Sizes the D2 discharge pipe from an unvented cylinder's temperature relief valve
tundish to its point of discharge, per Approved Document G3 Table 3.1.
D1 (valve to tundish) is simply the valve's own outlet size. This checks D2 against the run length and elbow count.
For design guidance only. Always verify with a qualified engineer.
About this unvented discharge sizing calculator
This free unvented discharge pipe sizing calculator sizes the D2 discharge pipe running from an unvented hot-water cylinder’s temperature relief valve tundish to its point of discharge, per Building Regulations Approved Document G3. It is aimed at plumbers, heating engineers and G3-registered installers checking that a chosen D2 pipe copes with the actual run length and number of bends before the discharge terminates safely. D1 (valve to tundish) is simply the valve’s own outlet size; this tool checks D2 against Table 3.1. Everything runs in your browser. Nothing is uploaded.
How the discharge pipe is sized
Approved Document G3 Table 3.1 gives, for each valve outlet size (G½, G¾, G1), a set of D2 sizes with a maximum resistance length and a per-elbow allowance. The calculator computes the equivalent length = actual run + (elbow count × per-elbow resistance) for the chosen D2, and it passes when that equivalent length is ≤ the Table 3.1 maximum for the size: algebraically the same as AD G’s own worked example. If the candidate fails, the tool walks up to the next larger D2 that passes; if none in the table accommodates the run, it directs you to a bespoke calculation under BS 6700 Annex D. Figures are for copper tube.
Frequently asked questions
What is the difference between D1 and D2?
D1 is the short pipe from the temperature relief valve to the tundish, and it is simply the valve’s own nominal outlet size. Nothing to calculate. D2 is the discharge pipe from the tundish to the safe termination point, which must be at least one size larger than D1 and is what this tool checks against the run length and bends.
How do elbows affect the sizing?
Each bend adds flow resistance equivalent to an extra length of straight pipe, and that allowance grows with pipe size in Table 3.1. The tool multiplies your elbow count by the per-elbow figure for the chosen D2 and adds it to the actual run, so several bends on a long run can push the equivalent length past the maximum and force the next size up.
Does it cover the tundish and termination rules?
It shows the AD G3 advisory notes, the tundish vertical and within 600 mm below the valve, at least 300 mm of vertical D2 before any bend, a continuous fall of 1 in 200, and a safe visible termination, but the numerical check is the D2 size only. It is design guidance based on copper tube; plastic pipe differs, so verify with a qualified engineer.