SAP 10 – Changes to Water Heating Calculations.

Original Article
May 10th, 2019


In this article, Dr Lisa Blake looks at the changes to the calculations for the hot water demand in SAP 10.

A bit of background
SAP is the Standard Assessment Procedure developed and maintained by BRE. SAP is used as the methodology to assess the energy performance of domestic buildings for both Building Regulations and Energy Performance certificates. The current version of SAP is SAP 2012[1].

SAP 10[2] was published by BRE in July 2018 alongside iSAP[3] which is their SAP software that can be used to assess the impact of the changes in the methodology. SAP 10 will not be adopted until there is an update to the Building Regulations, the consultation for which is due spring 2019. Following the Building Regulations consultation, SAP is likely to be refined. SAP 10 will first be used for new-build properties for around 6 months, after which it will be used for existing building assessment and the production of Energy Performance Certificates (Reduced Data SAP [RdSAP]).

The SAP methodology uses the heat loss characteristics of the building together with the heating/hot water system efficiency to calculate the energy requirements of the dwelling. This is then used with fuel costs to produce the SAP rating and with Carbon Emissions factors to produce the Dwelling Emissions Rate (DER) for Building Regulations compliance.

Hot water demand calculations in SAP

Hot water calculations become a more significant part of the energy requirement of a dwelling as the dwelling becomes more energy efficient. 

Therefore, the accuracy and sophistication of the hot water requirement methodology becomes more important. Figure 1 shows how the proportion of the energy requirement of the dwelling for hot water increases as the energy efficiency (SAP rating) increases.

Figure 1 – chart showing the increasing proportion of energy for hot water as the energy efficiency increases

As well as the rising importance of energy for hot water, the popularity of solar water heating (SWH) and waste water heat recovery systems (WWHRS) means it isn’t just occupancy that needs to be taken into account for hot water calculations. The volume of hot water used for both SWH and WWHRS has a significant impact on the energy yields. So, for an accurate calculation, the shower types in the dwelling and their flow rates are required.

Hot water calculations

SAP 2012 

The schematic below shows how the SAP methodology processes the calculation, beginning with the floor area of the dwelling through to the energy required for hot water.

Figure 2 – The SAP 2012 process for calculating hot water requirement

We can see from the flow chart that the base water requirement is simply a function of the floor area of the dwelling. However, hot water use for baths and showers makes up around two thirds of all hot water use, so the energy for hot water should consider the type of shower and its flow rate, as well as whether there is a bath present.

SAP 10: Showers

SAP 10 introduces the flow rate and number of showers into the calculations.

The flow rate of a standard mixer shower depends on the system feeding it. A combi boiler providing mains pressure hot water will have a higher flow rate than a shower feed from a regular boiler with a hot water tank. Of course, some showers feed from a hot water tank and have an additional pump (power shower), and these have an even higher flow rate. There is no evidence to suggest that someone using a high flow rate shower would spend less time in the shower, so the flow rate of the showers present will have an impact on the energy required for hot water.

In SAP 10, the number of showers will be included in the calculation, as well as the number of baths. The volume of water used for showers where a bath is present is lower than when there is no bath. Using the standard number of occupants, the number of showers per day is calculated using the following equations:

For example, a house hold with 2 occupants and a bath would be assumed to have 1.5 showers per day, having less than one shower per day each, taking account of the likelihood of having a bath on some days. If there was no bath present the 2 occupants would be assumed to shower once per day.

Each shower within the property is assumed to be used equally and so the number of showers per day is divided by the number of showers to get the use for each shower.

As well as the number of showers, the flow rate (volume of hot water produced per minute) is also a factor in determining the energy use for showers. The flow rate of a shower varies depending on the type of shower and the system feeding it. When SAP is used for assessing new build properties, the actual flow rate can be used in the calculations, bearing in mind that for Part G of the Building Regulations a maximum of 8 litres/minute for showers is required for compliance. For existing dwellings or when the flow rate is not known the following defaults would be used:

Hot water systemFlow rate (litres/minute)
Vented (gravity) hot water system A system with a hot water cylinder that is fed from a cold water tank in the loft space7
Vented (gravity) hot water system + pump A vented system that has either a pump specifically for the shower (power shower) or a pump that increases the pressure across the whole house12
Unvented hot water system This could be a combination boiler with mains pressure hot water or a system with a pressurised hot water cylinder fed directly from the mains11
Instantaneous electric shower (vented or unvented) An electric shower heats cold water itself, so uses no hot water0

As each shower is assumed to last for 6 minutes, the volume of hot water for each shower can be determined from the duration, showers per day and flow rate.

EST field trials in 2008[4] provide the cold water feed for each month ranging from 10.8°C in February to 21.3°C in July, together with the average temperature of domestic hot water of 52°C. Using the requirement of a shower to be at 41°C, this data can be used to determine the proportion of the volume of water from the shower that needs to be heated.

SAP 10: Hot water usage

We are now able to calculate the volume of hot water required for the shower use in the dwelling (under standard occupancy). Where there is a bath present a similar process is used to establish the volume of hot water for baths. The average volume of water used in a bath is 73l (almost the same amount of water as a 12l/min shower running for 6 mins). The number of baths per day uses the following equations, with more baths where a shower is not present.

For our 2 occupant household this would be 0.45 baths per day where there is at least one shower, and 1.2 baths per day if there is no shower.

Figure 3 illustrates the need for new hot water use calculations. When showers, flow rates and baths are taken into account, the hot water usage increases for most scenarios.

The red line is the current SAP 2012 daily hot water use. The floor area is used by both SAP 2012 and SAP 10 to arrive at the dwelling occupancy. We can see that for both SAP 2012 and SAP 10 the daily hot water usage rises quickly until around 100m2 and then has a gentler incline as the floor area increases.

The current SAP 2012 calculations have the lowest daily hot water usage, very close to the SAP 10 model where there are only low flow rate showers and no bath. When the dwelling has high flow rate showers and a bath, the increase is around 50%. The line for the low flow rate shower and bath illustrates the  impact of the flow rate on the hot water energy, the difference in flow rate producing a 20% difference in the hot water usage.

Figure 3 – SAP 2012 and SAP 10 hot water use variation by floor area

SAP 10: Electric showers

Electric showers which heat cold water instantaneously are also not considered in the current version of SAP. Using an electric shower will reduce the hot water demand but will increase the electricity used in the dwelling. Data from the Market Transformation Programme[5] indicates that around 45% of homes in the UK have instantaneous electric showers.

In SAP 10, along with including mixer shower data, electric showers are also considered. This will be particularly important when SAP 10 comes into force, as the carbon emissions factor for electricity has been halved, making electricity an attractive fuel to use in new-build housing.


Whilst some of the content of SAP 10 might be ‘tweaked’ following the consultation on Part L Building Regulations, it is clear the calculations for hot water will be more sophisticated. The inclusion of showers, both mixer and electric, will increase the accuracy of the hot water demand. As electric showers are becoming more popular, it is important that SAP takes the energy for these showers into account when calculating the running costs, especially as the proportion of energy for hot water starts to overtake space heating. Electric showers can be considered a hot water back-up for households with a combi boiler. Having at least one electric shower in the house means that if there is a problem with the boiler you can still have a shower, similar to the back-up electric immersion with regular boilers.