June 24, 2026
How Does a Tankless Water Heater Work?
What Every Des Moines Homeowner Should Know About Tankless Water Heaters
How does a tankless water heater work? Here’s the short answer:
- You turn on a hot water tap. Cold water begins flowing into the unit.
- A flow sensor detects the movement. This triggers the heater to activate.
- A gas burner or electric heating element fires up. Water passes through a heat exchanger and is heated on demand.
- Hot water travels to your tap. You get a continuous supply — no stored tank required.
- You turn off the tap. The heater shuts down completely, using zero standby energy.
That’s the core difference from a traditional tank-style heater: instead of keeping 40 to 80 gallons hot around the clock, a tankless unit heats only the water you actually use, only when you need it.
For homeowners in Des Moines and the Greater Iowa area, this matters. Iowa winters mean cold incoming groundwater, and an undersized or poorly installed unit can struggle to keep up. Understanding how the system works helps you make a smarter choice — and avoid surprises down the road.
How Does a Tankless Water Heater Work?
To understand how these systems operate, it helps to look inside the box. Unlike traditional water heaters that rely on a massive storage tank to hold and constantly reheat water, a tankless water heater functions as a high-powered thermal transfer station.
The primary magic happens within a small, wall-mounted metal cabinet. Inside this cabinet, several key components work in perfect harmony:
- The Flow Sensor: The gatekeeper of the system. It monitors incoming water movement and tells the system when to wake up.
- The Burner Assembly or Heating Elements: The heat source. In gas models, this is a high-powered burner that can produce up to 199,000 BTUs of heat. In electric models, these are heavy-duty electrical resistance coils.
- The Heat Exchanger: A winding path of copper or stainless steel tubing surrounded by heat-absorbing fins. As water winds through this tubing, it absorbs the heat generated by the burner or elements.
- The Control Board: The computer brain. It measures incoming water temperature, monitors the flow rate, and regulates the heat source to ensure the water exiting the unit matches your thermostat setting perfectly.
When you open a hot water tap anywhere in your home, cold water enters the unit through the cold water inlet. The flow sensor registers this movement instantly, sending a signal to the control board. The control board then ignites the gas burner or activates the electric coils.
As the cold water travels through the heat exchanger, the heat is transferred directly to the water. The water reaches its target temperature in seconds and flows straight to your shower, faucet, or washing machine.
To dive deeper into the overall design, advantages, and system options of these modern appliances, check out our Tankless Water Heaters Complete Guide.
How Does a Tankless Water Heater Work to Detect Water Flow?
The flow sensor is the unsung hero of the tankless system. Think of it like a highway on-ramp sensor: it doesn’t activate until a vehicle actually passes over it. In this case, the “vehicle” is the stream of water moving through the pipe.
When you open a faucet, municipal or well water pressure pushes water into the unit. The flow sensor, typically a small turbine or magnetic impeller, begins to spin. Once the water flow reaches a specific threshold—usually around 0.5 gallons per minute (GPM)—the sensor sends an electrical signal to the control board.
This activation trigger is vital because it prevents the heating elements from firing up when there is no water in the heat exchanger. If the burners fired up without water flowing through the pipes, the heat exchanger would melt or crack within seconds.
However, if your home has low water pressure or you only open a faucet a tiny trickle, the flow sensor might not register enough movement to trigger the system. This can lead to moments where the water suddenly runs cold, a phenomenon sometimes called a “cold water sandwich.” If you are experiencing fluctuating temperatures, read our guide on what to do when a Tankless Water Heater Inconsistent performance occurs.
How Does a Tankless Water Heater Work to Heat Water Instantly?
Once the flow sensor gives the green light, the control board springs into action to heat the water.
In a gas-powered unit, the control board opens the gas valve and ignites the gas burner. In an electric unit, it sends high-voltage electricity to the electric coils.
As the cold water winds through the heat exchanger, the metal tubes absorb the extreme heat from the burner or coils.
Because copper and stainless steel have exceptionally high thermal conductivity, the heat is transferred to the water almost instantly.
To ensure safety and comfort, a temperature sensor (or thermistor) at the outgoing hot water pipe constantly measures the temperature of the water exiting the unit. If the incoming groundwater is exceptionally cold—as it often is during an Iowa winter—the control board automatically adjusts the gas burner intensity or electrical output to maintain a steady temperature. If the temperature sensor or internal thermostat fails, it can cause the water to be scalding hot or ice cold. You can learn more about troubleshooting these issues in our article on Tankless Water Heater Thermostat Problems.
Gas vs. Electric Tankless Water Heaters
When choosing a tankless system, one of the first decisions you will face is selecting the fuel source. Both gas and electric tankless water heaters rely on the same fundamental science of on-demand heating, but their internal mechanisms, venting requirements, and output capacities differ significantly.
| Feature | Gas Tankless Water Heaters | Electric Tankless Water Heaters |
|---|---|---|
| Heat Source | Natural Gas or Propane Burner | Electric Resistance Coils |
| Venting Required | Yes (Sealed PVC or metal venting) | No venting required |
| Flow Rate Capacity | High (8 to 11+ Gallons Per Minute) | Moderate (2 to 5 Gallons Per Minute) |
| Electrical Needs | Standard 120V outlet (for control board) | Heavy-duty dedicated 240V circuits |
| Best Suited For | Whole-home use, large families | Single-use, point-of-use, small cabins |
Gas tankless water heaters are the heavyweights of the industry. Because burning natural gas or propane produces an immense amount of thermal energy quickly, gas units can achieve much higher flow rates than electric units. A high-efficiency gas model can easily produce 8 to 11 gallons of hot water per minute, making it capable of running multiple showers and appliances simultaneously.
However, because they burn fossil fuels, gas units require dedicated venting systems to safely exhaust combustion gases outside the home. They also require a condensate drain if they utilize condensing technology to capture extra heat. If you are located in Waukee, you can explore localized options in our guide on Tankless Water Heater Types Waukee IA.
Electric tankless water heaters, on the other hand, are highly efficient (often reaching nearly 100% efficiency) and require no venting at all. They are compact, simple to install, and quiet.
However, because heating water with electrical resistance takes a massive amount of electrical current, they require significant electrical upgrades. A whole-home electric unit may require up to four dedicated 40-amp, 240-volt breakers.
Additionally, because electric heating elements cannot transfer heat quite as rapidly as a gas flame, electric models usually top out at flow rates of 2 to 5 GPM. This makes them ideal for smaller households, guest suites, or point-of-use applications like a remote bathroom sink. If you live in Cumming, find out which system suits your property best by reading about Tankless Water Heater Types Cumming IA.
Sizing, Efficiency, and Iowa Winter Performance
Many homeowners are drawn to tankless technology because of its incredible energy efficiency. Traditional tank water heaters suffer from standby energy loss. No matter how well-insulated a tank is, the water inside slowly cools down over time. To keep the water at your set temperature, the heater must fire up periodically throughout the day and night, even when you are asleep or away on vacation.
Tankless water heaters eliminate standby energy loss entirely. Because they only consume energy when a hot water tap is open, they are vastly more efficient:
- For homes that use 41 gallons or less of hot water daily, tankless water heaters can be 24% to 34% more energy efficient than conventional storage tank models.
- For larger households using around 86 gallons per day, they are still 8% to 14% more energy efficient.
- Utilizing an ENERGY STAR certified condensing gas tankless water heater can save even more energy. These units feature a secondary heat exchanger that captures heat from the exhaust gases before they leave the vent, using 9% less energy than a standard non-condensing gas tankless model.
To learn more about how these efficiency numbers translate to real-world performance, take a look at our article on Tankless Water Heaters Efficient Choice.
The Reality of Iowa Winters and Temperature Rise
While the efficiency gains are impressive, sizing a tankless water heater in Iowa requires careful planning due to our freezing winter temperatures.
To understand why, you have to look at the concept of temperature rise. Temperature rise is the difference between the temperature of the cold groundwater entering your home and your desired hot water temperature (typically set to 120°F).
In the summer, groundwater in the Des Moines area might enter your home at a relatively warm 55°F or 60°F.
To reach 120°F, your water heater only needs to raise the temperature by 60 degrees.
In the dead of winter, however, that incoming groundwater can plunge to a freezing 38°F. To reach 120°F, your tankless heater now has to perform a massive 82-degree temperature rise!
Because the water must spend more time in contact with the heat exchanger to absorb that extra energy, the maximum flow rate of the unit will decrease. A unit that comfortably provides 5 GPM in July might only be able to provide 3 GPM in January. If you try to run the dishwasher, the washing machine, and take a shower at the same time on a freezing January morning, you might exceed the system’s capacity, resulting in lukewarm water.
This is why proper professional sizing is absolutely critical. We must calculate your home’s peak hot water demand alongside the worst-case winter groundwater temperatures to select a unit with the appropriate BTU or kilowatt rating. To see a detailed head-to-head breakdown of how these systems hold up compared to traditional options, read our Tankless vs Tank Water Heater Comparison.
If you are ready to explore the top-performing models designed to handle Midwestern winters, consult our regional guides:
Maintenance and Lifespan of Tankless Systems
One of the greatest benefits of a tankless water heater is its durability. While a traditional storage tank heater has an average lifespan of only 10 to 15 years, a high-quality tankless water heater can easily last more than 20 years if it is properly maintained.
Because tankless units do not store water, they are not subject to the constant rust and corrosion that eventually destroys traditional metal tanks. However, they do have one major enemy: hard water and scale buildup.
The water supply in Greater Des Moines contains high levels of dissolved minerals like calcium and magnesium. When hard water is heated, these minerals crystallize and precipitate out of the water, clinging to the hot metal surfaces inside the heat exchanger. This process is known as scaling.
Over time, scale acts as an insulating blanket inside the heat exchanger.
The burners have to run hotter and longer to heat the water through the layer of crust, which decreases energy efficiency and places immense thermal stress on the metal. If left unaddressed, scaling can cause the heat exchanger to overheat and crack, destroying the unit. To understand how to spot and prevent these mineral deposits, read our guide on Tankless Water Heater Scaling Problems.
The Importance of Annual Flushing
To protect your investment, tankless water heaters require regular maintenance, specifically an annual flushing and descaling.
During a professional maintenance visit, we connect a pump to the unit’s built-in service valves and circulate a food-grade descaling solution (usually a concentrated citric acid or vinegar solution) through the heat exchanger for about an hour. This solution safely dissolves and flushes away the calcium deposits, restoring the heat exchanger to pristine condition.
Along with descaling, regular maintenance includes cleaning the inlet water filter screen, checking the venting system for blockages, and inspecting the burner assembly. If your system is overdue for a flush or is showing an error code, professional help is just a call away. Explore our local repair resources to keep your system running smoothly:
- Tankless Water Heater Repair Des Moines IA Guide
- Tankless Water Heater Repair Deals Guide West Des Moines
- Tankless Water Heater Repair Guide Ankeny IA
Conclusion
Understanding how does a tankless water heater work is the first step toward upgrading your home’s comfort and energy efficiency. By heating water only on demand, these compact systems eliminate standby energy loss, provide an endless supply of hot water, and free up valuable floor space in your utility room.
However, because Iowa winters present extreme temperature drops, professional sizing, installation, and regular maintenance are absolutely essential to ensure your system performs flawlessly year-round.
At True Comfort Heating, Cooling and Plumbing, we serve homeowners throughout the Greater Des Moines area with honest assessments, dependable plumbing services, and flexible maintenance plans. Whether you live in Des Moines, West Des Moines, Ankeny, Urbandale, or any of the surrounding communities, our licensed and EPA-certified plumbers are here to help you select, install, and maintain the perfect hot water system for your family.
Ready to make the switch or schedule your annual system flush? Explore our localized installation and service guides to learn more:
- Tankless Water Heater Installation Des Moines Guide
- Tankless Water Heater Installation West Des Moines Guide
- Tankless Water Heater Installation Ankeny Guide
- Tankless Water Heater Urbansdale IA Guide
- Des Moines Tankless Heater Service Guide
- Tankless Water Heaters West Des Moines Guide
- Tankless Water Heaters Ankeny IA Guide
Don’t let cold winter groundwater catch your water heater off guard. Schedule plumbing service with True Comfort today and experience the luxury of endless, energy-efficient hot water!