Posted on

Hot Water Storage Cylinders, The Facts

Simply put, a hot water storage cylinder is a tank used to store hot water required for space heating or household use. It goes by other names such as hot water tank, heat storage tank, hot water cylinder, thermal storage tank or hot water thermal storage unit.

We don’t often think about this appliance as long as we get hot water on demand. But as soon as we’re greeted with that unwelcome cold shower, the realisation hits. The good news is hot water tanks are generally reliable and operate for many years without a glitch.

When that hitch finally comes, it’s good to have a background on this household convenience. It will also reduce your energy spending. Here are all the information you need to know when choosing the right hot water cylinder.

hot_water_storage_cylinder_mains_pressureTypes of storage cylinders

The main type of new installations in New Zealand are mains pressure storage water heaters, where the operating pressure is the same as the incoming supply (120-1,400 kPa, usually 350-700 kPa).

Besides that, other heaters operate as:

  • Low pressure, open-vented system, wherein pressure is provided by a cold water storage header tank (3-12 m head/30-120 kPa). This was traditionally used in Kiwi homes.
  • Low pressure system with a pressure-reducing valve to minimise the mains pressure water (3-12m head/30-120 kPa)

Here’s how the three types of systems compare:

Heater Type Mechanism Pros Cons
Low pressure, open-vented, header tank Cold water stored in a header tank is fed via gravity into the storage water tank located below the header tank. As the water is heated, it rises to the top of the cylinder where it can be drawn for domestic use.

The heating process causes water expansion, hence the open-vent pipe for releasing excess pressure. This pipe typically feeds back into the header tank supply.

  • Cheaper to install
  • Few valves required
  • Equal pressure system
  • Quiet operation
  • Suitable for wet back connection
  • Needs wider pipes
  • Needs grading to avoid air locking
  • Low pressure supply
  • Prone to pressure fluctuations
Low pressure, pressure reducing valve system

(open-vented or unvented)

This type works similar to a header tank system, but it uses a pressure-reducing valve to reduce the pressure from the mains supply to a pressure that can be maintained within the height of the vent pipe.

Supplies low pressure hot water and high pressure cold water to fixtures, hence called unequal pressure system.

  • Inexpensive installation and maintenance
  • Few valves required
  • Suitable for wet back connection (if open-vented)
  • Difficult to achieve a balanced flow to a shower
  • Needs larger diameter pipework for hot water supply
  • Prone to pressure fluctuations
Mains pressure, unvented Supplies mains pressure hot water to all outlets. It has an expansion vessel (internal or external) that allows the heated water to expand, but a pressure relief valve is recommended in case of expansion vessel failure.

This valve must be specified for the particular system used to achieve the required pressure rating.

  • More flexibility in locating the cylinder as header tanks and vent pipes are not required.
  • Equal pressure system
  • High pressure hot water
  • Quick hot water delivery
  • Smaller diameter pipework
  • Greater flexibility of pipework
  • More water wastage due to frequent heat-up cycles
  • Increased depreciation on pipes and fittings
  • Difficult to install
  • Subject to water hammer

Hot water storage tanks can also be classified based on how they are run: electric or fuel fired. The most commonly used fuel is gas, either natural or propane. But in some areas, an oil-fired heater is popular.

Fuel-fried models have vent pipe at the top that carry off exhaust gases, while electric units simply have a power cable connecting the heater to your electric service panel.

Indirect vs. Direct systems

This refers to how the water is heated in the tank. Most cylinders are indirect systems. They are heated using an external source such as a solar thermal or gas boiler, where water is heated then fed through a copper coil in the hot water tank. The heat is then transferred from the external heat source to the water inside the cylinder. In direct systems, an internal immersion heater directly heats the water inside the tank.

Methods for storing heat

  Method Advantage
Hot water tank with closed water circuit Heat is stored in the tank using external heat exchangers (coils) that can be directly tapped or used to power other heat exchangers The tank is not continually fed with cold water, which reduces limescale deposits especially in ‘hard’ water areas

Reduced oxygen levels, which minimises requirements for materials used in the tank, water circuits and pipework

Stratified hot water tank with closed circuit/ stratified thermal storage/ thermocline tank/ water stratified tank storage Mechanisms are put into place to maintain the vertical stratification of the water column, i.e. water at the top must be kept hot (90-95oC) and water at the bottom cold (20-40oC). Best for places with wide climatic range.

Design and construction

Water heater tanks are usually made of stainless steel, copper or vitreous (porcelain) enamel-lined carbon steel.

Electric and gas types are mostly similar in construction, sharing common components such as the drain valve, TPR valve, dip tube, internal anode rod and pipes and fittings. The main difference is that electric water heaters have a separate thermostat, while gas water heaters have a built-in gas control valve. In addition, gas models have an internal flue to vent gas and circulate heat; a heat limiting device to prevent overheating; and a special sensor called a ‘thermocouple’ to shut off the gas in case of emergency.

Most modern hot water tanks are made of stainless steel, which is lined with glass to prevent corrosion. Corrosion or rusting is the main reason for tank failure. An anode rod inside the tank also prevents the steel from rusting. It corrodes in place of the steel, so this part must be checked once a year and replaced if necessary. Otherwise, the steel will start to rust. Once a hole forms, the tank will eventually have to be replaced.

At the bottom of the tank is a drain cock that will empty the heater. A valve is fitted on the supply pipe to shut down the hot-water plumbing without interrupting the cold-water supply.

Because hot water tanks both heat and store water, they need to have some sort of insulation in order to maintain warm water in between heating cycles.


Water has a high specific heat capacity, which means it can store more heat per unit of weight compared to other substances. It’s one of the best medium for heat storage. In comparison, it can store four to five times as much heat as stone and up to ten times as much as most metals, per unit of weight.

When kept in an efficiently insulated tank, hot water will retain heat for longer. Thermal insulation significantly reduces standby heat loss and speeds up the heating process, which means you use less energy for heating water.

The most common insulating material for hot water tanks is fibreglass, which is typically fixed in place with straps, tape or the outer jacket of the water heater. For outdoor tanks, sprayed-on polyurethane insulation is common.

Water heaters have different insulation ratings. If you find the rating too low, you can add extra insulation on the outside of the tank to further reduce heat loss. In places with extreme weather conditions, the hot water cylinder can be fully enclosed in a specially built insulated space.

However, there is such thing as “too much” insulation. If you live in humid locations and you add more layers of insulation to an already well-insulated tank, condensation may occur which may lead to rust, mould or other operational problems. Air flow or combustion gas outflow, either by convection or fan-assisted, must be maintained to prevent condensation.

Heater effectiveness

Both the recovery rate and capacity of the tank affect your supply of hot water at home.

The recovery rate, or the speed at which a tank heats water, indicates how many gallons of water can be heated to 100oF in an hour. When you’re using water faster than it’s heated, you’ll eventually get colder water from the tap. However, since the tank also stores hot water, its capacity will also affect the ongoing supply at the tap.

Choosing a hot water tank with a suitable capacity and recovery rate depends on your household’s hot water demands. Electric heaters typically have low recovery rate but high tank capacity, which makes them suitable for intermittent use. On the other hand, fuel-fired tanks heat water faster but don’t necessarily have a large cylinder, which is good if you need hot water ready any time of day.


As mentioned earlier, tank capacity is a major consideration in choosing a hot water cylinder. Choosing a cylinder that’s too big will unnecessarily bump up your energy bill, while a tank too small will mean bouts of hot water shortage.

The right size depends on how much hot water your household uses on average. If you’re not sure, the table below will give you an idea on which size you need:

Family Size Gas Electric
1-2 30 30
2-3 40 40
3-4 40 50
5 or more 50 80

For big households or for commercial use, a tank with a higher recovery rate and capacity is ideal to ensure uninterrupted supply of hot water.

Space requirements

f can be installed in interior spaces of a property or in auxiliary areas such as a basement, garage or crawlspace. Ideally, they should be located closest to the plumbing system. Pipes running from the tank must be installed vertically for faster delivery of hot water into the home. The pipework may be adjusted when installing a hot water cylinder in less than ideal locations.

If standard-size water heaters do not suit any of the available spaces in your property, you have two other options to choose from.

Tall tanks (also called tall boys) range from 46 to 60 inches in height and 18 to 21 inches in diameter, and are typically installed in basements or garages where the height can be accommodated easily. They can hold up to Extra height is required to allow the connecting pipe to be installed on top of the tank.

Short tanks (also called low boys) range from 30 to 49 inches in height and 20 to 26 inches in diameter, and are suitable for crawlspaces, under cabinets or areas with low headroom. These are shorter and wider than the standard water tank, allowing them to hold the same amount of water (up to 50 gallons).

If you’re looking at hybrid water heaters, you’ll need extra space for proper installation. So that’s one thing to consider.

Safety issues

Water stored below 60oC can permit the growth of harmful bacteria, such as those that cause Legionnaire’s disease which is fatal to young children and those with compromised immune systems. This is why some jurisdictions impose a limit on tank water temperature. However, water at a temperature above 49oC can cause painful scalding injuries.

In order to prevent both dangers, it is recommended to use mixing valves that would automatically mix cold water with the hot water from the tank to maintain maximum temperature below 49oC. These valves are installed at outlets for sinks, showers or baths.

Features and Modern Technology

  • ENERGY STAR rating

Modern appliances are given star ratings to indicate their energy efficiency. The ENERGY STAR is a government-backed symbol that can help you choose and support energy efficient products and practices. Check the energy factor (EF) rating as well – it speaks of a product’s water-heating efficiency.

Products with higher ratings are more efficient. While they are typically more expensive upfront, they are designed to save you money in the long run. Not to mention, they have a smaller carbon footprint.

For easy comparison, check the EnergyGuide labels of the units you’re considering. This will give you an idea on how the product is expected to perform and how much its estimated annual operating cost.

  • Smart water heaters

Modern water heaters have intuitive technology that “learns” your household’s hot water consumption patterns and automatically adjusts the temperature and other operating features accordingly. These are obviously more energy efficient, which can reduce your energy bill.

Installing a hot water cylinder

Installation is best done by a qualified professional. An experienced plumber would know well how to run pipes or where to locate the tank, and would give helpful advice in choosing a suitable hot water cylinder for your home.