In fact, in simple terms :
You need a DIRECT hot water cylinder if you want the cylinder to heat the water itself, and…
You need an INDIRECT hot water cylinder if you have a separate boiler (such as a gas boiler or an oil boiler) and will be using that to heat the water.
And here are those answers in a bit more depth:
Direct Hot Water Cylinders
Direct units are used when the cylinder will be the only means of heating water for the property.
The direct system will heat water for use with baths, showers, kitchen sink, wash hand basins and any other hot water requirements in the property. Electrical Deals Direct stock a huge selection of direct cylinders to suit any requirement from domestic to full commercial use.
All direct cylinders contain one or more electrical elements (immersion elements) which heat the water stored in the cylinder. At a minimum, they will contain a single 3 kW heating element, but as the capacity increases, so does the option to add more elements (otherwise it would take an extremely long time to heat the water). For example the Megaflo 210 litre direct cylinder can have 4 x 3kW heating elements: this dramatically improves the recovery time after the water has been used.
Often, because these cylinders are very well insulated, they are wired into a night time tariff supply (such as Economy 7) to heat up much more cheaply at night, and the water is then used during the day.
Here are two of our most popular brands of Direct systems (size to suit your needs).
https://electricaldealsdirect.co.uk/water-heating/joule-water-heating/joule-direct-cyclone.html
Indirect Hot Water Cylinders
An indirect hot water cylinder is used in conjunction with a boiler (which may be powered by gas, oil, or indeed, electricity) and its function is to store the water once heated. The stored water in the indirect cylinder is not heated directly by the boiler, but is instead heated indirectly
Even though an indirect system has a separate boiler heating up the water, Most indirect systems also have a small (3kW) element as a backup heat source and can be used to top-up the heat during the day.
Electrical Deals Direct stock a huge selection of indirect systems to suit any requirement from domestic to full commercial use.
Here are two of our most popular indirect models (Size to suit your need).
https://electricaldealsdirect.co.uk/water-heating/joule-water-heating/joule-indirect-cyclone.html
As well as making sure you choose the right type of cylinder, you should also make sure you pick the right capacity: so for more advice please browse the other articles in this Advice Centre or just give one of our experts a call on 01934 253600.