A tankless heater has no tank of hot water waiting. Open a tap and it fires up on the spot — cold water runs through a copper coil while a burner licks the outside. Use the buttons to look inside and follow the water, the gas and the exhaust.
A tankless gas water heater is a wall-mounted box that heats water only while you are using it. It has no storage tank. The moment you open a hot tap, cold water starts flowing through the unit and a burner fires up to warm it on the way through. Close the tap and the flame goes out.
The path is short but precise. Cold water enters on the left, passes a flow sensor, and climbs through a serpentine copper coil. Below the coil a row of flames heats the metal from the outside, and the water picks up that heat as it snakes upward. By the time it reaches the outlet it is hot enough for the shower. An outlet sensor keeps reading the result and the control board trims the flame to hold your setpoint.
From the outside it is a plain white panel with a digital readout. All the action — the sensor, the gas valve, the burner, the coil and the blower — is packed behind the casing, so showing the shell only faintly makes it clear what happens between the cold pipe and the hot tap.
One idea ties the machine together: heat flows from the flame into the water, and the faster the water moves the more heat it needs. The control board simply balances the two by watching the water that leaves.
With a tankless heater there is no stored hot water. Everything begins when you open a tap and cold water starts moving through the inlet pipe. A small flow sensor sits right on that pipe, and the movement of water is what tells the unit a tap has been opened. No flow, no heat — that is why a tankless heater never runs when nothing is being used.
The control board reacts to the flow signal by opening the gas valve. This is a proportional valve, not a simple on/off switch: it can open a little or a lot, letting through a small flame or a large one. More flow through the unit means the valve opens wider to feed a bigger flame.
Gas flows into the burner, where it ignites across a row of jets. The flames reach roughly a thousand degrees and rise into the underside of the copper heat exchanger. Copper carries heat extremely well, so the coil quickly becomes as hot as the flame that is washing over it.
Meanwhile the cold water is threading upward through a serpentine coil — back and forth across the flame several times so it has plenty of chances to pick up heat. By the time it exits at the top it is the hot water you wanted. Fins packed around the coil give the hot gas more surfaces to give its heat away before it leaves.
Burning gas produces hot exhaust that must not be left around the burner. A blower fan forces air and burnt gas through the heat exchanger and up into the flue, which vents it outside. The same draft also feeds fresh air to the flame so it burns cleanly.
An outlet temperature sensor reads the water just as it leaves the unit. The control board compares that reading with the temperature you set: if the water is coming out too cool it opens the gas valve a little more, and if it is too hot it backs the flame off. That constant trimming is what keeps the shower temperature steady even when another tap opens somewhere else.
No. It has no tank. Water is heated only while it is flowing, so you get hot water on demand instead of a refilling cylinder.
A flow sensor on the cold inlet. The moment you open a tap and water starts moving, the sensor tells the control board to open the gas valve and light the burner.
Copper carries heat very well. The flame heats the copper coil, and the water running through it picks that heat up before it leaves the unit.
The burner produces hot exhaust gas. A blower pushes that gas through the heat exchanger and up the flue so it leaves the house instead of lingering around the flame.
An outlet sensor reads the temperature of the water leaving the unit. The control board compares it with your setpoint and trims the gas valve, so the flame is bigger or smaller as needed.
A tankless heater usually lasts longer than a storage-tank model, often around 20 years. The exact life depends on your water quality and how well the unit is maintained.
Minerals in hard water build up as scale on the inside of the heat exchanger. That scale lowers efficiency and shortens the unit's life, so the heater needs periodic descaling.
Because it heats on demand, a tankless heater is limited by its maximum flow rate. Running several hot taps at once splits that flow and the outlet temperature drops.