An assembly for a thermostatic mixing valve includes a valve body including a first fluid channel having a first inlet port for receiving a first fluid from a first fluid supply line, a second fluid channel having a second inlet port for receiving a second fluid from a second fluid supply line, and a cartridge chamber having a first layer, a second layer offset from the first layer, and an opening for receiving a thermostatic mixing cartridge. A first removable insert guides the first fluid to the first layer when inserted into the first fluid channel and guides the second fluid to the first layer when inserted into the second fluid channel. A second removable insert guides the first fluid to the second layer when inserted into the first fluid channel and guides the second fluid to the second layer when inserted into the second fluid channel.