How a direct cylinder differs from an indirect one, and what single-feed and double-feed mean.
Stored hot water cylinders come in two main kinds, and Level 2 expects you to tell a direct cylinder from an indirect one and to know the single-feed and double-feed variants.
A direct cylinder heats the water that will be drawn off, commonly with an electric immersion. Older direct systems can circulate that water through a suitable non-ferrous solid-fuel back boiler. Do not identify the design just from the presence of boiler connections.
In an indirect system, a heat exchanger passes heat from the primary circuit to the stored domestic water while keeping the two water circuits apart. A coil inside the cylinder is the familiar arrangement. A tank-in-tank design uses the wall of an inner vessel, and another arrangement can use an external plate exchanger.
Where a boiler serves both heating and hot water, primary water can circulate through the radiators and the cylinder’s exchanger. The domestic water stays in its own circuit in normal operation; temperature, turnover and water quality still need attention.
In traditional vented indirect cylinders, single-feed and double-feed describe how the water circuits are supplied. A double-feed arrangement gives the primary circuit its own feed and vent. A single-feed, self-venting cylinder uses one cistern and an internal air bubble to separate the circuits; losing that bubble can allow contamination. Do not add chemical inhibitor to such a primary circuit. Identify the actual cylinder before choosing a filling or treatment procedure.
When inspecting a cylinder, trace three things separately: how heat enters, where the domestic supply comes from, and how pressure and expansion are controlled. Direct or indirect does not by itself tell you whether the vessel is vented or unvented. Use the particular cylinder’s instructions for its connections and controls.
Isolating an immersion heater
An immersion heater is a fixed electrical appliance, so before any maintenance or replacement its circuit must be switched off, secured and proved dead. Electrical isolation stops that heating input, but the water can remain hot. It does not isolate the water supply, empty the cylinder or remove stored pressure; follow the water-side precautions needed for the work as well.