A heat trap is a device on a water heater's hot and cold connections that stops heated water from drifting up and out of the tank when no one is using it. Usually a pair of special nipples with an internal ball, it blocks the slow heat loss called convection, cutting standby energy waste.
A heat trap is a small energy-saving device on a water heater, and it solves a problem most people never think about: a tank slowly loses heat even when no hot water is being used. That happens because of convection. Hot water is lighter than cold water, so the heated water in the tank naturally tries to rise up and out through the pipe connections at the top, while cooler water drifts down to replace it. This slow circulation, called thermosiphoning, carries heat out of the tank and up into the pipes, where it is wasted. A heat trap blocks that flow.
Most heat traps take the form of a pair of special fittings called heat-trap nipples, screwed into the hot outlet and the cold inlet on top of the tank. Inside each is a small, light ball or flap. When water is standing still, the ball settles against the opening and seals it, so heated water cannot creep up and out. The moment you open a tap and water actually flows, the pressure pushes the ball out of the way and water passes through normally. The hot side and cold side use slightly different balls so each seals in the right direction.
The payoff is lower standby heat loss, the energy a heater wastes just sitting there keeping water warm between uses, the same waste that insulating hot water pipes also targets. A heat trap can cut that convection loss substantially, which shows up as a modest but steady saving on the energy bill over the life of the heater. For example, a family that draws hot water a few times a day still leaves the tank idle for many hours, and a heat trap quietly protects the stored heat during all of those idle stretches.
Many modern water heaters come with heat-trap nipples installed at the factory, so there is nothing extra to add. On an older unit without them, they can often be added when the heater is serviced. A side benefit is that heat-trap nipples are usually built to also separate dissimilar metals, which helps guard the tank against the galvanic corrosion that happens when copper pipe meets a steel tank.