Are we talking a thermostatically controlled valve, or just one with a separate temp and volume control? It does make a difference. Depending on the rough-in valve, Delta has three different types of shower valves: pressure balanced single handle, separate temp and volume pressure balanced, and two knob thermostatically controlled. A properly working thermostatically controlled valve should remain pretty consistent temperature. Either of the other two will vary based on the WH outlet temp and the incoming cold water temp. Maybe a call to Delta would help sort out the problem.