We have a Bosch gas on demand water heater. In the summer time (mostly) I can't get warm water in the shower in the master bath. When I turn the lever to the cooler side to cool down the hot water, I have warm water for about a minute, then it gets hot again and stays that way until I turn it so far to the cooler side the heater is no longer heating the water. How do I get warm water? I don't have a problem with the kitchen faucet and it is also a single lever faucet. Any help you can give would be appreciated. dpmath (Dan)