A grey water heat recovery apparatus has first and second passes arranged in counter-flow orientation. It has a hot side for grey water, and a cold side for fresh water supplied under pressure. It extracts heat from the grey water drains of a building. The fresh water is carried in tubing modules in series immersed in a grey water sump in a cylindrical plastic or stainless steel pipe. The coils have a return leg such that both ends of the fresh water coil are carried out through 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 grey water flow paths. The apparatus has a leak detection circuit and co-operable bypass valves. The entire assembly may be enclosed in a unitary external housing with easily accessible connection fittings.