Sizing hot water storage for low-temperature heat source systems

The CIPHE has been working closely with the industry for the last two years looking at the future of domestic hot water storage design to ensure that the transition from high-temperature to low-temperature heat sources has consumers at its heart.

The industry has been cautiously hanging its hat on the old BS 6700 method for sizing hot water storage for many years and has not fully recognised its limitation when applied to current technology. In the early part of 2022, the CIPHE will be reconvening its Technical Working Group in Hot and Cold Water Services to tackle the current confusion surrounding hot water storage design. The following is a list of some comparisons between the two standards:

BS 8558

This standard utilises the old simplified BS 6700 formula for calculating both the usable volume of hot water within a cylinder and reheat times. This method does not realistically account for the effects of water mixing within the cylinder as the cylinder is being discharged/charged, as turbulence occurs when the cold water mixes with the hot water, reducing the temperature of the DHW as it leaves the cylinder during draw-off periods over time.

Additionally, this standard does not account for the effects of lower heat generator flow temperatures and how this affects the cylinder coil effectiveness/heat input during the reheat phase. In a mixed storage cylinder with an internal coil, the full heat output of the coil is achieved when the water surrounding the coil is at its coldest, for example 10°C. The greater the difference between the heat generator flow temperature within the coil and the surrounding stored water, the greater the power available through heat exchange. Figure 1 illustrates the effect of lowering the flow temperature in a mixed storage system from 65°C to 55°C.

BS EN 12831 Part 3 2017

• Makes allowance for the incoming water to mix during the charging phase.

• All calculations are based upon an energy balance equation and calculated on a minute-by-minute basis. The energy equation allows for various factors such as cylinder heat loss and variable DHW draw off over time.

• This standard reduces the available water that can be drawn from the cylinder before the temperature falls below a usable minimum capacity.

• There is also a calculation for the effects of heat transfer where the power is reduced as the water approaches its final storage temperature. Full heat generator power is not applied to the cylinder coil throughout the cylinder reheat phase for mixed storage systems.

• There is a differentiation between the available power during the reheat phase between a charged hot water system versus a mixed hot water cylinder. Figure 2 illustrates both a direct charged and mixed hot water storage cylinder. The direct charged hot water system allows for full power during reheat, whereas the available power in the mixed hot water storage cylinder suffers a drop in available power as the temperature of the water surrounding the cylinder rises.

Note: heat exchanger design varies for different manufacturers with different coil locations and types, so you should always consult the manufacturer for test and performance data.

Understanding the basics of usable hot water in a cylinder

Using a simplified version of the equation stated in BS EN 12831 part 3 2017 that calculates an amount of usable energy in the cylinder, you can see the amount of available mixed point of use hot water at 40°C for a cylinder stored at 60°C from the tables in figure 3.

How does this relate to customers’ hot water usage?

The obvious key factor for correctly sizing a hot water cylinder is to ensure there is enough usable stored hot water available to deal with the peak demand from the household, and with sufficient power available to reheat the cylinder during periods of disuse.

The key challenges with the future of hot water design are tackling the lower power and flow temperatures available from heat generators such as heat pumps. Installers should always consult manufacturers as there is a wide variety of different technologies on the market to help deal with the challenges of low power and flow temperatures. Figure 4 illustrates a 24-hour consumer hot water profile of 225 litres at a mixed outlet point of use of 40°C throughout the day, with a peak evening usage of 130 litres. In the profile in figure 4 there is a small amount of usage between the morning and afternoon, so it would be wise to always leave some additional capacity within the cylinder for small draw-offs such as the kitchen sink and basins. In the event of a property with high usage throughout the day, then the reheat time becomes important between periods of disuse, so the sizing of the cylinder becomes even more critical.

Selecting a suitable cylinder

Referring to both the table in figure 3, the simple energy equation derived from the amount of usable energy between 40°C and 60°C, there is the equivalent of 167 litres of DHW at 40°C mixed at the outlets for a volume of 250 litres to cover the peak load.

The MCS HW cylinder selection guide also gives the installer a useful table to help aid the cylinder selection (figure 5) based upon the number of rooms and bathrooms. However, the guide also encourages the installer to consult the cylinder manufacturer to ensure the heat exchanger/coil is sized sufficiently for lower flow temperatures and higher flow rates. Considering the previous example of a four-bedroom property, with a bath and en suite, the MCS table recommends a minimum of a 250 litre cylinder for a 3kW to a 6kW heat generator.