A modular manifold for a tankless water heater includes a first cavity member and a second cavity member. The first cavity member includes a first opening, a second opening, and a first base wall. The second cavity member is coupled to the first cavity member and includes a first opening, a second opening, and a second base wall. Two of the openings are configured to receive a first conduit and a second conduit. The first and second base walls and the two openings that receive the first and second conduits define a fluid flow path through the modular manifold.