A gray water heat recovery apparatus has first and second passes in counter-flow orientation. The hot side is gray water. The cold side is fresh water. It extracts heat from the gray water. The fresh water is carried in tubing bundles in series immersed in gray water sumps in a unitary cylindrical plastic, mild steel, stainless steel or copper pipe section that defines multiple flow passages. Both ends of the fresh water bundle assembly extend from the same upper end pipe closure, without a pressurized line wall penetration in the walls of the pipe. There is a non-electrically conductive barrier between the fresh water and gray water flow paths. The apparatus has a leak detection circuit and co-operable bypass valves. The lower manifold has gray water passages between the centering ears. The entire assembly is enclosed in a unitary external housing with axially accessible connection fittings.