Continuous Flow Hot Water Heater: How Does a Gas Water Heater Work?
How does a gas hot water heater work?
A gas continuous flow hot water heater works by heating the water as it passes through the unit.
Also called an LPG instant hot water system, a continuous flow hot water heater is the most popular type of gas hot water heater in Australia.
The reasons are simple — an LPG instant hot water system never runs out of hot water and they are energy efficient.
But how do they work?
How Does a Gas Water Heater Work?
An LPG instant hot water system works when you open a hot water tap and a sensor detects the water flow.
When the continuous flow hot water heater is working, the gas burner fires and the heat exchanger works to heat the water. See image.
Some models of tankless LPG instant hot water system work with a secondary heat exchanger to pre-heat the water, using heat from the continuous flow hot water heater exhaust gases.
A thermostat works to regulate the amount of gas that goes to the gas hot water heater burner, maintaining the pre-set water temperature.
The gas burner fires and the heat exchanger warms the flowing water.
Computer Controlled LPG Instant Hot Water System
At the heart of an LPG instant hot water system 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 a modern continuous flow hot water heater, 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.
Continuous Flow Hot Water Heater 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 continuous flow hot water heater, and the heating begins.
A slow leaking tap is not enough flow to start the gas hot water heater.
However, leaky water pipelines and hot water system repairs require immediate inspection.
Some LPG instant hot water system 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 fails to light within a few seconds, the ECU shuts down the flow to prevent any unsafe accumulation of gas.
The LPG Instant Hot Water System Process
In an LPG instant hot water system, 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 hot 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 of a Continuous Flow Hot Water Heater
As the flow rate and temperature of the incoming water can vary, the ECU modulates the amount of gas that goes to continuous flow 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 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 gas 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.
Gas Hot Water Heater Tank
Different from an LPG instant hot water system, a gas hot 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 gas hot water heater works by having the burner run until the temperature recovers to the set point.
Power Supply to an LPG Instant Hot Water System
To power the flame igniter, most continuous flow hot water heater units are connected to your home’s power supply.
However, there are some LPG instant hot water system 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 Gas Hot Water Heater Works – Summary
Cold water flows into the tankless gas hot 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 of LPG Instant Hot Water System
Please note that there are many manufacturers and even more models of LPG instant hot water system, 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.