As stated above, depending on the length and size of your service line pulling too much flow really drops pressure. For simplicity sake we’ll assume you have 100’ of 5/8" type k copper feeding your system. 7 GPM yields a loss of ~12.5 psi. If you run another zone concurrently with the same flow 7+7 gpm through that same service line has a loss of ~45 psi!
