This is a problem that typically arises during the colder (winter) months when the incoming cold-water temperature starts to drop due to the colder ground temperatures. In most cases the hot water to the house being affected is supplied by a tankless (on demand) hot water system.
Tankless (on demand) hot water heating systems are capable of producing a fixed amount of hot water per minute based on certain incoming cold-water temperatures. To provide a comfortable shower or tub/shower temperature during the colder winter months a larger amount of hot water is needed to compensate for the lower cold-water temperature being mixed in the valve. Also depending on the area, a tankless water heater must raise the cold-water temperature from 40°F to 120°F in the winter months, whereas in the warmer (summer) months the cold-water temperature might be 65°F. This increase in demand for hot water typically outstrips the ability of the water heater to produce enough hot water resulting in a luke-warm shower or tub temperature.
The reason that the bathroom faucet appears to provide adequate hot water is because the faucet contains an aerator that restricts the amount of water that flows between 0.5 gallons and 2.0 gallons per minute. While the Temptrol valve will produce 5 to 7 gallons per minute from the tub spout and 2.5 gallons per minute if equipped with a newer flow-restricted showerhead.
You can conduct a test to see if the overall temperature of your hot water decreases during continuous use. First, turn on the hot water in your bathroom faucet and take a temperature reading with a thermometer. Then while leaving the faucet running, open your Temptrol valve to the hottest position. Let both run for five minutes and monitor the temperature in both. You'll notice that the temperatures may start out quite different; this is due to the Temptrol valve always mixing cold with the hot water. But over time, you'll observe that the hot water coming from both outlets will start to decline in temperature due to the condition described above.
The only solution we can offer to help with this water heater problem is to supply a reduced flow spindle (TA-10-3HL), which cuts down the flow of water from the valve. This may help with the symptoms of the problem but will not correct the underlying cause of the problem.