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What are the parts of a hot water cylinder?

What are the parts of a hot water cylinder?

Shop for Immersion Heater Spanners

   Diagram of a standard hot water cylinder and an economy 7 hot water cylinder with key parts labelled

Immersion heater element

Image showing a standard hot water cylinder, with the immersion heater element at the top

Standard hot water cylinder

A standard hot water cylinder is heated by one immersion heater element which will usually be found at the top of the tank.

Most often, this type of element will have a 685mm (27″) coil.

Image of an economy 7 hot water cylinder, with two immersion heater elements

Economy 7 hot water cylinder

An economy 7 hot water cylinder is heated by two immersion heater elements. These elements are inserted across the width of the tank and they are, therefore, shorter than the elements used in standard tanks, measuring 457mm (18″).

Image showing a happy piggy bank due to the owner of an economy 7 hot water cylinder saving so much money The element at the bottom of the tank is the main heater element, and is often referred to as the night setback. This is connected to a separate power supply to take advantage of off peak energy rates. This immersion element heats up the whole of the tank.
Image of a DIYer spending a little longer in the bath thanks to the top up from their booster immersion heater element The one at the top is a booster element that can be used to top up the hot water in the tank during the day. This immersion heater heats only the top half of the tank. It will not heat the water below the element since hot water rises.
 Wonkee Donkee asks the DIYer what kind of hot water cylinder they have so that they will know which parts of the guide to changing their immersion heater element are relevant later on

What is a copper boss?

Image to show how wide the copper walls are on a hot water cylinder The walls of a hot water cylinder are very thin – only 3mm (1/8″) thick.

Immersion heater elements are considerably deeper and require something more substantial than just the wall thickness of the tank to screw into to ensure a tight fit.

Image of immersion heater copper flange showing screw thread To enable this, hot water cylinders are fitted with a copper boss (also known as a head) that is soldered into a hole in the tank. This provides a  housing for your immersion heater element.

A 21/4” brass flange, as shown here, can be retro-fitted to a tank as a replacement or for installing an additional immersion heater. These have a backnut to allow the flange to be tightened onto the cylinder rather than soldering or brazing. This job is best left to the experts.

 Wonkee Donkee explains that excessive force can cause damage to your copper boss and hot water cylinder

Hot water outlet

The hot water outlet pipe is located at the top of the hot water cylinder. This pipe carries hot water to the sink and bath taps.

Cold water inlet

Image showing how a cold water inlet pipe works The cold water inlet pipe is located at the bottom of the hot water cylinder. This pipe allows cold water to enter the cylinder to be heated by the element coil.
Image of a drainage valve attached to a cold water inlet pipe The cold water inlet pipe should have a drainage valve attached. This allows you to drain the tank, when required.

Foam insulation

Image of a polyurethane foam insulated hot water cylinder Most tanks are insulated with polyurethane foam which helps to prevent heat from escaping the tank.

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