Do plumbers offer a written warranty? It varies from property to property hatches should be at least xmm and positioned within . metres of any joint in the flue system the number of hatches you need will depend on how long your flue is. This is an estimate of the time it will take to complete the most relevant course associated with this occupation it’s based on the amount of time taken to complete this course and doesn’t take into account personal circumstances or barriers. Heating and hot.