We have two upstairs baths and lately, neither are getting hot water. The water pressure is fine, but the temperature will only get warm. Both showers have a separate hot and cold handle as do the sinks. The strange thing is that sometimes the water will be hot, but most times not. Our house was built in 1994 and has copper piping. Water heater is the original one that came with the house in '94. Is this a water heater issue (bad bypass valve?) or a pipe issue?