How Does a Gas Water Heater Work?
Tankless continuous flow water heaters are the most popular type of gas hot water heater (gas geyser).
The reasons are simple — they never run out of hot water and they are energy efficient.
But how do they work?
How Does a Continuous Flow Gas Hot Water Heater Work?
A continuous flow hot water heater, sometimes known as tankless gas heater, works when you open a hot water tap and a sensor detects the flow.
The gas burner fires and the heat exchanger warms the flowing water.
A gas water heater tank works by heating water and storing it until needed.
Hot water is drawn from the top of the tank and replaced with cold water at the bottom because warm water rises.
When the tank water temperature drops below the thermostat set point, the water heater works by having the burner run until the temperature recovers to the set point.
Some tankless instantaneous gas hot water systems work with a secondary heat exchanger to pre-heat the water, using heat from the flow through hot water heater exhaust gases.
A thermostat regulates the amount of gas that goes to the flow through hot water heater burner, maintaining the pre-set water temperature.
Computer Controlled
At the heart of most modern tankless continuous flow hot water heaters is an Electronic Control Unit (ECU) that controls how the gas flows through the hot water heater.
The ECU controls all of the functions of instantaneous gas hot water systems, including temperature control, ignition, gas flow, and modulation.
It is also responsible for safety, with a flame sensor and automatic gas cut-off in the event of a problem.
Getting Started
When you turn on a hot water tap anywhere in your home, a flow sensor connected to the ECU detects the water flow through the hot water heater, and the heating begins.
A slow leaking tap is not enough flow to start the gas water heater.
However, leaky water pipelines and hot water system repairs require immediate inspection.
Some instantaneous gas hot water systems models have a combustion fan that draws air for combustion.
At the same time, the ECU starts the gas flow via a solenoid gas valve and triggers the igniter.
The igniter sparks until the flame sensor detects a flame.
If the gas flow hot water heater fails to light within a few seconds, the ECU shuts down the gas flow to prevent any unsafe accumulation of gas.
The Water Heating Process
The gas burner applies heat to the heat exchanger that the water flows through.
The course of the water is typically a serpentine pattern, allowing the water to absorb as much heat as possible from the walls of the heat exchanger.
Some continuous flow hot water heater models have a secondary heat exchanger to pre-heat the water, using the heat from the burner exhaust gases.
This is often called a “condensing” model.
The advantage of these gas water heaters is that they provide even greater energy efficiency; however they tend to have a higher purchase price than standard continuous flow hot water heater models.
Many non-condensing models are already 6-star rated.
Controlling the Temperature
As the flow rate and temperature of the incoming water can vary, the ECU modulates the amount of gas that goes to hot water heater.
This modulation of the gas keeps the water at a constant temperature.
This functionality is based on a water temperature sensor on the exit side of the heat exchanger.
Some continuous flow through hot water heaters have temperature controllers that allow the user to select a temperature and others are factory pre-set.
These controllers can be located on the flow-through hot water heater or, wired to a remote location like a kitchen or bathroom.
The ECU takes the controller’s input to modulate the gas flow, keeping the water at the user’s desired temperature.
Most continuous flow hot water heaters come with their maximum temperature pre-set in the factory.
This is a safety precaution to help prevent possible scalding.
Power Supply
To power the sparking igniter, in order to ignite the gas, most continuous flow units are connected to your home’s power supply.
However, there are some units that use energy created by the water flowing through a small water turbine to provide the power for ignition (see graphic).
Lastly, there are also some units that just use a simple battery.
The turbine and battery powered units have a distinct advantage over the connected models, inasmuch as they will operate even during a power outage.
The disadvantage of the turbine powered type is that the entire unit is physically larger than a comparable plug-in model, as well as more expensive.
How a Tankless Water Heater Works – Summary
Cold water flows into the water heater when a hot water tap is opened inside your home.
The water flow is detected by a sensor that ignites the gas burner, to heat the water in the heat exchanger.
The water typically follows a serpentine pattern through the heat exchanger, absorbing as much heat as possible.
An electronic control unit modulates the gas burner to maintain a set water temperature.
This is the simple explanation of how does a gas water heater work.
Many Manufacturers & Models
Please note that there are many manufacturers and even more models of tankless continuous flow hot water heaters, including both LPG and natural gas.
Therefore, there can be variations in features, functionality and operation from the general descriptions provided above.
You should always consult the specific manufacturer’s information when making a purchase selection or other decision.
You should also always consult your owner’s manual for advice on safe operation and maintenance.