Conservation of fuel and power: Approved Document L, Volume 2 — Buildings other than dwellings

O N L I N E V E R S I O N L1(b), L2 NOTE: For boilers with an output of more than 400kW, the manufacturer’s declared efficiencies should be used. Multiple-boiler systems in new buildings 6.5 For multiple boilers in new buildings, the four-step method described below should be used to calculate the overall boiler seasonal efficiency. a. Step 1: Determine the load on each boiler for each of the three system part load conditions of 15%, 30% and 100%. NOTE: For example, if the total system output is made up of three equally sized boilers, at 15% of system output the lead boiler will be operating at 45% of its rated output with the other two boilers switched off. b. Step 2: Determine the efficiency of each boiler for the above operating conditions. NOTE: Linear interpolation should be used to determine efficiencies between manufacturers’ declared efficiencies at 30% and 100% load. If efficiencies at below 30% are required and unavailable, the boiler efficiency may be taken as equal to that at 30% load. c. Step 3: Calculate the overall operating efficiency at each system part load condition using equation 6.2. η = Q /Σ(q /η ) equation 6.2 p p b,p b,p Where: η is the system efficiency at part load condition p, i.e. 15%, 30% and 100% of system rated output p Q is the system heat output at part load condition p p q is the individual boiler heat output at system part load condition p b,p η is the individual boiler efficiency at system part load condition p. b,p d. Step 4: Calculate the overall boiler seasonal efficiency (η ) as the weighted average of the OBSE efficiencies at the three load conditions, using equation 6.3. η = 0.36η + 0.45η + 0.19η equation 6.3 OBSE 15% 30% 100% Multiple-boiler systems with non-identical boilers replacing existing systems 6.6 In existing systems, equation 6.4 should be used to calculate the overall boiler seasonal efficiency if both of the following apply. a. More than one boiler is installed on the same heating system. b. The efficiencies of the boilers are not identical. NOTE: All boilers should be used in the calculation, including any that are identical. Σ(η × R) BSE η = equation 6.4 OBSE Σ R Where: η is the gross overall boiler seasonal efficiency – an average, weighted by boiler output, of the OBSE individual seasonal boiler efficiencies η is the gross boiler seasonal efficiency of each individual boiler calculated using equation 6.1 BSE R is the rated output in kW of each individual boiler (at 80/60°C flow/return temperature). 41 Building Regulations 2010 Approved Document L Volume 2, 2021 edition O N L I N E V E R S I O N