Originally Posted by Craig
The brass T just below the flame arrestor is the heater shutoff. The plastic T that runs in between the port manifold riser and the t-stat housing came from the PO and has never been connected to anything since I have owned the boat.
The plastic T could possibly have been a cold water supply for a shower. My boat has a similar T, but it is on the hose between the raw water pump and the mixer thermostat housing.