I live in a new house (2 yrs old) and the upstairs master bedroom bathtup and hallway bathroom bathtub have extremely low hot water pressure. The master bedroom bathtub fixture is a "Kohler" two handle (1 cold and 1 hot); the hallway bathroom bathtub fixture is a "Kohler" single handle. When I turn on the cold water in the hallway bathroom bathtub, the water flows great. But once I turn it to hot, I get dribbles of water. Same scenario in the master bedroom bathtub; when I turn the cold water nob I get great water flow. But once I turn the hot water knob, I get very little water flow. The shower in the master bedroom is a "Kohler" single handle and works great; hot or cold water. What do I need to do to fix my proble?:confused:

Your house is a little too new for the crud to be building up in your water heater but something is in your valves. You need to shut down your main and pull the cartridges out to check for debris. I'd drain the water heater while you are doing this work too. Put a hose on the boiler drain and let it run for a few minutes while the supply is still on.

The house has a tankless water heater

Double check the valves on the inlet and the outlet of the tankless unit. Possible someone has bumped into them and partially close one of them. You could disconnect the tank at one of those valves just to verify that you are getting full volume/oressure to and from the unit.