Isaac Occhipinti, Head of External Affairs at the Hot Water Association, talks about all things cylinder sizing, including the importance of tempering water to meet demand.

Many homeowners remain under the impression that having a hot water storage cylinder means that at times they may run out of hot water, and therefore have to wait for the cylinder to reheat. Those in the industry know that this is not necessarily the case. 

If the cylinder is sized correctly according to the needs of the household, this should not happen. But what happens when the needs of the household increases?

In this article, I would like to provide a quick guide to sizing and take a look at how you can meet a change in demand after the cylinder has been installed by providing more water without increasing the size of the cylinder.

Obviously, larger houses will have larger cylinders. When selecting the size of the hot water cylinder, a simple rule of thumb is that, for a typical domestic household, you should allow between 35l and 45l for every occupant. That said, a mains pressure system can use about 18l of water per minute at 40°C if a decent quality showerhead is used. Certain brands of shower can use up to 25l of water per minute. 

However, it must be noted that personal habits also play a big part in total hot water use. Two households of the same size can use completely different amounts of hot water.
In determining how much hot water is required, you should always consult with the occupants, and consider the following:

  • A typical bath uses 100l of hot water per minute at 40°C (equating to 60lpm at 60°C)
  • Showers can use about 18l of hot water per minute at 40°C (equating to 11lpm at 60°C).

The following average consumption values can be used as a general rule (hot water requirements per person per day):

  • Low consumption = 20-30l
  • Average consumption = 30-50l
  • High consumption = 50-70l.

On this basis, a typical four person household would often use around 200l of hot water a day.

This does not necessarily mean that a 200l cylinder is required as, dependent on the heating system, the cylinder may be partially reheated during the day. It is up to the installer to match the correct size of cylinder relative to boiler (or electrical) input to avoid running out of hot water.

So, now that we have covered correct sizing, what happens when an en-suite is added or a dependant returns home? The answer? Tempering water! 

In essence, this means increasing the temperature of water in the cylinder, mixing it with a tempering valve to achieve a lower water temperature at the taps, thus producing more hot water from the same size cylinder. 

Most cylinders store water at approximately 60-65°C. If this was increased to 70°C, you get more water delivered at 55-60°C as the valve mixes both hot and cold water, which increases the cylinder’s water capacity. Mixing the hot water with cold means that less hot water is drawn from the cylinder, therefore boosting its capacity and the amount of usable hot water for the home. 

As a general rule, at 70°C storage, the amount of usable hot water that you can generate is approximately 20% more than at a storage temperature of 60°C. Of course, this is dependant on the primary flow being capable of reaching a higher temperature to heat the stored water to 70°C; electric water heaters or solar PV are ideally suited for this. 

The Distribution Tempering Control valve (DTC) scheme by NSF operates as a third-party approval scheme to verify that tempering valves comply with the performance criteria of both Part G of the Building Regulations and the European Standard EN 15092:2008.

The humble domestic hot water cylinder can meet household hot water demand from multiple outlets simultaneously, even when demand increases, offering a favourable solution in comparison to other products on the market which cannot cope with increased or simultaneous demand in this way and would simply need to be replaced or ‘put up with’. 

Of course, when quoting for a hot water system it is always wise to ask the homeowner if their demand will remain the same but, as we all know, change can happen unexpectedly and tempering water offers a solution.