Delta uses a temperature control system on their 1300/1400 and 1700/1800 series bath and shower taps. This feature prevents accidental scalding in the event the tap handle is accidentally turned too far to the hot water side. Adjusting a Delta spigot temperature depends on which model you have and requires testing the water temperature prior to making any adjustments. Adjusting the temperature does not require turning off the water to access the adjustment dial.
Hook a common household thermometer to the side of a large plastic tumbler or container. Turn on the tap and allow the hot and cold water to mix. Turn the tap to the full hot position and place the container under the water flow. Watch the temperature reading on the thermometer. Ideal readings vary by local plumbing codes but are generally in the 43 to 49 degree C (110 degree to 120 degree F) range.
Remove the set screw securing the handle to the tap with an Allen key, if you have a 1300/1400 series Delta tap. If your tap has a smaller knob on top of the handle, you have a 1700/1800 series. Pry the smaller knob off the top of the tap handle with a flathead screwdriver.
Pull the temperature control dial toward you with your fingers and rotate it counterclockwise to decrease the water temperature, if you have a 1300/1400 series tap. There are teeth that the dial rotates against. Each tooth is approximately a 3.5 degrees C (6 degrees F) temperature difference. If the water is too cool, rotate the control dial clockwise.
Rotate the dial clockwise to decrease the temperature, if you have a 1700/1800 series faucet. For every tooth you move the dial, the temperature changes by 1.1 degrees C. To increase the temperature, rotate the dial counterclockwise.
Snap the smaller temperature control knob back over the handle, if your tap is a 1700/1800 series. Reattach the handle if you have the 1300/1400 series tap. Test the water temperature again and adjust if necessary.