Differences Between Gas Storage Water Heaters & Gas Continuous Flow


A decision that many householders have to make in today’s age is to do with the choice of the hot water system. One such decision that regularly needs to be made is whether to replace their existing gas water heater with another storage tank or whether to convert to a new gas continuous flow unit. Most people however don’t really know the differences between a storage tank and a continuous flow unit. So the purpose of this article is to list those differences so as to make it easier for people to decide between the two options of water heaters.

  • Size of Water Heater
  • Continuous Hot Water
  • Star Rating
  • Cost of Gas / Savings in Gas Used
  • Warranties
  • Cost of Repairs
  • The Need for a Tempering Valve

Hot water heaters diagram
Difference between an electric and gas hot water system.

Size of Water Heater

The first obvious difference is the physical size of the two units. Whereas a gas storage tank generally has a height of either 1.6 metres or 1.9 metres, a gas continuous flow unit is generally around 530mm tall. Therefore, a common reason for someone to opt for the smaller continuous flow unit is to save space outside in the garden. By removing the old cylinder from say a side pathway and replacing it with the smaller continuous flow, much space is saved and more of a walkway created. Likewise the same saving in space can apply indoors. For example, sometimes an indoor tank is removed from a laundry and replaced with a smaller unit, allowing for more cupboard space to be created. So a desire for more space can be one reason for making the move away from a gas storage tank.

Continuous Hot Water

One of the most common reasons for people to change from a gas hot water tank to a continuous flow unit is because they have found themselves running out of hot water with their current set up. For example, most gas tanks provide anywhere from 100 litres per hour to 200 litres per hour. However, that equates to approximately 20-25 minutes of showering each hour or the filling up of a large bath. So, for households who use baths regularly, the problem of running out of hot water is there for the next person to hop into a shower (in the same hour that the bath has been filled up). With people living busy lives in today’s fast paced world, having to wait up to an hour for the hot water tank to reheat is very frustrating. Therefore, for households that experience this problem of running out of hot water regularly, the option becomes converting to a gas continuous flow unit.

With a gas continuous flow unit, the most common size unit is the 26 litre per minute one. This equates to 1560 litres per hour, which is up to 15 times more hot water than a storage tank can provide. So, it’s this difference in available hot water and recovery rate that makes this style of water heater very popular for large hot water using households. The above problem of running out of hot water after filling up a bath is simply non-existent with the continuous flow water heater.

Star Rating

Whereas the highest star rating for a gas hot water tank is five star, the highest rating for the continuous flow unit is currently six star. For people who are very conscious of energy efficiency, then this greater star rating can be the reason for them to change over to a continuous flow unit.

Cost of Gas / Savings in Gas Used

The savings in gas usage between a storage tank and a continuous flow unit is hard to quantify. However, for a average hot water using household, the difference in cost can be anywhere from $20 per year to $40 per year. This is because the continuous flow unit only heats up when any hot water tap is turned on.


For gas storage tanks the warranties on the cylinder vary from 5 years to 7 years and finally to 10 years depending on the brand and model. What this warranty means is that if the cylinder ruptures and therefore leaks, the tank will be replaced free of charge (i.e. a new tank is supplied at no cost). The parts connected to the storage tank are covered by a 1 year warranty. The reason being that the parts are constantly working every day and therefore can break down with excessive use.

In contrast, the warranties for continuous flow units are generally either 10 or 12 years on the heat exchanger. This means that if the heat exchanger was to break down and leak, a new one would be supplied free of charge. The other warranty is 3 years on the printed circuit board. So again if the printed circuit board faulted within the first 3 years, it would also be replaced free of charge.

Cost of Repairs

The most common repairs required for a gas storage tank are break down of parts. Examples of parts that may need replacing during the life of the water heater are the thermocouple, gas control, piezo ignitor, and the pressure temperature relief valve. Costs of replacing any one of the thermocouple, piezo ignitor and pressure temperature relief valve generally range from $275 to $375. Replacing the gas control is a little bit more expensive generally ranging from $400 to $500.

A thermocouple will often need to be replaced at least once during the tanks lifetime. The same would apply to the pressure temperature relief valve. However, the gas control is less likely to break down during the life of the tank, however still is a chance of maybe needing replacing in the later years.

With continuous flow units, the most common repairs involve either the clogging up of the system, and the breakdown of parts, in particular, the printed circuit board. These units, being prone to clogging up, therefore should have a regular service (at least once every 3 years). This service should cost anywhere from $200 to $300. The printed circuit board is a much more expensive item to replace, costing anywhere from $800 to $1000 on average. If this goes after the first 3 years, it is then a cost that the owner will have to bear, however normally they don’t go until after 6 or 7 years and hopefully not until after at least 10 years. Nevertheless, it is possible for them to go soon after the 3 year warranty period and therefore, this needs to be taken into consideration if one is very wary about future repair / maintenance cost.

The Need for a Tempering Valve

With the installation of a gas storage water heater in domestic locations, a tempering valve is now compulsory. What this does is take the hot water as it immediately leaves the tank, and reduces the temperature to a safe 50 degrees (as determined by the authorities). However, just like with any external valve, there can be issues with tempering valves, requiring service from time to time. Typical reasons for service may be the cleaning out of the tempering valve or the adjustment to the temperature being generated from the tempering valve. Occasionally, the whole tempering valve will need replacing, however this isn’t very common and one would have to be unlucky if this was required within the first 5 years of installation.

In contrast to the storage water heater, the continuous flow unit comes in two preset options – a 50 degree preset and a 60 degree preset. For domestic installations, when installing a 50 degree preset, there is no need to install a tempering valve. This can be an advantage because there is no need to worry about the repairs to the tempering valve.

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