest single cost in the conversion.
  • For electric: read the main breaker. The Stiebel Eltron Tempra 24 Plus specifies a minimum 150 A service and draws 100 A; the EcoSmart ECO 27 draws 113 A across three 40 A breakers; Rheem states a 200 A service requirement for the RTEX-24. If your panel is 100 A or 125 A, whole-house electric tankless is a service upgrade first.

Start with the roundups

The decisions inside the category

A close view of a shower head with water running

Sizing

Tankless sizing chart

GPM by fixture count and temperature rise, with real published rise-versus-flow tables.

Gas or electric

Gas versus electric tankless, the honest comparison
GasElectric
Whole-house capabilityYes, in any climateYes in warm-inlet climates; marginal in cold ones
Typical output5.5 GPM at a 70°F rise (199,900 BTU unit)3.0 GPM at a 54°F rise (24 kW unit at 240 V)
Supply requirementGas line sized for up to 199,900 BTU/h100 A+ of draw, 150 to 200 A service
VentingRequired. PVC if condensing, stainless if notNone
CondensateYes on condensing unitsNone
Install complexityHighMedium, unless the panel needs upgrading
Running cost per gallonUsually lowerFollows the electric rate
Annual maintenanceDescaleDescale

The short version: in a cold-inlet climate, whole-house tankless means gas. Electric tankless is excellent as a point-of-use or single-bathroom unit, and in the southern half of the country it can serve a small whole house. Gas vs electric works through the cost of ownership.

What tankless owners actually complain about

Reading owner reviews at volume, three complaints repeat, and all three are solvable at install time:

  1. The wait for hot water got longer. A tankless unit does not start heating until flow begins, so the delay at a distant fixture is the pipe volume plus the ignition delay. The fix is a recirculation system — either a unit with one built in, or a separate pump.
  2. The cold-water sandwich. Stop and restart a draw quickly and you get a slug of cool water. A unit with a small buffer tank, like the Navien NPE-240A2, is specifically designed around this.
  3. It went lukewarm in winter. Almost always a sizing error: the unit was sized from a headline GPM measured at a low temperature rise. The sizing chart prevents this.

Maintenance is not optional

A storage tank tolerates neglect for a decade. A tankless unit does not. Scale builds on the heat exchanger, flow drops, the unit works harder, and eventually it faults. The manufacturer-recommended interval for residential descaling is about once a year, more often on hard water.

Practically, that means two things you should buy at install time: isolation valves on the hot and cold connections (without them there is nothing to hook a flush hose to), and a flush kit. If your water is hard, read tankless for hard water before you choose the unit.

What goes in with it

A water heater is a system, not a box. These are the parts that go in with it, and the ones people discover they need halfway through the job.

  • Flush kit

    The annual descale is not optional on a tankless unit, and you need a pump, hoses and a bucket.

  • Descaler solution

    What actually dissolves the scale once the kit is hooked up.

  • Recirculation pump

    Fixes the cold-water delay that makes new tankless owners call a plumber.

Questions people actually ask

›Are tankless water heaters worth it?
They are worth it when your gas line can already feed a 199,000 BTU unit, you value the floor space, and you will stay in the house long enough to recover the install cost. They are usually not worth it as an emergency same-day replacement, because the conversion work dwarfs the appliance. Tankless vs tank.
›How long do tankless water heaters last?
Up to 20 years, consistently across the manufacturer and retailer sources we checked, against 10 to 15 for a storage tank. That figure assumes the annual descale actually happens — a neglected unit in hard water does not reach it.
›Do tankless water heaters need to be flushed?
Yes, about once a year for residential use, more often on hard water. You need isolation valves fitted, a pump, two hoses, a bucket and a descaling solution. A flush kit is all of that in one box.
›Can a tankless water heater run two showers at once?
A 199,000 BTU gas unit can — around 5.5 GPM at a 70°F rise covers two showers plus a sink. A 24 kW electric unit can in a warm-inlet climate but not in a cold one. Best tankless for 2 bathrooms.
›Why is my tankless water heater slow to deliver hot water?
Because it only starts heating when flow starts, so you wait for the pipe to clear plus the ignition delay. The fix is recirculation: either a unit with it built in, or a separate recirculation pump.
›Do I need a bigger gas line for a tankless water heater?
Very often, yes. A whole-house unit wants up to 199,900 BTU/h where your old tank wanted 40,000. Have the gas supply sized by a licensed professional before you buy the unit, because this is frequently the biggest line item in the project.