Drain-Water Heat Recovery
Drain-Water Heat Recovery Saves Energy
Bird's eye view
A simple device saves significant amounts of energy
Most of the energy that's used to heat water goes right down the drain as the water is used. Drain-water heat recovery is a way of capturing heat that would otherwise be wasted — the equivalent of 350 billion kWh of electricity each year in the U.S., according to the U.S. Department of Energy.
Installation costs are not especially high, and a simple system can save 16% or more of the energy used to heat water.
Most systems have no tank
There are two main types of drain-water heat recovery systems. In both cases, warm drain water is used to heat the cold-water supply; where they differ is in their ability to store the heat.
Field studies point to savings of 16% to 30%
Drain-water heat recovery devices are usually 4 to 6 feet long, and must be installed vertically. They can't be installed in a single-story home with a slab foundation, but they work well in a two-story home with upstairs bathrooms or in any home with a basement.
Several studies have looked at potential savings in the energy required to heat water resulting from the installation of a drain-water heat recovery device. The data from three studies show that in a house where occupants prefer showers to baths, the following savings in energy used to heat water can be expected:
[Sources: Energy Design Update, September 1997, September 2000, March 2001, October 2006]
Follow manufacturers' instructions
Many plumbers are unfamiliar with the installation of a drain-water heat recovery device. However, manufacturers of the units provide detailed installation instructions.
While there are no specific code previsions dealing directly with drain-water heat recovery devices, all installations must comply with Water Supply and Distribution (Chapter 29) and Sanitary Drainage (Chapter 30) regulations. Installation must also be in accordance with the manufacturer’s installation instructions.
DRAIN-WATER HEAT RECOVERY
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 exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank. (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:
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