My hot water is brown only when I use my back boiler any suggestions

My back boiler feeds a direct feed storage tank which also has an immersion heater, could this cause me to have brown hot water. The back boiler is built into the back of a log burner

Have you thoroughly drained out ALL of the water in the HW heater? A nice build up of brown gunk is your problem inside the tank. Your tank needs to be drained every year or 6 months depending on how bad the build up is.