Water is heated in big quantities via an element in an insulated tank and stores at a constant temperature.

What are tank storage units/direct fired cylinders?

These are insulated containers designed to heat water and/or store it. Storing large quantities of hot water can be useful when there are large demands at particular times - for example in a hotel. Due to the storage and constant heating of hot water for long periods of time, these systems can prove to be wasteful in terms of energy usage.

How do tank storage units/direct fired cylinders work?

In the majority of cases, water enters the tank under mains pressure and is heated by a gas burner. As the water heats up it rises to the top, where it is drawn off.

What are the pros and cons of tank storage units?

Advantages Disadvantages
Storage tank systems provide hot water with minimum ramp-up time Once out of hot water, it can take some time to reheat the next batch of water
The temperature is consistently maintained while the hot water lasts Mild steel tanks require maintenance at least once every 5 years to change the sacrificial anode. Which means down time
  Hot water tanks occupy a considerable amount of space and can be very heavy when full
  As the system is constantly in operation these system prove wasteful in terms of energy usage through heat loss occurring
  Limited capabilities to satisfy peak hour demand
  Lower performance from prolonged exposures to poor quality untreated water
  Consideration needs to be given to available gas supply