How does a tankless gas water heater work, and is it worth it?

How a tankless gas unit works

A tankless (instantaneous) water heater has no storage tank. When you open a hot tap, a flow sensor detects water moving through the unit, the burner lights, and a heat exchanger heats the water as it passes. When the tap closes, the burner shuts off^[1]. Because nothing sits hot between uses, there is almost no standby loss^[1].

To heat water that fast, the burner has to be big. DOE's selection guide notes that gas tankless units are rated at 50,000 to 200,000 Btu/h input, while residential gas storage heaters fall under 75,000 Btu/h^[2]. One example: Rinnai lists its RU199eN condensing model at 15,000 to 199,000 Btu/h, with a maximum flow of 11 gallons per minute and a UEF of 0.95^[3]. Maximum flow is a best case; real output depends on how much the water must be heated.

Flow rate is the sizing number

A tankless unit never runs out, but it can only heat so many gallons per minute. The colder your incoming water, the fewer gallons it can deliver at shower temperature. Size it by the fixtures you run at once and your winter temperature rise, using the maker's flow chart, not the maximum on the box. See tankless sizing by groundwater temperature and our GPM calculator.

Condensing vs non-condensing

	Non-condensing (conventional)	Condensing
Efficiency in DOE's guide	EF about 0.82[1]	EF 0.92 or higher[1]
Heat exchanger	Typically copper, more prone to scale corrosion[1]	Stainless steel second stage that recovers heat from exhaust[1]
Venting	Hotter exhaust; venting per the maker	Cooler exhaust; condensate must be drained[1]
Upfront cost	Lower	Higher; lower operating cost[6]

DOE's guide uses the older Energy Factor (EF) metric; current labels show UEF. ENERGY STAR sets the bar for gas tankless at a UEF of at least 0.95 and at least 2.8 gallons per minute over a 67 F rise, with a warranty of at least 6 years on the heat exchanger and 5 years on parts^[4]. More on the trade-off in condensing vs non-condensing.

Gas line and venting

This is where most conversion budgets go.

• Gas supply. DOE's guide says the fuel needs of a gas tankless unit may require a larger gas line from the meter or a dedicated line, and that in most cases the line serving the unit will need to be 3/4 inch^[1]. Sizing depends on the length of the run and every other gas appliance on the line. That is licensed gas work. See gas line sizing for tankless.
• Venting. HomeGuide notes that an old tank's vent cannot be reused for a tankless unit^[6]. Vent material, length and termination clearances come from the maker's manual and local code.
• Condensate. Condensing units need a drain for acidic condensate^[1], and many installers add a neutralizer.

None of these are DIY tasks. If you smell gas at any point, leave the house and call the gas utility or 911 from outside^[11].

What it costs

HomeGuide's January 2026 installed ranges^[6]:

Fuel	Installed
Natural gas	$2,100 to $5,600
Propane	$1,600 to $4,500

Upgrades drive the high end: a larger gas line runs $350 to $2,000, and new venting $100 to $600^[6]. See tankless water heater cost.

There is no federal tax credit for 2026 installs. The IRS says the 25C credit is not allowed for property placed in service after December 31, 2025^[10]. Some gas utilities still pay rebates for ENERGY STAR units.

Is the government banning gas tankless?

No. DOE finalized a stricter gas tankless standard in December 2024, but Congress disapproved it under the Congressional Review Act, signed into law as Public Law 119-6 in May 2025^[8]. DOE has published a notice withdrawing the disapproved rule^[9]. Any claim that gas tankless units are being phased out is out of date.

Upkeep

Scale is the main enemy. Minerals in hard water coat the heat exchanger, cut flow and, on most brands, trigger a service reminder code. See error codes. DOE's guide says units should be flushed per the maker's instructions to prevent calcium buildup^[1]. HomeGuide's 2025 range for a professional flush and descale is $75 to $300^[7]. See how to descale a tankless water heater.

Lifespan

Rheem puts tankless life at 15 to 20+ years with proper care, compared with 8 to 12 years for tanks^[5]. That longer life is a big part of the value case if you plan to stay in the home.

Who a gas tankless unit suits

• Homes with gas service and room on the gas line, or a budget to upsize it.
• Households that run out of hot water with a tank, or want wall space instead of a tank.
• Owners who will keep up with descaling, especially in hard water areas.

It is a weaker fit where the gas line and vent would both need major work, or for a short stay in the home. Compare tank vs tankless.

When to call a pro

• Any tankless install or tank-to-tankless conversion: gas sizing, venting and condensate are licensed work.
• Ignition or flame error codes that return after one reset with gas confirmed on.
• Vent or condensate codes that return after the outside terminal and drain line are clear.
• Any gas smell: leave first, then call from outside.