Main Pressure Hot Water Cylinder

Main pressure hot water cylinder have become far more popular than low-pressure hot water cylinders in recent years, the main attraction being a sharp increase in hot water pressure. 

Electric or Gas?

Mains pressure hot water cylinders can be electric or gas heated. Most homes in New Zealand have an electric hot water cylinder, older cylinders tend to be smaller and low pressure. Most modern cylinders are mains pressure and have a capacity of 180 litres or bigger. Older electric hot water cylinders are often poorly insulated, which will lead to heat loss. It is worth wrapping the cylinder with insulation if the electric hot water system is less than “A” grade. Gas hot water storage systems have a faster heat recovery time than a comparable electric hot water cylinder. Gas cylinders need to be installed in a well-ventilated area and flued to remove exhaust gases. This leads to long pipe runs and means that gas hot cylinder have higher heat losses than electric hot water cylinders. It is also not safe to wrap a gas hot water cylinder. 

Working of Main Pressure Hot Water Cylinder

The components of a mains pressure hot water cylinder are the storage tank, thermostat, element (electric system) or the burner (in the case of gas systems), temperature relief valve, anode rod and drain valve. The storage tank will usually have an inner steel cylinder which is lined with glass or vitreous enamel, insulation and a steel outer lining. Vitreous enamel is a thin layer of glass fused to the surface of the metal at a very high temperature. The thermostat acts as a thermometer and controls the water temperature inside the water storage tank. Power or gas  is only used when heating is required, meaning the thermostat is fully automatic. In electric hot water cylinders one, sometimes two, immersion elements are located within the storage tank, these create the heat required to heat the water. In gas systems, the burner is located at the bottom of the cylinder with a flue running through the centre of the tank to exhaust toxic emissions. Some tanks have two elements controlled by separate thermostats, only heating the water when the water temperature goes below a predetermined temperature. The temperature-pressure relief valve exists to protect the hot water cylinder from excess pressures and temperatures, avoiding major damage to the cylinder or your home. The anode rod protects the cylinder from corrosion. The anode rod is suspended in the tank to reduce corrosion over time and prolong the lifespan of the cylinder. The drain valve is used to drain water out if the cylinder should repairs be required. 

Hot water cylinders are very simple systems, which makes them a great hot water solution. Similar to an electric jug, electric hot water cylinders make use of an element inside the storage tank to heat and maintain the water to 60 degrees. Cold water will enter the cylinder storage tank when hot water exits, this way the water temperature in the bottom of the cylinder storage tank is colder than the water at the top. This triggers the thermostat, allows the element to activate and heat the temperature to the set temperature again. This process repeats 24/7 to ensure hot water is always available. Gas hot water cylinder work in a very similar way, the only difference being they burn gas to heat water at the bottom of the tank and do not have an element in the water. 

Mains pressure, open vented, storage water heater supply systems supply mains pressure hot water to all outlets; hot and cold water is delivered to outlets at the same pressure. These systems have an internal or external expansion vessel that allows the heated water to expand, the system must also include a pressure relief valve in the event the expansion vessel fails. All mains pressure hot water cylinders (>120 kPa) require this pressure relief valve to be both temperature and pressure operated. If a mains pressure unvented system is used, valves must be specified for the particular system to achieve the required pressure rating. The problem with only having a single valve for relieving pressure is that hot water is wasted as the hot water expands during the frequent heat-up cycles. To minimise wasting hot water, an expansion control valve, set at slightly less pressure, is installed on the inlet side of the cylinder and will relieve cold water during the heat up-cycle. The expansion control valve can normally drip up to 5 litres of water per day. Both valves must have the appropriately-sizes copper drain line that discharges to a safe and visible location. 

Advantages 

  • Equal pressure of supplied hot and cold water
  • High-pressure hot water
  • Quick hot water delivery
  • Smaller diameter pipework can be used
  • The system allows for greater flexibility of pipe layout

Disadvantages

  • It can be noisy and subject to water hammer
  • There can be more water wastage
  • More valves are required in the system
  • There is increased wear and tear on the pipes and fittings
  • It can be difficult to install a wetback alongside this system

Benefits of an electric mains pressure hot water cylinder over a gas mains pressure system

  • Electric powered systems are more energy-efficient than gas systems, conventional electric hot water cylinders have an average energy efficiency of 90%, while a conventional gas hot water cylinder rates at about 60%.
  • Electric hot water systems can easily be shut down if you go on holiday, you only need to flick a switch should you need to turn it off.
  • The upfront costs are relatively low when compared to other water heating solutions.
  • These systems have been around for a long time time, which means the components have been refined and perfected over the years improving the reliability and extending the life expectancy.
  • Hot water cylinders are flexible and can be future-proofed, they can be ready for adding to solar power and are suited for hot water heat pumps. This allows for greater adaptation if your hot water needs change.
  • Some models can be installed outside, which frees up space inside the house and allows for better access if maintenance is required.

Cylinder Capacity

Cylinders come in a range of sizes from 30L to 300L, keeping in mind that one person uses approximately 50L hot water per day, you’ll be able to determine the capacity needed to sustain your household. Contact us today for a consultation to discuss your options. 

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