Tank in Tank Hot Water Cylinder Alternatives
The tank in tank hot water cylinder system is promoted as hot water storage with a quick recovery solution, delivering a good flow rate via a comparatively smaller tank size.
For domestic projects, it can often provide an effective and efficient solution for your hot water requirements, however, the approach does have a number of issues which must be taken into consideration.
What is a Tank in Tank Hot Water System?
The tank in tank system is, as you would expect, a solution which features one tank inside another. The inner tank is manufactured from stainless steel which is immersed in outer tank manufactured from steel.
The outer tank tends to be manufactured from steel and contains primary water which is heated via the respective energy source. The inner tank is always manufactured from stainless steel and contains the cold water feed and outlet.
The wall of the inner tank acts as a heat exchanger, drawing the heat from the external tank and heating the cold water feed as it flows through the internal tank.
The system is designed to contain a smaller volume of hot water to meet the initial demand with the remaining demand being met by the heat transfer from the outer tank.
When working at optimum performance, the tank in tank approach minimises the risk of legionella and build up of lime scale.
Disadvantages of Tank in Tank Hot Water System
The very nature of the tank in tank hot water system presents a number of major disadvantages which must be taken into consideration when deciding on the optimum hot water storage solution.
This main issues with the tank in tank system are its inability to incorporate an inspection manhole to provide access for essential maintenance and the failure to be able to offer a temperature gauge to the cooler section of the tank to provide critical information on the performance of the hot water system.
Both issues are key requirements of British Standard 853 which provides the specification guidelines for calorifiers and storage vessels for central heating and hot water supply.
If you are working on a project which must features a hot water cylinder which conforms to the BS853 standards, the tank in tank system is not for you.
On many models there is no option of adding in a back-up Immersion – so if the boiler fails – no hot water!
One other point to note, is that while the internal tank may have a 25 year warranty the external tank has only 5 years – one needs the other!
What are the Alternatives to a Tank in Tank Hot Water System?
Depending on the key requirements of your project, there are a number of alternatives to the tank in tank hot water system, especially when using copper hot water cylinders.
For projects which demand a quick recovery hot water cylinder solution, McDonald Water Storage can design a copper cylinder, which not only incorporates significantly more coil than normal stainless steel cylinders, but also more surface area than the Tank In Tank heat exchanger. This is achieved using finned copper coil with excellent surface area and the wonderful heat transfer properties that copper provides.
For commercial projects where clients are looking for a solution with a relatively small storage capacity with a quick recovery, we tend to offer our PLATEflow Commercial Hot Water Storage solution which features a buffer vessel with external plate heat exchanger.
The PLATEflow approach is perfect for projects, such as hotels or leisure centres, which can experience a sharp increase in demand for hot water at certain times of the day. The core demand of hot water is supplied by the Plate Heat Exchanger, with peak periods of demand drawing on the hot water contained in the buffer vessel.