Architect
Kingspan Advice on Sizing Solar to Cylinder and Dwelling

 Solar cylinder sizes should be selected, following the same approach as used for standard fossil fuel hot water cyclinders. The following six steps outline the pathway that should be followed for correctly sizing a solar system in domestic hot water systems   

 
 
1: Determine Daily Hot water Demand 
 

Two useful reference points for selecting the daily hot water demand are BS 6700: 2006 and the Standard Assessment Procedure (SAP)
 
Bs6700: 2006 details volumes of water used per appliance and these figures should be assessed against the building design. From SAP 2005, the demand for hot water is derived from the floor area of the dwelling and is specified in Table 1 of the SAP guide
 
Whilst the SAP supports the Part L Building Regulations in England and Wales, it is also a useful reference point for Northern Ireland and Scotland. SAP has already been revised and SAP 2009 is due this Autumn. There are changes to the SAP method for calculating the daily hot water demands so the following guidance may eventually be revised but it is currently valid
 
 
 
2:  Determine the Backup Storage Volume
 
This should be calculated on the hot water requirement, taking into consideration the property design and draw-off-requirements. For example, a 170m2 property with two baths requires from SAP 159 litres / day. Assuming 6KW heat input to water from BS 6700 minimum storage capacity would be 140 Litres. The larger of these two figures should be used.
 
 
 
3. Determine Solar Storage Volume (I.e Using Sap)
 

Dwelling Type
 
Typical Net Panel Area m2
 
Minimum Compliance Vs
 
Enhanced Practice Vs
 Based on 25 Litres /m2
 
Based on Sap
Based on 35 Litres/ m2
Based on SAP
Small 50m 2 Floor Area
 
 2
50
64
72
80
Medium 75m2 Floor Area
 
4
100
80
144
100
Large 110m2 Floor Area
6
150
95
216
119

 
 
4: Size the Cylinder
 
Chose the Cylinder from the preferred manufacturer’s product ranges, selecting the next size available greater than the calculated values.
 
 
5. Size the Collector Area
 
 
“Capturing Soalr Energy” from the CIBSE knowledge series states, “the system should be designed to supply 100% of the buildings’ hot water demand during the summer months to gain full benefit from the installation”.
 
When sizing a solar system for domestic hot water, it is typical to size the system to achieve an annual solar fraction of 55%. Energy required can then be calculated using the following equation – QHW = Volume of Daily DHW X Cw X (T). (Key- Volume of hot water from data calculated, Cw = Specific heat capacity of water (1.16 wh/KgK) T= Temperature difference between incoming and desired water temperature.
 
 
  1. Confirm Cylinder and Collector Compliance
 
From SAP a minimum of 25 Litres per net square metre of solar collector storage volume is required, therefore the example would require 75 litres of dedicated solar volume.
 
Check this against the data provided by the cylinder manufacturer to confirm all requirements are met.
 
By following these six steps, you will be able to provide a domestic hot water system that utilises all the benefits of solar technology.