Can you tell me why the maximum distance for the D1 pipe in Building Regulations G3 is 600mm?

Question from John Potter, via email:
Can you tell me why the maximum distance for the D1 pipe in Building Regulations G3 is 600mm? I work for Barratt Homes and in some of our cylinder cupboards we struggle to achieve this dimension.

Answer from Martyn Bridges, director of marketing and technical support at Worcester, Bosch Group:
The D1 distance is quoted as maximum 600mm in Building Regulations, because with this measurement there is:

• No measurable back pressure exerted on the Pressure Relief Valve
• No likelihood of blockages or restrictions
• The tundish remains in the same vicinity of the cylinder and it is very noticeable if it has discharged

Bear in mind that this is only guidance. However, to prevent questions and issues arising with any Building Control inspection, I would advise it is probably simpler to comply with the guidance.