Dimensioning hot water storage vessels to comply with Part L 2021 of the Building Regulations

• 27-11-2024


• Joris Bracke


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The “new” Part L of the Building Regulations for both residential and non-residential properties came into full force in June 2022, with a raft of improvements to meet the UK Government’s net zero carbon ambitions.

There was a 12 month “grace” period for projects which were approved under the previous Part L Documents (Part L1 and L2 2013 as amended), provided building works were commenced before 15th June 2023. As this window has now closed, all new projects which did not start, and all new developments whether it be new builds, conversions, extensions or renovations must now fully comply with the latest Regulations.

What are the changes?

It is a Building Regulations requirement that new and replacement domestic hot water systems for dwellings (Volume 1 of Approved Document L 2021), and buildings other than dwellings (Volume 2), are sized for the anticipated domestic hot water demand of the building, based on BS EN 12831-3. The Regulations also state that systems should not be significantly oversized.

This is a common-sense approach, as installing a hot water vessel which is going to store an excessive amount of energy which will not be required will produce excess carbon emissions, and also be unnecessarily expensive to operate.

What is BS EN 12831:2017?

BS EN 12831 is European Standard adopted by the United Kingdom, and discusses the energy performance of buildings, and the method for calculation of the design heat load. The Standard consists of 4 parts;

• Part 1 = Determination of space heating load
• Part 2 = Explanation of BS EN 12831-1
• Part 3 = Domestic hot water systems load and characterisation of needs
• Part 4 = explanation of BS EN 12831-3

BS EN 12831-3 is applicable for storage systems which include domestic hot water storage tanks with internal heat exchangers, and also larger storage systems with external heat exchangers, or charging storage systems. These scenarios apply to both residential, and non-residential buildings.

How do I size hot water storage vessels in accordance with the required Standards and Regulations?

Dimensioning a hot water storage vessel capacity is not always easy, especially for large buildings or very well insulated homes that only need a small heating capacity.

In recent years, however, the heating capacity has steadily become lower due to improved energy retention of the storage vessel and pipework, and the capacity required for domestic hot water heating has remained constant or increased slightly due to new dwellings having more bathrooms. This makes it important to ensure that the domestic hot water comfort requirements are always fulfilled.

This is checked in BS EN 12831-3 and this part of the standard describes how to draw up a graph as follows:

On the X-axis, we can see the hours of the day, starting at midnight (00:00 Hours). At 07:00, people start consuming hot water (for example by showering), and here we see the first peak in the blue consumption curve. Throughout the day, the consumption of domestic hot water increases cumulatively, and in the evening, we again see a peak in consumption before people go to bed.

The red curve indicates how much energy is stored in the boiler tank. At 00:00 Hrs, the generator (boiler or heat pump) heats up the hot water vessel, which then slowly cools down over time due to standing losses from the vessel, and at around 08:00 Hrs, it is reheated. The upper orange line indicates the maximum amount of energy that can be stored in the hot water vessel, and the lower orange line indicates the minimum comfort limit. The minimum comfort limit is the water temperature provided at an outlet (i.e. a shower), which is defined as 42°Celcius in BS EN 12831-3. If the red line in the graph falls below the lower orange line, the storage vessel is unable to provide water at the required temperature (i.e. 42°C) at the outlets, and the comfort requirements are therefore no longer met.

Example: an apartment block with 20 flats, 2 residents per flat and a communal boiler room with a heat pump of 50kW that heats a hot water vessel with a storage volume of 500 litres, to 55°C. This gives us the graph below;

We see that the domestic hot water comfort requirements are not met since the red line falls below the lower orange line at approximately 08:00 Hrs, and again at 21:00 Hrs. This means that the water temperature provided by the hot water vessel at the outlets is less than the minimum comfort level of 42°C during these periods.

To solve this problem, we can do two things. Firstly, we can increase the size of the storage vessel, and secondly, increase the capacity of the heat pump. If we work with a 2000 litre instead of a 500 litre cylinder, we get the following situation.

Now the red line is within the orange zone and all comfort requirements are met. We can also see that the red line starts to rise at around 08:00 Hrs as the heat generator starts to heat up the water in the storage vessel, and it takes until approximately 12:00 Hrs for the vessel to be fully reheated. If we want this to happen faster, we can increase the capacity of the heat pump from 50kW to 70kW. Now we can see that the storage vessel is fully heated up at approximately 10:00 Hrs as shown below;

This approach allows us to change the parameters until we reach a situation where all requirements are met. The BS EN 12831 Standard states that the building space heat loss must first be calculated, and only then is it determined whether the domestic hot water comfort requirements can also be met with a storage vessel. If this is not the case, the vessel should first be enlarged, and if the requirements can still not be met, the output of the heat generator can be increased. What is not allowed for example, is to calculate the capacity for heating and then add 200W per person to compensate for the generation of domestic hot water.

The Heat-Box domestic hot water module assists users to create this graph for a project. Users can set the flow rates, bath capacity and number of delivery points to accurately model individual projects;

Users can also indicate when the domestic hot water is consumed. In the example below, 27.5% of all domestic hot water is consumed between 08:00 Hrs and 09:00 Hrs;

In addition, an adjustable simultaneity factor is calculated, and the software allows particular variables to be entered. These include;

• incoming cold water temperature
• minimum desired hot water temperature
• capacity and flow temperature of the heat generator
• maximum temperature of the vessel
• volume and height of the storage vessel
• vessel temperature sensor height
• hot water circulation system option (creates additional distribution losses)

To demonstrate compliance for example with Building Regulations, Heat Box produces a report detailing the required information in a simple, easy to understand format.

This module is particularly useful for the dimensioning of hot water storage vessels in large buildings (i.e. apartment blocks with communal boiler rooms, sports halls, hotels or care homes) or for houses that are very well insulated and will be heated with a low-power heat pump (7kW or less).