How does an instant gas hot water system work?

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Home 9 Residential LPG Blogs 9 How does an instant gas hot water system work?

27 Mar, 2023 | Residential LPG Blogs

In this article:

Discover how tankless LPG instant gas water heaters can enhance efficiency, energy ratings, and overall benefits, reducing operational costs compared to traditional gas storage tank water heaters. We will explain the operating mechanism of gas continuous flow water systems, particularly how its Electronic Control Unit (ECU) regulates temperature and controls all functions. Aside from controls, you’ll learn about why the right hot water system capacity matters based on household usage patterns and why having an expert install and maintain your instant gas hot water system for safety and compliance with policies is important.

Continuous flow hot water heater systems are the most popular type of gas hot water heater. The reasons are simple – they are energy efficient and being tankless means the hot water just keeps on coming. 

But how does a continuous flow 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.

Electronic Continuous Flow Gas Hot Water Heater 

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.

Turning on your hot water system

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 Continuous Flow Hot Water Heater 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. 

What to look for when buying a Continuous Flow Hot Water Heater

There are many manufacturers of tankless continuous flow hot water heaters, each offering different models including both LPG and natural gas options.

There can be variations in features, functionality, and operation so always consult the specific manufacturer’s information when making a purchase selection or other decision. Always consult the owner’s manual for advice on safe operation and maintenance.

Popular continuous flow hot water heater brands and models include:

  • Stiebel Eltron Series
  • Rheem Metro and Rheem Pronto Series
  • Rinnai Infinity Series
  • Vulcan Hot Water Systems
  • Bosch Hot Water Systems
  • Thermann Hot Water Systems

Don’t Calculate by Number of People or Bathrooms

The sizing information available can be confusing because manufacturers and suppliers are trying to provide a simple answer to what can be a complicated question.

Some suggest that you should use the number of bathrooms in your home to calculate the size of system that you need, while others recommend you consider the number of people in your family.

In some cases, a worst-case scenario is used to recommend the size of the system required, and this can result in a much higher hot water consumption calculation than your actual water use.

Buying Too Large Wastes Money

The result is that many people end up buying a larger system than they need. This costs them extra money when they buy the hot water system and will, in many instances, increase the ongoing operating costs.

Factors to Really Consider

Simultaneous use is one of the fundamental determining factors.

The key is not only how many people are in your home, but how and when they use hot water. For example: 

  • Do they all take showers at the same general time of day?
  • Do they take showers simultaneously in different bathrooms?
  • Do the showers have WELS 3 Star Rated Showerheads (maximum 9L/Min) or standard 18 to 25L/Min showerheads?
  • Do some prefer baths which use more hot water? And if so, is the bath or spa oversized, requiring even more hot water?
  • Is the washing machine run using hot or warm water, and is it run while people are showering?
  • Is your dishwasher connected to the cold or hot water? And if hot, is it run while the hot water is being used elsewhere in the house?

Showers are a key consideration when it comes to hot water use

Showers use the most hot water in a home. People generally shower for 5 to 10 minutes. Assuming you have a 3 Star water-saving shower head and you mix some cold water with the hot, the typical hot water consumption would probably be around 7 litres per minute. 

This means that an 8-minute shower uses 56 litres of water per shower when using a water-saving shower head, or 224 litres for a family of four. 

Using a non water efficient showerhead can increase water usage significantly, between 12 – 22 litres per minute. 

Therefore the use of WELS (Water Efficiency Labelling and Standards) 3-star rated water-saving shower head is an important factor when it comes to saving water and energy, and also in reducing the size and cost of the hot water system required.

Hot Water Tank Example

In the above example, if all four people take their showers in the morning, you would only need a storage hot water system with a 224-litre first-hour capacity, regardless of how many take showers at exactly the same time (assuming no other hot water devices are running at the same time).

The use of additional hot water after the shower period would depend on the recovery time required for the hot water system to achieve full temperature. So, with a larger family or house guests, some scheduling may be required to avoid the need for a larger unit.

Off Peak Electric Issues

Off peak electric storage units are a completely different story. As they only heat water at night, they must be quite large to ensure you don’t run out of water during the day.

So, some of the electric savings are offset by needing to heat a larger tank.

The bigger tank would also cost more to purchase.

Continuous Flow Hot Water Heater Example

For a tankless continuous flow unit, the size you need depends on how many showers are being used simultaneously.

If you have WELS 3-star rated water-saving shower heads installed, they will use approximately 7 litres per minute. So a unit with a minimum flow rate of 14L/min would be sufficient if you are running two showers simultaneously. With 3 people showering at the same time you would need a minimum of 21L/min. Note that this does assume that you are not using other hot water devices simultaneously.

Recovery time is not an issue with continuous flow units, so even a large family with guests would not run out of hot water as long as your unit is sizes for the number of hot water using appliances that would be running simultaenously.


Frequently Asked Questions

What maintenance is required for gas hot water heaters?

Here are essential maintenance procedures you can do or arrange with a licenced professional to keep your gas hot water units in their best shape:

1. Routine Professional Inspection

We recommend having an accredited hot water system technician perform routine inspections at intervals recommended by the appliance manufacturer. They are skilled and certified to inspect, repair, and replace faulty components, such as gas fittings, heat exchangers, valves, thermostats, and flue vents.

2. Cleaning and replacing air filters

If your continuous flow gas hot water system has an air filter in its air intake system, you must inspect it every few months for dust and debris and replace it when it’s clogged beyond cleaning.


Heavily clogged air filters in continuous flow hot water systems reduce the amount of air available for combustion, reducing the system’s efficiency and increasing the risk of carbon monoxide production.

3. Checking the gas fittings and piping

Most instant gas hot water heaters are installed on the house’s outer wall for easy installation and maintenance. For homeowners, you can look at the gas valves and gas pipelines and check for something unusual. If you find any possible signs of a broken valve, or damaged gas hoses or smell the slightest gas or rotten egg odour, don’t hesitate to call the ELGAS emergency hotline at 1800 819 783 for immediate repairs.

4. Keeping the area around the hot water unit clear

Always ensure the area around your continuous flow hot water system is free from debris, obstruction, clutter, or rubbish. It’s important to have good ventilation for the system to operate at maximum efficiency.

5. Watching for leaks and drips

Always check if the cold water intake and hot water output are leaking or dripping in your gas continuous flow hot water system. You can inspect the inlet and output at the bottom of the unit, check for leaks, and have a professional address any issues found immediately.

How long does a gas hot water system last?

Most gas continuous flow hot water systems have optimal performance for around 8-12 years. Unlike gas storage hot water systems, the components in instant gas hot water heaters place different stress levels on their heating components.


The actual life of your hot water system will also depend on realistic factors such as:


Heavier use and continuous operation will wear out the system’s components and heating elements over time. While the heat exchanger, hot water pipes, and combustion chamber are all built to withstand significant temperature changes, all materials have their own limits.

Water quality

Water quality—that is, the hardness of water determined by present minerals (dissolved calcium and magnesium)—can affect your hot water system’s lifespan. When the unit processes hard water 24/7, it may lead to scale formation and corrosion in your water pipeline and heat exchanger, reducing the tank’s lifespan.


You can extend the hot water system’s life by more frequent maintenance to counteract this.

How much does it cost to install a gas hot water system?

When installing a gas hot water system there are two main costs – the cost of the unit itself and the installation cost for the unit. 


Factors affecting the installation cost include the brand/model chosen, the unit’s capacity, and the complexity of the installation given the home’s utility layout and water pipeline.


Always ask your chosen licensed professional about their rates before you have the unit supplied and installed. Installing a new system is generally going to cost more than replacing or upgrading an existing one, however you can expect to pay between $200 and $400 for most systems.

What are the benefits of using gas hot water heaters?

Continuous flow gas water systems are popular for several reasons:


  • They are generally cheaper to run than an electric hot water system
  • Gas rates are more stable and do not fluctuate or vary throughout the day
  • Gas hot water units with good energy ratings are more cost-effective than solar and electric storage hot water systems
  • Gas hote water units are energy-efficient

What are the disadvantages of a gas hot water system?

No hot water system is perfect, and no single type is going to fit everyone’s unique needs and available resources. Here are some disadvantages to using a gas hot water system:


  • Gas hot water systems that use electric ignition cannot be used during blackouts
  • You must have a flue vent installed if the system is located inside

What safety precautions should I consider with gas hot water systems?

Consider these safety precautions if you have a gas hot water system at home:


  • Always keep the vicinity of the unit free from paper, dust, and other light/flammable materials
  • Turn off the pilot light in the heater unit before using aerosol sprays or flammable liquids in the vicinity
  • If the gas hot water system is in the garage, ensure the unit is installed 18 inches (about 50cm) above the floor to prevent igniting gasoline vapour that collects near the floor
  • Ask your gas hot water technician to include an automatic gas shutoff valve in the unit. This valve prevents a dramatic increase in gas flow, preventing fires due to earthquakes and other natural disasters
  • We recommend insulating the first 2 metres of hot and cold water pipes to prevent heat loss and sweating on the pipelines

How long does a gas hot water system take to heat up?

Unlike storage water heaters, which take 30-40 minutes to heat stored water in tanks, most continuous-flow hot water systems powered by LPG gas can almost instantly reach the desired water temperature.


That’s because the cold intake water passes through a heat exchanger in a series of coils, heating it to its desired temperature as soon as it exits the unit.

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