Technical Booklet P Sanitary Appliances Unvented Hot Water Storage and Scalding·Page 16
Worked example for discharge pipe sizing
Example calculation demonstrating how to size a discharge pipe (D2) for a G1/2 temperature relief valve with 4 elbows and 7 m length, showing that 28 mm copper pipe is required.
The example below is for a G1/2 temperature relief valve with a discharge pipe (D2) having 4 No. elbows and a length of 7 m from the tundish to the point of discharge.
From Table 2.1. Maximum resistance allowed for a straight length of 22 mm copper discharge pipe (D2) from a G1/2 temperature relief valve equates to: 9 m. Subtract the resistance for 4 No. 22 mm elbows at 0.8 m each = 3.2 m. Therefore the maximum permitted length equates to: 5.8 m. 5.8 m is less than the actual length of 7 m therefore calculate the next largest size.
Maximum resistance allowed for a straight length of 28 mm pipe (D2) from a G1/2 temperature relief valve equates to: 18 m. Subtract the resistance for 4 No. 28 mm elbows at 1 m each = 4 m. Therefore the maximum permitted length equates to: 14 m. As the actual length is 7 m, a 28 mm (D2) copper pipe will be satisfactory.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet P - Sanitary Appliances Unvented Hot Water Storage and Scalding.pdf