A direct hot water cylinder heats up water using the heat sources that are already present in the cylinder. These heat sources can be electric immersion heaters. In general, each of these cylinders has two immersion heaters to enable the user to take advantage of the off-peak energy tariffs, thereby minimising the total running costs. This is why a direct hot water cylinder does not have a heat exchanger. They are apt for the Economy 7 electricity tariffs, which offer cheap electricity at night for seven hours. Using the cheap night tariff, the lower immersion heater heats the whole cylinder, and the other immersion heater maintains the temperature of the water as it is drawn off throughout the entire day. These plumbing heating systems come in handy in locations or premises that have no access to gas, particularly upper-level flats and apartments.
How Is an Indirect Hot Water Cylinder Different from A Direct Hot Water Cylinder?
An indirect hot water cylinder uses an external energy source, such as a solar thermal system or a traditional central heating boiler. These external sources are connected to the cylinder via a coil or heat exchanger, enabling the water to get heated ‘indirectly’. Few of these hot water cylinders come with an in-built electric immersion heater as a backup in case the primary energy source becomes unavailable or fails. This temporary measure enables households to get hot water even when the cylinder does not work. This might have a huge impact on the electricity bill. Depending on the project requirements, a direct or indirect hot water cylinder can be included in a vented and unvented cylinder.
General Working of Indirect Hot Water Cylinders in Households
In a household with a feed and expansion cistern or a cold water storage cistern in the loft, a regular gas or oil boiler has a hot water cylinder that contains a coil of copper pipe, which acts as a heat exchanger. A thermostat is attached to the edge of the cylinder. The role of the thermostat is to signal when hot water from the boiler must be pumped through the coil, heating the rest of the water in the cylinder. The thermostat is usually set at 55-60°C. An important thing about indirect hot water cylinders that has to be noted is that the two bodies of water – the water from the boiler and the domestic hot water – never come in touch or get mixed. (If it happens, it is most likely due to a broken coil.)
Also, the heat exchanger does not always have to be a coil. In the case of plumbing systems such as a prismatic hot water cylinder (which is different from the standard indirect hot cylinders), water is supplied by the cold storage cistern for both the cylinder and the central heating. The heat exchanger has an airlock that separates the two bodies of water. As the cylinder gets filled with water, the airlock forms. This process is called priming the cylinder, hence the name.