Typically, 80-90% of the energy used to heat water in the home goes down the drain and into the sewer system. Drain water heat recovery systems can capture much of this wasted energy.
The two main types of systems are storage and non-storage systems. The storage type system uses a small tank and a copper coil inside the tank. The drain water from the home goes through the tank while the water supply to the home goes through the coil. This is uncommon in homes due to the expense of the system and installation costs.
Non-storage systems are the most common and inexpensive. These systems replace a portion of the home's drain line with a device called a gravity-film heat exchanger. This is a 3 or 4 inch diameter copper pipe with 1/2 inch copper coils attached around the outside. Typically the warm water from a shower will run through the copper pipe. This will form a thin film of warm water on the inside of the pipe.
The cold water supply to the home's hot water heater passes through the copper coils on the outside. Heat is transferred to the cold water and sent to the storage tank. This preheats the water supplied to the water heater and can typically save 30-50% of the cost of heating water for the home. These systems normally cost from $200 to $500 but the payback is usually from 2 to 5 years. (If you have several kids, the payback time can be even less!) Some utility companies now offer incentives to install these systems.
The drain water heat recovery system installation is an easy diy project. A vertical section of drain pipe is removed and the heat exchanger replaces it. They normally are connected using rubber connections and require minimal tools.
The coil can be connected two basic ways. The first way is called balanced flow. In this setup, the main water supply to the home is connected to the coil. This typically is the most efficient at recovering the heat. The main disadvantage of this type of installation is that the cold water to the fixtures in the home is tempered. This means that the cold water coming out of the faucets is not as cold as it would be without the system.
The second method of installation is called unbalanced flow. In this case, the cold water supply line to the water heater is connected to the coil on the drain water heat recovery system. This preheats the water that is supplied to the water heater without affecting the cold water to the rest of the home. These systems not only save energy but they also minimize the water heater's recovery time. This can in effect make the water heater provide more hot water than it is rated for. In some case where the water heater in the home is too small, these systems can help to minimize or even cure the problem.