An assembly for a thermostatic mixing valve, the assembly comprising: a valve body including: a first substantially cylindrical fluid channel extending completely through the valve body, the first fluid channel having a first insert opening located on a first surface of the valve body at a first end of the first fluid channel and a first inlet port for receiving a first fluid from a first fluid supply line, the first inlet port located on a second surface of the valve body opposite the first surface at a second end of the first fluid channel; a second substantially cylindrical fluid channel extending completely through the valve body, the second fluid channel having a second insert opening located on the first surface of the valve body at a first end of the second fluid channel and a second inlet port for receiving a second fluid from a second fluid supply line, the second inlet port located on the second surface of the valve body at a second end of the second fluid channel; 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 configured to be insertable into the first fluid channel and the second fluid channel, wherein the 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 configured to be insertable into the first fluid channel and the second fluid channel, wherein the 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; and first and second connection ports (145, 147, 143, 149) located along circumferential faces of each of the first fluid channel and the second fluid channel, wherein the thermostatic mixing cartridge controls an amount of fluid permitted to enter the first layer and the second layer by variably opening and closing the connection ports. a valve body including: a first substantially cylindrical fluid channel extending completely through the valve body, the first fluid channel having a first insert opening located on a first surface of the valve body at a first end of the first fluid channel and a first inlet port for receiving a first fluid from a first fluid supply line, the first inlet port located on a second surface of the valve body opposite the first surface at a second end of the first fluid channel; a second substantially cylindrical fluid channel extending completely through the valve body, the second fluid channel having a second insert opening located on the first surface of the valve body at a first end of the second fluid channel and a second inlet port for receiving a second fluid from a second fluid supply line, the second inlet port located on the second surface of the valve body at a second end of the second fluid channel; 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 substantially cylindrical fluid channel extending completely through the valve body, the first fluid channel having a first insert opening located on a first surface of the valve body at a first end of the first fluid channel and a first inlet port for receiving a first fluid from a first fluid supply line, the first inlet port located on a second surface of the valve body opposite the first surface at a second end of the first fluid channel; a second substantially cylindrical fluid channel extending completely through the valve body, the second fluid channel having a second insert opening located on the first surface of the valve body at a first end of the second fluid channel and a second inlet port for receiving a second fluid from a second fluid supply line, the second inlet port located on the second surface of the valve body at a second end of the second fluid channel; 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 configured to be insertable into the first fluid channel and the second fluid channel, wherein the 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 configured to be insertable into the first fluid channel and the second fluid channel, wherein the 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; and first and second connection ports (145, 147, 143, 149) located along circumferential faces of each of the first fluid channel and the second fluid channel, wherein the thermostatic mixing cartridge controls an amount of fluid permitted to enter the first layer and the second layer by variably opening and closing the connection ports.