We often don’t give much thought to the heat energy that goes down the drain while we’re taking a shower or washing dishes. Typically, we spend money to heat our water, use the hot water for washing, and discard it while it’s still hot, sending a lot of heat energy that we’ve paid for out to the onsite sewage disposal system, or municipal sewers. A drain water heat recovery system can capture a significant portion of that heat and return it to the water heater to be used again.

Drain water heat recovery systems consist of a large copper drain pipe that is part of the main household drain, and a coil of smaller copper pipes that wind around the outside of the larger drain. Hot water from the house drains through the large copper pipe, naturally flowing down the inner surface. Clean cold water from the mains or well runs up the smaller coil, and heat is transferred from the large pipe to the coil. The clean water then returns the heat to the water heater. This means that the water heater does not have to use as much energy to heat more water for household use.

Manufacturers claim that drain water heat recovery systems can reclaim a third of the heat from discarded hot water, lowering the costs of heating water by a third. This technology works best when:

• It can be installed in a vertical drain pipe (thus reclaiming heat from all around the copper pipe rather than just the bottom surface. These systems don’t work as well in homes built on a slab because of this.)
• The drains for showers, baths, and the kitchen are all fairly close to the unit.
• The water heater is fairly close to the unit.

In our home, the kitchen is at one end of the house and both bathrooms are at the other. The water heaters (electric and solar) are in the middle at the front, and the main drain is at the back. This is far from an ideal situation for a drain water heat recovery system, but at least we can comfort ourselves that heat lost from pipes running inside the house stays inside the house and will contribute slightly to home heating during the colder months.