What Is a Tankless Water Heater?

1. The Short Answer: A Water Heater With No Tank

A conventional water heater keeps 40 or 50 gallons hot around the clock, reheating it every time it cools, whether anyone needs it or not. A tankless water heater, also sold as an on-demand, instant or instantaneous water heater, stores nothing. Cold water enters, passes through a compact heat exchanger, and leaves hot. When the tap closes, the heater shuts down and goes back to idling on a few watts of standby power for its electronics.

Because there is no stored volume to run out, a tankless heater can supply hot water indefinitely. A fourth back-to-back shower comes out exactly as hot as the first. What it cannot do is supply unlimited hot water at the same moment. Its output is measured in gallons per minute, and every fixture you open draws from the same fixed budget. Endless in duration, finite in rate: that is the whole trade in one line.

The idea is not new. Gas tankless heaters have been the default in Japan and much of Europe for decades, where small homes and expensive energy made a 50-gallon tank in the basement an odd choice. In North America they spread more slowly, mostly because a big gas or electric tankless unit needs far more fuel or power at its peak than a tank does, and older houses were not plumbed or wired for that.

If you are still working out which family of heater suits your home, our guide to the types of water heaters sets tankless beside tank, heat pump, solar and combination systems. This page stays on the tankless unit itself: what is inside it, how it decides how hard to fire, and which numbers on the spec sheet actually predict whether your showers stay hot in February.

2. What Happens Inside When You Open the Tap

Everything starts with a flow sensor on the cold-water inlet, usually a small turbine that spins as water moves past it. When you open a hot tap, water starts to flow through the heater, the turbine spins, and the control board reads the flow rate. Nothing happens until that flow crosses the unit's activation threshold, which on most whole-house gas models is somewhere around 0.4 to 0.5 gallons per minute, and lower on some newer designs.

On a gas unit, the board then runs a short ignition sequence. A combustion fan starts and purges the chamber, the gas valve opens, an igniter lights the burner and a flame sensor confirms it is lit. On an electric unit there is no ignition at all; the board simply switches power to a set of resistance elements wrapped around or immersed in the water path. Either way, heating starts within a few seconds of the tap opening.

The water then passes through the heat exchanger, a coil of copper or stainless tubing with a lot of surface area packed into a small space. The burner's hot combustion gases, or the elements, transfer heat into the water as it travels through. Because the water is only inside the heater for a few seconds, the heat source has to be very powerful: a large gas tankless unit is typically rated at up to 199,000 BTU per hour, roughly four to five times the burner on a typical 40-gallon gas tank.

Finally, a temperature sensor on the outlet tells the board how hot the water leaving the unit actually is. The board compares that to your set point and continuously adjusts the gas valve or the element power to hold it there. When you close the tap, flow drops below the shut-off threshold, the burner or elements cut out, and the fan runs briefly to clear the chamber before the unit goes quiet again.

3. Flow Rate and Temperature Rise: The Two Numbers That Rule It

Every tankless water heater has a fixed maximum heat output. How many gallons per minute it can deliver depends on how much that water has to be heated, which is called the temperature rise. Rise is simply your set point minus the temperature of the water coming in from the street or well. Heating 50-degree water to 120 degrees is a 70-degree rise; heating 70-degree water to the same set point is only a 50-degree rise.

That is why the big number on the box is almost never your number. A unit advertised at 11 GPM reaches that figure only at a small rise, usually in a warm climate. The standard rule of thumb is that it takes about 500 BTU per hour to raise one gallon per minute by one degree. A 199,000 BTU unit at around 95 percent efficiency therefore gives roughly 5.4 GPM at a 70-degree rise, but closer to 8.4 GPM at a 45-degree rise.

Groundwater temperature changes with season and region. Northern states often see incoming water in the high 30s or low 40s in winter, while the Gulf Coast and Southwest rarely drop below the 60s. The same heater can comfortably run two showers in Phoenix and struggle with one shower plus a dishwasher in Minneapolis in January. You size a tankless heater for your coldest month, not your average one.

When demand exceeds what the heater can handle, it does not run out the way a tank does. Most units either let the outlet temperature sag, or restrict flow with an internal valve to protect the set point, so the shower gets weaker rather than colder. Our step-by-step guide to what size tankless water heater you need walks through adding up fixture flow rates and finding your winter rise so you can do this math for your own house.

4. Gas vs Electric Tankless: Same Idea, Very Different Limits

Gas tankless heaters, whether natural gas or propane, are the whole-house workhorses. Their burners deliver far more heat than any practical residential electric unit, so they can supply several fixtures at once even in cold climates. The catch is everything around the unit: a burner that size usually needs a larger gas line than an old tank used, a dedicated vent run to the outside, a 120-volt outlet for the fan and electronics, and in condensing models a drain for condensate.

Electric tankless heaters have no flame, no vent and very few moving parts, which makes them simple to place and quiet to run. Their limit is the electrical service. One kilowatt equals about 3,412 BTU per hour, so even a large 27 kW electric unit produces around 92,000 BTU, less than half a big gas model. At a 70-degree rise that works out to roughly 2.5 to 3 GPM, which is about one shower in a cold-climate winter.

That 27 kW unit also draws around 112 amps at 240 volts, typically spread across three double-pole 40-amp breakers with heavy-gauge wire. Many homes with 100-amp or even 150-amp service simply cannot add that load without a panel upgrade. In warm climates where incoming water is 65 degrees or more, though, an electric whole-house unit can work well, and it avoids the venting and gas-line costs entirely.

The practical rule is that gas suits whole-house tankless in most of the country, and electric suits warm climates, small homes and single fixtures. Our electric tankless water heater picks are sized by amperage and rise for exactly this reason, and our top tankless water heaters list ranks the gas units by real winter flow rather than the brochure figure.

5. Condensing, Venting and Where the Efficiency Comes From

Gas tankless heaters come in two designs. A non-condensing unit has a single heat exchanger, and its exhaust leaves hot, often above 300 degrees. That hot, slightly acidic exhaust needs a stainless steel Category III vent, which is expensive per foot. These models are typically rated around 0.81 to 0.82 UEF, the Uniform Energy Factor the Department of Energy uses to compare water heaters.

A condensing unit adds a second heat exchanger that pulls more heat out of the exhaust, cooling it enough that water vapor condenses into liquid. That recovered heat pushes efficiency up to around 0.93 to 0.96 UEF, and the exhaust is cool enough to vent through ordinary PVC pipe. The trade is a condensate line: the liquid is mildly acidic, so it has to run to a drain, usually through a small neutralizer cartridge first.

The venting difference often matters more than the efficiency difference. A condensing unit costs more to buy, but cheap PVC venting and more flexible placement can make it the less expensive installation overall. We break the numbers down in condensing vs non-condensing tankless, including when the condensate drain becomes the deciding problem.

Where does the efficiency of tankless come from in the first place? Mostly from eliminating standby loss, the heat a tank leaks through its walls and flue all day while it waits for you. The Department of Energy estimates that for homes using about 41 gallons of hot water a day or less, demand water heaters can be 24 to 34 percent more energy efficient than conventional storage heaters, and 8 to 14 percent more efficient for homes using around 86 gallons a day. Heavy users save less because a tank's standby loss is a smaller share of their total.

6. Whole-House, Point-of-Use and Recirculation

A whole-house tankless heater replaces your tank in the same general location and feeds every fixture through the existing plumbing. A point-of-use tankless heater is a much smaller unit, usually electric, mounted under a sink or near a single shower to serve just that fixture. Point-of-use units draw far less power, and because they sit right next to the tap they deliver hot water almost instantly.

Point-of-use heaters are often the right answer for a remote bathroom, a garage sink, a basement bar or an addition that is a long pipe run from the main heater. They can also supplement a tank rather than replace it. Our guide to choosing a point of use water heater covers sizing them by fixture and the circuits they need.

Recirculation is the other piece. A tankless heater does not shorten the wait for hot water at a distant tap. In fact, the few seconds of ignition add a little to it, because the cold water sitting in the pipes still has to run out first. A recirculation system fixes that by keeping hot water moving through the lines so it is waiting at the fixture when you open it.

Some tankless models have a recirculation pump built in and can run on a schedule or learn your usage pattern. Others work with an external pump and either a dedicated return line or a crossover valve under the farthest sink, which uses the cold line as the return. Recirculation costs some energy, so schedules or on-demand buttons are worth using rather than running the loop around the clock.

7. What Changes Day to Day, and Who It Suits

Living with a tankless heater mostly feels like living with a tank that never runs out. A few quirks are new. If someone turns a tap off and on again quickly, a slug of cooler water can pass through before the burner relights, the so-called cold-water sandwich. Very low flows, like a trickle to rinse a razor, may not reach the activation threshold at all, so the water stays cold until you open the tap further.

Maintenance changes too. There is no anode rod to swap and no tank to drain, but minerals in hard water build up on the hot surfaces of the heat exchanger. Most manufacturers call for flushing the unit with a descaling solution every year or so, more often with hard water, and skipping it is the most common reason units fail early. Our guide on how to flush a tankless water heater shows the pump-and-bucket method with isolation valves.

Power outages matter more than with an old atmospheric gas tank. Gas tankless units need electricity for the fan, igniter and controls, so no power means no hot water unless you have a backup source. Outdoor models also rely on electric freeze-protection heaters, and they can be damaged if the power fails during a hard freeze without the unit being drained.

The payoff is lifespan and space. The Department of Energy notes that most tankless heaters have a life expectancy of more than 20 years, against 10 to 15 for a typical tank, and a wall-mounted unit frees up the closet or corner a tank occupied. Whether that justifies a higher upfront bill depends on your house; our full breakdown of tankless water heater pros and cons weighs it honestly, and the tankless water heater cost guide shows the installed numbers.

If you have decided to switch, our walkthrough on how to replace a water heater with tankless covers the gas line, venting and electrical work the swap involves, and our rundown of water heater brands explains who makes what and where each one is sold. When you are ready to choose a unit, start with our tested tankless water heater rankings.