My kitchen sink has a peculiar setup I haven't seem before. The faucet has 3 pipes and there is an electric water heater under the sink. The first pipe brings cold water into the faucet. The second pipe bring cold water from the faucet into the water heater. The third pipe brings warm water from the heater back to the faucet. Why can't the cold water be supplied directly to the water heater without the initial trip to the faucet?