How does a tankless water heater work? Instead of storing heated water in a tank, a tankless unit uses a high-power burner or electric element to heat water instantly as it flows through a heat exchanger. When you turn on a hot water tap, cold water travels through a pipe into the unit, and the flow sensor triggers the heating mechanism. This process delivers a continuous supply of hot water—without the standby heat loss of a traditional storage tank. Understanding how does a tankless water heater work is the first step for any homeowner considering an upgrade in 2026.
On-Demand Heating: The Core Principle
A tankless water heater, also called a demand-type water heater, activates only when water is flowing. Here’s the step-by-step process:
- Flow sensor activation: When you open a hot water faucet, the flow sensor detects movement and sends a signal to the control board.
- Burner ignition (gas models) or element energizing (electric models): A gas burner or electric resistance element turns on. Gas units require a vent pipe to exhaust combustion gases.
- Heat exchange: Incoming cold water passes through a coiled copper or stainless steel heat exchanger. The heat exchanger absorbs thermal energy from the flame or element.
- Temperature modulation: The control board adjusts the burner intensity or element wattage to maintain the set output temperature, typically between 100°F and 140°F.
- Shutoff: When the faucet closes, the flow sensor stops the signal, and the heating source turns off within seconds.
Gas vs. Electric Tankless Water Heaters
| Feature | Gas Tankless | Electric Tankless |
|---|---|---|
| Heating source | Natural gas or propane burner | Electric resistance elements |
| Flow rate (typical residential) | 5–11 gallons per minute (GPM) | 2–5 GPM |
| Installation requirements | Vent pipe (PVC or stainless steel), gas line, condensate drain | High-amperage circuit (often 60–150 amps), larger wire gauge |
| Energy factor (EF) | 0.82–0.96 (condensing models up to 0.98) | 0.99–1.00 |
| Best for | Whole-house applications, multiple simultaneous uses | Point-of-use or small homes with moderate demand |
According to the U.S. Department of Energy’s Energy Saver guide, tankless water heaters can be 8–34% more energy-efficient than storage tank models, depending on usage patterns. Energy.gov’s water heating page provides a national overview of efficiency ratings and rebate programs.
Key Components of a Tankless System
Heat Exchanger
The heat exchanger is the metal core where the actual heat transfer occurs. Copper is common in lower-priced models because of its excellent thermal conductivity, but it can be vulnerable to acidic water (low pH). Stainless steel or specially coated exchangers are more durable if your water is aggressive. The International Association of Plumbing and Mechanical Officials (IAPMO) publishes standards for heat exchanger materials used in tankless units.
Flow Sensor and Control Board
The flow sensor uses a paddle wheel or magnetic hall-effect sensor to measure water movement. The control board receives the flow rate data and calculates how much power to apply. If the flow rate is too low (below the unit’s minimum startup flow, usually 0.5–1.0 GPM), the heater may not activate—a common “cold water sandwich” phenomenon.
Vent System (Gas Models)
Gas tankless heaters require a sealed combustion vent to expel exhaust outdoors. Power-vent models use a fan to push exhaust through a small-diameter PVC pipe, which can exit through a side wall instead of a rooftop. This flexibility makes retrofits easier. Always follow manufacturer and local code requirements; the EPA WaterSense program includes tankless water heater guidelines for efficiency and installation.
Installation Considerations for US Homes
Switching from a tank to a tankless heater is not a simple swap. Your home’s gas line or electrical panel may need upgrades. A typical gas tankless unit requires a ¾-inch gas line (many older homes have ½-inch lines) and a dedicated condensate drain. Electric tankless models often demand 60–150 amps of electrical service—older 100-amp panels may be insufficient. Always work with licensed professionals for these modifications; MyTownPlumber offers drain cleaning and pipe repair services that can help prepare your home for a new installation.
Water quality also affects performance: hard water can scale the heat exchanger over time, reducing flow and efficiency. Installing a water softener upstream of the tankless unit can extend its lifespan and maintain warranty coverage.
Maintenance and Longevity
Annual flushing of the heat exchanger with a descaling solution (white vinegar or a commercial descaler) is recommended to remove mineral deposits. The process involves closing the shutoff valves, connecting a pump to the service ports, and circulating the solution for 45–60 minutes. Most manufacturers also recommend replacing the inlet water filter and checking the vent system for blockages. With proper care, a tankless water heater can last 20+ years, whereas a storage tank often fails after 10–12 years.
Common Troubleshooting Issues
- Cold water sandwich: A brief slug of cold water when the heater cycles off and on. Solution: install a recirculation pump or set the unit to a higher temperature and use a mixing valve.
- Insufficient hot water: Unit undersized for simultaneous demands. Check the incoming water temperature (colder winter water reduces flow rate).
- Error codes on display: Many units show codes for ignition failure, overheat, or fan speed faults. Refer to the manual; error often indicates need for professional service. MyTownPlumber can assist with water heater repair for both tank and tankless models.
Frequently Asked Questions
How long does a tankless water heater last?
With proper maintenance (annual descaling, filter cleaning), a tankless unit can last 20 years or more. Storage tank heaters typically last 10–12 years.
Can a tankless water heater supply hot water to two showers at once?
Yes, but only if the unit’s flow rate rating (GPM at a specific temperature rise) meets your household’s peak demand. For example, two showers using 2.5 GPM each at a 70°F rise requires a unit capable of at least 5 GPM at that rise. Consult a sizing worksheet or a plumber.
Do tankless water heaters need electricity?
Gas tankless models still need electricity to power the control board, fan, and ignition—they will not work during a power outage unless connected to a backup generator. Electric models also require a dedicated circuit.
What size tankless water heater do I need?
Sizing depends on your home’s maximum simultaneous hot water demand, the incoming groundwater temperature (colder in northern states), and the desired output temperature. A single-point-of-use unit (for one sink) may be 2–4 GPM; a whole-house unit for a 3-bedroom home often ranges 7–10 GPM.
Is a tankless water heater worth the higher upfront cost?
The average installed cost in the US ranges from $1,500 to $4,500 (prices vary by region and model), significantly higher than a tank heater. However, the energy savings—typically 8–34%—and the longer lifespan can offset the investment over 10–15 years. The EPA WaterSense page notes that ENERGY STAR certified tankless models can save a typical family $100 or more per year.
Can I install a tankless water heater myself?
Because of gas line sizing, venting, and electrical requirements, professional installation is strongly recommended and required by many manufacturers to keep the warranty valid. Improper installation can also lead to carbon monoxide risks in gas units. MyTownPlumber offers sewer line repair and toilet repair as well, but tankless installation is best left to licensed pros.
Ready to upgrade your home’s hot water system? Contact MyTownPlumber today for a free estimate on tankless water heater installation and start enjoying on-demand hot water.
— MyTownPlumber Editorial Team