Replacing a Hot Water Cylinder?

Most homes have a hot water storage cylinder, but how many of us even give it a second thought until it is time to upgrade or replace the hot water cylinder

There are two types of hot water cylinders found in homes today. The newer pressurised cylinders with unvented hot water tanks, and the older style cylinders with vented hot water tanks. Water heater tanks are usually made of stainless steel, copper, or vitreous (porcelain) enamel-lined carbon steel. The most popular hot water cylinder brands are Rheem or Rinnai.

250l Rheem Mains Presser Cylinder installed

Unvented hot water cylinders

In an unvented system there is no cold water tank – instead, the sealed hot water cylinder is fed directly by the cold water mains. Since they are operating at mains pressure, they offer much better flow rates, meaning your shower and bath performance should be higher. Since you aren’t relying on gravity to move the hot water around the home, the unvented cylinder can be located pretty much anywhere in your property.

The major issue with unvented hot water cylinders is that since hot water flow depends on the cold water main pressure; if for any reason the mains water is turned off, your home will be without access to any hot water.

Vented hot water systems

In this system, the cylinder’s tank is fed by cold water from a header (storage) tank (located in the roof) and gravity is used to drive the hot water around the home. A vent pipe links the vented hot water cylinder and the cold water in the header tank.

The hot water pressure tends to be determined by the height of the water tank above the tap or shower feed. This means that although on the ground floor of the home the pressure might be excellent, in rooms on upper floors the pressure will be lower. As a result, many showers in homes with vented hot water tanks use electric pumps to drive the hot water to the shower at increased pressure.

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.

When to Replace your Hot Water Cylinder?

Hot water cylinders generally last a very long time and will only need replacing if they are damaged (rust and corrosion) or leak, or if you are upgrading to a bigger capacity or updating to a new model. Life expectancy of modern copper or stainless steel cylinders is 20 to 40 years, but mains pressure glass-lined steel is shorter at 12 to 20 years. There are generally a few tell-tale signs that you need to replace your hot water cylinder.

  • Leaking valves or pipes are easily repaired; however, if the cylinder’s tank is leaking, you’ll need to get it replaced.
  • Rust or corrosion is a clear sign that your cylinder needs to be replaced. Most modern hot water tanks are made of stainless or carbon steel and lined with glass or vitreous enamel to prevent corrosion. To prolong the lifespan of the storage tank and reduce corrosion over time, an anode rod is suspended within the tank. 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.
  • If your cylinder was made before 2002, you should have a cylinder wrap fitted on it. While this can easily be done, we strongly recommend you consider replacing the cylinder with a modern, more efficient model.

Installing a Hot Water Cylinder

Installation must only be done by a licensed plumber. FlowFix’s plumbers are experienced at this and provide expert advice in choosing a suitable hot water cylinder for your home. It is possible to upgrade your hot water cylinder from a vented to an unvented system, and we can specify the work involved. 

Contact us today to discuss your options for repairing, upgrading or replacing your hot water cylinder. Call Colette on 027 265 4949 or email her at