Low Pressure Hot Water Cylinder

Low pressure hot water cylinders are extremely common in New Zealand; they tend to be smaller and are generally older.

There are two main types of low pressure hot water cylinder tanks

  • A low pressure, open-vented, header tank system
  • A low pressure, pressure-reducing valve system.

These systems are assembled from copper with a galvanised outer casing and are easily distinguished by the copper pipe protruding through the roof of a house. 

Low pressure, open vented, header tank hot water system

In this system, cold water is stored in a header tank located at a higher level than the storage water cylinder. Cold water is gravity fed to the bottom of the storage water cylinder in order to deliver adequate hot water supply pressure. The water rises to the top of the cylinder as it is heated, allowing it to be drawn off through taps or shower outlets and replaced from the header tank at the bottom of the cylinder. The gravity feed provides the water pressure that pushes the water to the outlets, as long as they are at a lower level than the stored water. Heating the water in the cylinder causes it to expand; the open vent offers an outlet for excess pressure. 

Advantages of low pressure open vented hot water system are

  • The relatively low cost of installation
  • Few valves are required; and it is an equal pressure system
  • It also operates quietly and can be connected to a wet back system

Disadvantages of the low pressure hot water system include

  • The large diameter pipework required
  • It requires grading to avoid air locking
  • Hot water is supplied at low pressure
  • Can be prone to pressure fluctuations

Low pressure, pressure-reducing valve system

This works in the same way as the header tank system, but uses a pressure reducing valve to reduce the high pressure water from the mains supply down to a pressure, or head, that is able to be maintained within the height of the vent pipe, which usually discharges above the roof. This system supplies low pressure hot water and high pressure cold water to fixtures, it is commonly known as an unequal pressure system. 

Advantages of low pressure valve hot water system include

  • The relatively low cost of installation and maintenance
  • Few valves are required; and can be connected to a wet back system if open vented.

Disadvantages of low pressure valve hot water system are

  • The large diameter pipework required for hot water supply pipes
  • Low hot water supply pressure; and this system is also prone to pressure fluctuation.
  • A major disadvantage of this system is how difficult it can be to achieve a balanced flow to a shower. 

Mains pressure vs Low pressure hot water cylinders

Mains pressure water heater systems deliver hot water to all outlets at mains pressure; this means that both the hot and cold water is delivered at the same pressure. As this system does not require cold water storage header tanks or vent pipes, it allows for more flexibility in locating the cylinder. 

A lot of older homes in New Zealand will have low pressure hot water systems and there are some ways to check if this applies to your home. If there is a noticeable difference in the water pressure between your hot and cold taps, with the cold tap having a much higher pressure, you most likely have a low pressure system. You can also check your cylinder for labels; a low pressure cylinder may not necessarily have a label but, mains pressure cylinders generally have a large label. If you can see a vent pipe sticking out of your roof or over a gully trap, this also means you have a low pressure water cylinder as main pressure systems are close-vented and do not required a vent pipe. 

The advantages of choosing a mains pressure hot water cylinder include

  • A large proportion of single lever tapware in the current market requires mains pressure
  • Water is delivered to the whole house at the same pressure so the system can cope much better with multiple hot water demands
  • It also reduces the temperature variations in showers. This is an inherent issue with most low pressure systems. 

While Low pressure hot cylinders become more unpopular as they compete with other water heating systems, such as mains pressure hot water cylinders, heat pump hot water heaters or gas continuous flow water heaters, there is still use for them and continue to be supplied and installed. A few examples of instance where they are preferred are in older homes, rental properties, secondary farm properties or commercial businesses where hot water use is much lower. The cost of installing these systems or replacing an old one with a “like-for-like” model ranges between $2,000 and $3,000. 

Sumich Plumbing can help you in deciding which hot water cylinder will be efficient for you contact us now for a free quote.

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