A tankless water heater stores no hot water. When a hot tap opens and enough water flows through it, a sensor tells the control board to fire a gas burner or switch on electric elements, and the water is heated as it passes. That makes its limit heat per minute, not gallons in a tank: the Department of Energy’s archived guide says tankless units typically deliver 2 to 5 gallons a minute, with gas models giving more flow than electric ones. So a tankless heater is sized by flow and temperature rise, it needs descaling where the water is hard, and when a gas model stops it usually tells you why with a code.
General information, not professional advice. Gas, 240-volt power, scalding water and a pressurised tank can hurt you. Follow the label and manual on your own heater; where they say to stop and call a qualified technician, stop. Codes and licensing rules are set locally.
Water Heater Handbook sells, installs and recommends nothing, and names no models to buy. The guides below take their steps and figures from the makers’ own manuals and bulletins, named and dated, and from federal texts. Read them in order: how the heater works first, because sizing, care and codes all follow from it.
If you smell gas or a CO alarm sounds
If you have a gas tankless heater and smell gas, don’t look for the cause and don’t restart the heater: the federal pipeline safety agency, PHMSA, says make no flame, don’t use a phone and don’t switch any light or electrical switch on or off; leave and call 911 from somewhere well away (PHMSA). If a carbon monoxide alarm sounds or people feel ill, CPSC says get outside to fresh air at once, then call 911 (CPSC). If any part of the heater has been under water, don’t use it. Our guide to a gas smell or carbon monoxide near a water heater covers alarms, symptoms and shut-off.
1. How a tankless heater works
Inside, the sequence is short. Water starts to move, and nothing happens until it reaches the model’s activation flow: Rinnai lists 0.4 gallons a minute for its N and RE series (0.26 on the two largest RE models), and Rheem about 0.25 for two small electric models. Sensors read the incoming and outgoing water temperature. The burner lights, with no pilot in the Rinnai gas models we read, and turns up or down; the largest Rinnai models run up to 199,000 Btu an hour. When the tap closes, the heat goes off and there is no stored water to keep hot.
That design explains the everyday quirks. A trickle from one tap can run cold or swing between hot and cold, because the flow is below or close to the minimum. A gas model needs its own vent, not the old chimney, and a gas line sized for its full demand plus every other gas appliance; Rinnai says a trained and qualified professional must install the gas supply. An electric model needs no vent but draws heavy current, so its breaker and wire sizes come from the rating plate and the maker’s manual. Our guide to how a tankless water heater works goes through each step from the makers’ manuals, with condensing and non-condensing gas models side by side, relief valves and freezing.
2. Sizing: flow and temperature rise
A tank heater is sized by its first hour rating. A tankless heater is sized by two numbers: the gallons a minute you run at the same time, and how many degrees the heater has to add. DOE’s archived sizing page works an example: a 0.75 gallon-a-minute tap plus a 2.5 gallon-a-minute shower is 3.25 gallons a minute, and 120°F wanted minus 50°F coming in is a 70°F rise. DOE says a 70°F rise is typically possible at about 5 gallons a minute through gas-fired tankless heaters and about 2 through electric ones; those are typical figures, not limits (DOE, archived 2 July 2026).
Two things trip people up. Winter water: with the same shower and tap and water coming in at 40°F, the rise becomes 80°F and the rated flow to look for goes from about 3.4 to about 3.9 gallons a minute. And the label: the federal test measures a tankless heater’s maximum flow at a 67°F rise, from 58°F water to a 125°F outlet (10 CFR 430, Appendix E), so the label figure only fits your house if your rise is about the same. Our tankless size calculator adds up your own fixtures and incoming temperature, restates the need on the label’s basis, and explains how to read a maker’s flow-and-rise chart.
3. Looking after it: descaling and filters
A tankless heater has no tank to drain of sediment. Its care is descaling: pumping a mild acid, usually white vinegar, through the heat exchanger before hard-water scale builds up, plus rinsing the inlet water filter and, on indoor condensing units, the air filter. No maker prints one interval for every home. Navien ties it to water hardness, from no flushing for soft water to once a year in a home from 4 grains per gallon up; Rinnai’s SENSEI heaters show an “SS” reminder at the interval the installer set; Rheem has an optional code 88. Rinnai’s manual says damage from lime build-up isn’t covered by its warranty.
The procedure differs by maker and series, and every one we read needs isolation valves with service ports under the heater. The makers also differ on who does it: Rinnai’s manual says maintenance is for a trained and qualified professional, while Navien gives owners the steps but calls flushing “a somewhat complicated procedure”. Our tankless maintenance and descaling guide gives Rinnai’s and Navien’s steps as written, each maker’s schedule and reminder codes, the yearly checks, and the stop points.
4. When it shows a code
Gas tankless heaters report faults as codes, and their meanings can differ by maker and series, so start from the manual for your model. We start with Rinnai’s. In Rinnai’s manuals, code 11 means no ignition: the heater tried to light and didn’t get a flame. Code 12 means the flame went out or couldn’t be sensed. Code 10 means the air supply or exhaust is restricted, and on condensing models it can also mean the condensate trap is full because its drain line is blocked or frozen.
The first checks are ones you can see without opening the heater: is the gas on, is there propane in the tank, is the vent outside clear of snow, ice or debris, and is the condensate line flowing? Most of what comes after that, Rinnai reserves for a trained and qualified professional. Our page on Rinnai codes 10, 11 and 12 sets out each code in Rinnai’s own words, how to read the last nine codes, the owner checks in order and where Rinnai says to stop and call.
Is tankless the right choice?
DOE’s archived tankless page said these heaters can be 24% to 34% more energy efficient than storage tanks for homes using 41 gallons of hot water a day or less, falling to 8% to 14% at around 86 gallons a day. It also said most have “a life expectancy of more than 20 years”; that is DOE’s statement, and the page cites no field data (DOE, archived 30 June 2026). You may read that gas tankless heaters are being banned: DOE issued a stricter standard in December 2024, and Congress voided it on 9 May 2025 (Public Law 119-6). To weigh the options, read tankless vs tank and heat pump vs tankless.
Your next step
- Find the rating label on your heater and write down the maker and full model number, then download that model’s manual from the maker’s own site. Series differ, even within one brand.
- Hold a thermometer under a cold tap in winter, as DOE suggests, and note the reading for sizing.
- Ask your water utility how hard your water is, or have a private well tested, so you know where you sit on your maker’s descaling table.
- For a code, a vent, the gas line or wiring, call a qualified technician; for a gas smell, leave and call 911 or the gas utility from outside. Ask your building department about permits before a heater changes size, fuel or type, and get written quotes.
Sources checked on 9 November 2026.
