How a Tankless (On-Demand) Water Heater Works

A tankless water heater makes hot water in the few seconds it takes to travel from the unit to your tap, and it makes none the rest of the day. There is no reservoir sitting at temperature, no pilot-warmed tank, no 40 or 50 gallons held hot around the clock. Instead, the heater waits. The moment you open a hot tap, water starts moving through it, a sensor notices that movement, a burner or an electric element switches on, and the water that comes out the other side is hot. Close the tap and the whole thing shuts back off.

That single design choice, heating on demand instead of storing, is what gives a tankless unit its nickname and explains almost everything else about how it behaves. The sections below walk through the mechanism: what happens inside the box, why a tankless unit is rated by flow rather than by gallons, the quirk known as the cold-water sandwich, and how a gas model differs from an electric one. For a side-by-side verdict on whether tankless is right for your home, see our comparison of tankless vs. tank water heaters (062), and for choosing a capacity, see our sizing guide (063). This post stays on the mechanism.

What “On-Demand” Means: No Tank, No Standby Loss

On-demand means the heater only runs when water is flowing, so it never reheats a stored supply. A conventional storage heater keeps a tank of water hot continuously, which means it loses some heat through the tank walls and reheats to make up for it even when no one is using hot water. That wasted background heat is called standby loss. According to the U.S. Department of Energy, tankless heaters avoid those standby heat losses precisely because there is no stored water to keep warm.

The practical payoff scales with how you use hot water. DOE estimates that for homes using 41 gallons or less of hot water per day, demand water heaters can be about 24 to 34 percent more energy efficient than conventional storage tanks. For homes that use a lot of hot water, around 86 gallons per day, the advantage narrows to roughly 8 to 14 percent. DOE also notes one catch: a gas model with a standing pilot light burns a little fuel continuously, which can offset some of the standby savings.

What “on-demand” does not mean is unlimited instant gallons. The supply is continuous, but it is metered by the unit’s heating power, which is the subject of the capacity section below.

Inside the Unit: Flow Sensor, Burner or Element, and Heat Exchanger

Inside, three parts do the work in sequence: a flow sensor detects water movement, a heat source fires, and a heat exchanger transfers that heat into the passing water. ENERGY STAR describes the gas version this way: when a hot tap opens, cold water is drawn in, a flow sensor activates the gas burner, the burner warms a heat exchanger, and the incoming cold water encircles that hot exchanger and leaves at the set-point temperature. Combustion gases then exit through a dedicated, sealed vent.

It helps to picture the heat exchanger as a coil of metal tubing or a finned chamber that the water threads through. The burner or element heats the metal; the water flowing past it picks up that heat in one pass and exits hot. Nothing is stored. The water you are using right now is being heated right now.

An electric tankless unit follows the same logic with a different heat source. Instead of a gas burner under a heat exchanger, electric resistance elements sit in the water path and heat it directly as it flows by. DOE notes that either a natural gas burner or an electric element does the heating; the rest of the on-demand concept is identical.

Many units also modulate. DOE notes that some tankless heaters are thermostatically controlled and can vary their heat output based on the water’s flow rate and the temperature of the incoming water. In plain terms, the unit measures how fast water is moving and how cold it starts, then turns the burner or elements up or down to hold your target temperature instead of running flat out. That is why a well-set tankless unit can deliver a steady temperature whether you crack the tap or open it wide.

Why Flow Rate and Temperature Rise Define Capacity

A tankless heater is rated by how much it can heat per minute, not by how many gallons it holds, because it holds none. Its true ceiling is a pair of numbers: flow rate in gallons per minute (GPM) and temperature rise, meaning how many degrees it can raise the water as that water passes through.

Those two numbers trade off against each other. The faster water flows, the less time each drop spends near the heat, so the smaller the temperature rise. Slow the flow and the same unit can raise the temperature further. DOE puts concrete figures on this: a 70 degree Fahrenheit temperature rise is possible at about 5 GPM through a gas-fired demand heater, and at about 2 GPM through an electric one. DOE describes typical tankless output as roughly 2 to 5 GPM overall, with gas models generally reaching higher flow rates than electric models.

Temperature rise is the gap between your incoming water and your target. If your groundwater arrives at 50 degrees and you want 120-degree water, you need a 70-degree rise. The colder your incoming water, the harder the unit works for the same output, which is why the same model effectively delivers fewer hot GPM in a cold climate than a warm one. Working out the rise and flow your household actually needs is a sizing exercise, and the regional cold-water factor is covered in our sizing guide (063).

The capacity limit becomes obvious when several fixtures run at once. DOE points out that demand heaters have output limitations under simultaneous use, and one common fix is to install more than one unit or to dedicate a separate heater to a high-use appliance. A single shower may stay perfectly hot while a shower plus a dishwasher plus a laundry load may exceed what one unit can heat at once.

The “Cold-Water Sandwich” and Minimum Activation Flow

The cold-water sandwich is a brief slug of cool water you sometimes feel a few seconds into a tankless shower, sandwiched between hot water at the start and hot water after. It happens because of how on-demand heating starts and stops. When you turn the tap on, you first get the hot water already sitting in the pipe from a recent draw. Behind it comes water that entered the unit during the moment between the burner firing and the heat exchanger coming fully up to temperature, so it is cooler. Once the exchanger is hot and steady, normal hot water follows. Hot, then a short cool patch, then hot again. That is the sandwich.

It is most noticeable when hot water was used a short time earlier, such as someone finishing a hand wash right before you step into the shower. It is a normal characteristic of the start-stop cycle, not a malfunction, though some units and recirculation setups reduce it.

A tankless unit also will not fire at all until enough water is moving through it. Every tankless heater has a minimum activation flow: the flow sensor needs to see at least a threshold amount of water before it tells the burner or elements to switch on. ENERGY STAR describes the gas burner being activated when the unit’s minimum water draw is met. The practical consequence is that a very low trickle, like a tap barely cracked open, may not be enough to trigger heating, so the water stays cold until you open the tap a little wider. The exact threshold varies by model and brand, so check the manufacturer’s specification for any particular unit rather than assuming a single figure.

Gas-Fired vs. Electric Tankless: How the Heating Source Changes the Picture

The heating source changes the unit’s flow capacity and its installation needs more than it changes the basic on-demand concept. DOE notes that gas-fired tankless heaters generally produce higher flow rates than electric ones, which is the difference behind that 5-GPM-gas versus 2-GPM-electric figure for a 70-degree rise. A gas burner can pour a lot of heat into the exchanger quickly, so it can keep up with more simultaneous demand.

Electric models are simpler in some respects. They have no combustion, so they need no gas line and no flue to carry exhaust away. The trade-off is electrical demand. DOE notes that large residential electric tankless units can draw roughly 120 to 160 amps, and that most older homes will need an electrical service upgrade to handle one. That is a significant requirement, since a typical older home’s whole panel may be rated at or near that figure.

Gas brings its own demands: a properly sized gas supply and a dedicated vent for combustion gases, both covered in the next section. Neither fuel is universally “better.” Gas tends to win on raw flow capacity for whole-home use, while electric avoids combustion and venting but can demand a service upgrade. The full fuel-choice comparison, including operating cost, is its own topic in our gas-vs-electric guide (059), and the head-to-head decision against a storage tank lives in 062.

What Venting, Gas Supply, and Electrical Demand a Tankless Unit Needs

A tankless unit’s hookups, the venting, gas line, and electrical service, are code-governed work that a licensed professional should size and install, not a DIY project. This is the part of “how it works” that most often surprises people switching from a tank, because a tankless heater rarely drops into the same connections the old unit used.

For a gas model, two things commonly need attention. The first is the gas supply. A tankless burner fires at a high rate only while it runs, and an undersized gas line cannot deliver that input without the pressure dropping below what the unit needs, so the line and sometimes the meter may have to be upsized. The second is venting. Combustion gases must leave through a dedicated, sealed vent designed for that unit, as ENERGY STAR describes; the old tank’s vent is frequently not suitable. Gas supply sizing and venting are governed by fuel-gas and plumbing codes that vary by jurisdiction, so the right materials and clearances depend on local rules and the manufacturer’s instructions.

For an electric model, the central question is whether your electrical service can supply the unit’s draw, which as noted can be very large for whole-home units and may require a panel or service upgrade.

Because all three, gas, venting, and electrical capacity, involve combustion safety, code compliance, or high current, the safe path is to have a licensed plumber confirm gas supply and venting and a licensed electrician confirm electrical capacity before any tankless unit is installed. There are no homeowner step-by-step instructions for that work here, by design.

Frequently Asked Questions

Does a tankless water heater give truly unlimited hot water?
It gives continuous hot water, not unlimited simultaneous hot water. As long as demand stays within the unit’s flow-and-temperature-rise capacity, it will keep producing hot water indefinitely. Push past that capacity by running several fixtures at once and the water turns lukewarm, because the unit can only heat so many gallons per minute.

Why do I sometimes get a burst of cold water in the shower?
That is the cold-water sandwich. It is a short slug of cooler water caused by the brief gap between the burner firing and the heat exchanger reaching full temperature, usually right after hot water was used a moment earlier. It is normal to the start-stop cycle, not a fault.

Is a gas tankless heater more powerful than an electric one?
Generally yes, in terms of flow. The Department of Energy notes gas-fired tankless heaters reach higher flow rates than electric ones, which is why gas is often chosen for whole-home use. Electric models avoid gas lines and venting but can demand a large electrical service.

Does a tankless heater use no energy when no one is using hot water?
It avoids the standby heat loss of a storage tank because there is nothing kept hot. The one exception is a gas model with a standing pilot light, which burns a small amount of fuel continuously. Most modern units use electronic ignition instead.

Can I install a tankless unit myself to save money?
The unit’s gas supply, venting, and electrical service are code-regulated and involve combustion or high current, so this is licensed-professional work. Have a licensed plumber confirm gas and venting and a licensed electrician confirm electrical capacity.

This article is general information, not professional advice; have a licensed plumber or electrician evaluate your specific home and confirm code requirements before installing or modifying a water heater.

Sources

• U.S. Department of Energy, Energy Saver: Tankless or Demand-Type Water Heaters: https://www.energy.gov/energysaver/tankless-or-demand-type-water-heaters
• U.S. Department of Energy, Energy Saver: Sizing a New Water Heater: https://www.energy.gov/energysaver/sizing-new-water-heater
• ENERGY STAR: Whole-Home Gas Tankless Water Heaters, How It Works: https://www.energystar.gov/products/wholehometanklessgaswaterheaters/how-it-works
• ENERGY STAR: Whole Home Tankless Gas Water Heaters (program overview): https://www.energystar.gov/products/wholehometanklessgaswaterheaters