Consumer demand for hot water

In a residential application it has become common practice to replace a like-for-like hot water system; however, it would be prudent to first check whether the existing system is actually delivering to meet the consumer’s expectations before any replacement system is installed. It is also worth noting that the consumer will have adjusted their behaviour to the benefits and limitations of the existing hot water system, based upon the operation of the system and their behavioural patterns. For example, a family of four who all wish to shower between 6pm and 7pm in the evening may have been forced to plan how they shower at their peak hot water usage period, in between the cylinder recovery. Delivering lifestyle benefits for your customers along with new system improvements will make you stand out as a reliable professional.

There are many factors that will have inadvertently affected their behaviour, such as: the volume of domestic hot water (DHW) storage available; the available flow rates at the shower outlet; and the size of heat input. Figure 1 illustrates the relationship between hot water storage volume, time of use, usable DHW volume at 400C and outlet flow rates. The graph in figure 1 also demonstrates that with a 300 litre hot water cylinder, the available usable* DHW is 150 litres, and the time period before the cylinder’s heat needs to be recovered is 15 minutes.

Understanding demand

Before the type of DHW system is selected, the volume of water required during a building’s peak period needs to be assessed so that the peak demand can be calculated. This peak demand could be over an hour, or even shorter, depending on the type of building, occupancy and activities being carried out. Traditionally, peak demand has been based upon a heat recovery time period of two hours, which has been deemed acceptable. However, when assessing the hot water demand for a residential property, it is important to advise the consumer about all of the options available to them based upon their expected needs, and to carry out a rational assessment. For example, a residential property with two occupants may require enough DHW for two consecutive showers at 10 l/min between a peak hour period in the morning. If each of the two occupants had a shower for 5 minutes, each at 10 l/min, then the specific peak DHW demand for that hour would be 100 litres at 400C.

Referring to figure 1, the required DHW cylinder size (without the use of a coil) is 300 litres, and the time before the outlet temperature drops below 400C is 15 minutes. Nevertheless, in reality, the majority of cylinders are indirect, with an integral heating coil that would prolong the amount of usable DHW. Figure 3 reveals that the example above of 15 minutes use can be extended to 30 minutes with a 12 kW heating coil.

The control of legionella

While consumer satisfaction is being considered, it is important to also take into account the system’s effect on water safety, including legionella risk. To ensure health and safety requirements are met, the DHW must not be stored at a temperature below 600C, with a minimum secondary return temperature of 500C (figure 2). For example, the Water Regulations Guide stipulates that 500C must be achieved at all outlets within 30 seconds of draw off.

In a residential application within a single dwelling, it may be suitable to install an instantaneous hot water system, such as a combination boiler. However, depending upon the number of outlets to be used during the peak period, this may not be viable. Another thing to consider is the type of heat source being used – a low carbon solution, such as an air or ground source heat pump, will typically require a stored DHW cylinder, such as an unvented hot water cylinder. If an unvented hot water cylinder is installed then the installer must be qualified and competent to install cylinders to Part G3 of the building regulations predominantly up to 500 litres and 45kW power input.

Calculating hot water heat recovery time

The amount of power required to raise the temperature of a known volume of water can be calculated using the following formula which highlights that for the 300 litre cylinder in figure 1, that a power input of 17.45kW is required to heat up the cylinder from 100C to 600C in an hour:

Q= Quantity of heat required (kW)

dt= Temperature difference (initial temperature – final temperature)

C= Specific heat capacity (kJ/kg.K) 4.19 used for simplification

t= Recovery time expressed as a decimal in hours

m= Mass quantity of water (kg) fixed density used for calculation (1kg = 1l)

Building a low carbon future through design

With the Committee on Climate Change’s recommended date of 2025, when all new-build properties will need to be heated by some form of low-carbon technology such as heat pumps, being able to understand system design will be critical. This will ultimately lead to a more skilled workforce and increased consumer confidence. The review of Part L of the building regulations, focusing heavily on increasing insulation levels and the delivery of low temperature heating, means that it has never been more important for installers to carry out CPD to upgrade their skills. There is a real economic opportunity for plumbing and heating professionals to prepare themselves for the future.

Find out more

For information on a career as a plumbing engineering designer, contact You can get advice on what qualifications and experience are expected to achieve Engineering Council registration for Engineering Technician (Eng Tech), Incorporated Engineer (IEng) and Chartered Engineer (CEng).

This article first appeared in the May/June 2019 issue of P&H Engineering, the magazine for members of the CIPHE. Find out how to join here.

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