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Continuous Flow Hot Water - Instantaneous - Instant - Tankless

No matter what you call them, they are great gas hot water systems. 

Actually, all three terms are very descriptive of this increasingly popular style of water heater.

How a Tankless Water Heater Works

Schematic of continuous flow hot water heaterIn 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.

See all the details on How a Tankless Hot Water Heater Works

Tankless Hot Water

Rinnai Infinity 26 LPG gas hot water system

They are “tankless”.  

Instead of having a traditional storage tank full of water, which has to be kept at a minimum of 60°C to prevent bacterial growth, tankless water heaters only heat the water when you need it. 

A tankless water heater detects when you turn on the hot water tap and heats the water as it passes through the unit. 

When you turn off the tap, the tankless water heater also turns off. 

This helps save energy and money.

Being tankless also means that these units are much smaller. 

Instead of a big bulky tank, you get a small box that is installed on your wall.  

The units are available for both internal and external installations. 

In Australia, most are installed outside the home. 

Continuous Flow Hot Water

Bosch 26e Highflow LPG gas hot water system

“Continuous flow” accurately describes how they work. 

You never run out of hot water because, unlike storage tank systems, it does not rely on pre-heating a quantity of water. 

You could have a house full of visitors, be last in the shower queue, and still have hot water. 

Just think, no more cold showers!

Instantaneous Hot Water

They are also virtually “instant”, as the hot water starts flowing within seconds of turning on the tap. 

The length of time does depend on the length of pipe between the hot water heater and the tap.

Additional Advantages

There are other advantages, too. 

The water may be cleaner, as there can be no rust in the water from rust build up in the tank.  

You are also reducing your greenhouse gas emissions by about 75%, when compared to an electric storage unit. 

All these advantages help make tankless, instant, continuous flow gas hot water heaters a great choice for your next hot water system.

Health & Safety

Unlike storage tank units, there is no risk of health issues from organism growth, such as Legionella bacteria, as there is no water being stored in which they can grow.

Quality Manufacturers

Rinnai and Bosch are the two leading brands in Australia. 

Rinnai Infinity Gas Hot Water Systems are the best sellers, with the Rinnai Infinity 26 being the leading model.  

Bosch Highflow Gas Hot Water Systems feature great German engineering and reliability. 

They also have their Bosch HydroPower range, which requires no electrical connection.  

Both offer models with remote temperature controls. 

Best of all, you can’t go wrong with either brand.

Instant Gas Hot Water Systems Prices - Instantaneous Hot Water Price

Instant (instantaneous) gas hot water system prices are typically from around $700 to $1500*. The instantaneous hot water heater price difference depends on the flow rate which is, in turn, dependent on the number of showerheads and taps.

Storage tank style gas hot water heaters range in size from around 130L to 350L, with gas hot water system prices from approximately $800 to $1450*.

Efficiency star ratings also influence the instant water heater price, with higher star ratings typically cost more in both types.

*Note that instant (instantaneous) gas hot water system prices (instant water heater price) are indicative only, as of July 2019

What Size Hot Water System Do I Need?

Don't Calculate by Number of People or Bathrooms

The sizing information available is very confusing because manufacturers, and others, are trying to provide a simplified answer to a complicated question. 

Some suggest that you calculate by the number of bathrooms in your home while others say to go by the number of people in your family. 

Also, in many cases, their recommendations are based on the worst case scenarios, assuming higher consumption than your actual water use.

Buying Too Large Wastes Money

The result is that many people end up buying a larger system than they actually need. 

This costs them extra money when they buy the hot water system and will increase the ongoing operating costs, in many instances.

Factors to Really Consider

Simultaneous use is one of the fundamental determining factors

The number of people in your home combined with how and when they use hot water is the key. 

  • 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 water saving shower head (maximum 9L/Min) or standard 18 to 25L/Min showerheads?

  • Do some prefer baths, which use more hot water? Is the bath or spa oversized, requiring even more hot water?

  • Do you run the washing machine, using hot or warm water, while people are showering?

  •  Is your dishwasher connected to the cold or hot water? If hot, do you use it while hot water is being used elsewhere?

Showers Are the Key

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 in some cold water with the hot, the typical hot water consumption would probably be in the 7 litres per minute range. 

If you also assume an 8 minute shower, water saving shower head will consume about 56 litres of hot water per shower or a family of four would consume 224 litres. 

The use of WELS (Water Efficiency Labelling and Standards) 3 star rated water saving shower head is an important factor in saving water and energy whilst reducing the size and cost of the hot water system required.

Hot Water Tank Example

Rinnai HOTFLO 170 gas hot water system

In the above example with water saving shower head, if all four people take their showers in the morning, you would only need a storage hot water system with 224 litre first hour capacity, regardless of how many take showers at exactly the same time.  

This all presumes that you are not using any other hot water devices at the time.  

The use of additional hot water, after the shower period, would depend on the hot water system’s recovery time required to achieve full temperature.  

So, with a larger family or house guests, a bit of 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. 

Inasmuch as they only heat water at night, they need to be quite large to make sure you don’t run out of water during the day. 

So, some of the electric savings is offset by the fact that you need to heat a larger tank. 

The bigger tank would also cost more to purchase.

Tankless Hot Water Example

Bosch 21e tankless gas hot water systemFor a tankless continuous flow unit, it would depend on how many showers are being used simultaneously. 

Using the example above, if it’s two showers, then you only need a unit with a minimum flow rate of 14L/min. 

If you have 3 people showering at exactly the same time, you would need a minimum 21L/min.  

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.


Compare Gas Hot Water Heater Prices

Now you can compare prices for all the major brands of gas hot water systems including Rinnai, Bosch, Kelvinator, Rheem, Dux and AquaMAX.

Click on your category of interest to compare gas hot water systems prices:

Tankless Instantaneous Continuous Flow Gas Hot Water Systems Prices

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The information in this article is derived from various sources and is believed to be correct at the time of publication. However, the information may not be error free and may not be applicable in all circumstances.