Sizing Storage

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The following example describes the tools we use for sizing hot water or thermal storage.

This method applies to stores with decent top-down reheating from source, and not to coil type systems.

28 Apartments – 2 or 3 No. 2 Bedroom, the rest will be 1 Bed and Studio apartments.  (max. 50 People)
+ 6 W.C.’s with 2 Kitchen Sinks in the communal area.

There are two aspects of the design to look at:

  • Peak flow rate
  • Peak volume delivery

Peak Flow Rate

This is determined by diversity calculations. In other words, we need to decide how many taps can run at the same time.

The following graph shows two commonly used diversity graphs, that provide a standardised approach.


The graph mentions number of dwellings. Our enquiry calls for 28 properties, so the diversity would be around 20%.

In other words, we are to design for 20% of the peak load each property can draw.

With each property having a bathroom, and kitchen, the peak draw would be a bath and a sink, totaling 24 litres/minute (18+6).

Peak site flow = 28 x 20% * 24 = 135 litres/minute

This particular design is for unvented water storage, so the 135 litres represents water flow through the store. As such we will size the cold inlet and hot water outlet to 1.5" connections. A suitable inlet control set will also be selected.

Storage Size

As the cylinders are to be reheated by a heat source, the amount of stored hot water required is derived from an estimate of how much hot water will be drawn in the busiest (most concentrated) period of the day. This will almost always be showering in the morning between 6:30 and 8:30, with a large lunchtime and evening load.

To get a feel for real life use, the following graph provides real-time data from a site:

We would typically size to provide 50%-65% of occupants with a shower or bath in one sitting.

Using our Waterload Calculator (see below) we arrived at a balance of hot water storage and boiler size that best suits.


As 500 litre stores and 40kW boilers are standard sizes, this appeared to best match. We could go larger on the boiler, and smaller on the store, but this would push the boiler into a commercial size that would cost significantly more. A similar story for the storage size, except there would be an option to deploy a couple of stores, and use a significantly smaller boiler.

Waterload Calculator

To take the donkey work out of calculating the required storage, we have developed the Waterload tool. It is flash based, so apologies if you are on a non-compatible device. There are more up-to-date versions, but none as user friendly as the original as as embedded below.

Download executable version here...