Simple systems have speedy payback
The energy department estimates that energy losses in hot water amount to 80% to 90%, a good argument for installing a drain-water heat-recovery (DHR) system.
In a gravity-film heat exchanger (GFX), drain water flows through a copper pipe that is wrapped in smaller diameter copper tubing. Heat from the drain water is transferred to incoming water. Heat exchangers are made from solid copper so they are durable, and there are no moving parts to wear out.
Exchangers without storage. In the simplest type of system, the heat exchanger preheats cold water coming into the house. This arrangement does not include a way of storing the recovered energy so it’s only really effective when hot water is being drawn and drained at the same time — as it would be during a shower, for example.
The most efficient systems are balanced, which means all of the incoming water is piped through the heat exchanger. In one study reported by the energy department, electricity savings were between 800 kWh and 2300 kWh per year. Payback for the $500 investment was in as little as two years. (With recent increases in copper prices, a drain-water heat-recovery device is more likely to cost $800 or $1000 these days.)
Systems with storage tanks. The main disadvantage of non-storage systems is that they won’t recovery energy lost in bathwater or water drained from a washing machine or dishwasher because there’s no incoming water moving through the heat exchanger.
The alternative is expensive: a heat-recovery system with a storage tank. One heat exchanger inside the tank captures heat from the drain water while a second heat exchanger for incoming cold water picks it up.
Manufacturers of drainwater heat recovery devices include: