My company installed a number of balancing valves (Tour & Andersson, STAD) in a central heating system. These valves are used to measure the flow (in this case water). According to the valve's specs it should be installed with a certain distance of straight pipe both before and after the valve in order for the measurement to be correct. The only problem is that they aren't installed according to this. One valve has a 90° almost immediately on each side. I'm wondering if someone has any idea how much this might affect the measurement of flow?