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Drain water heat recovery (DWHR) is a relatively simple technology to reuse household hot water energy and to prolong the availability of hot water during periods of high demand or continuous use.

In 2007, CMHC completed a study (07-116 Canada Mortgage and Housing Corporation, 2007) on this technology. CMHC explains that DWHR units take advantage of the fact that as water drains it clings to the sides of vertical drainpipes due to surface tension. This creates a very high surface-contact-to-volume ratio, allowing heat to be recovered from the drain water by wrapping the incoming cold water supply pipe around the vertical drain.

Figure 1 – CMHC drawing of Drain Water Heat Recovery System

In most homes, drain water will come in three types:

• Hot – from dishwashers, showers, bath tubs
• Cold – from toilets
• Hot or cold – from clothes washers and sinks

The steadiest source of  hot drain water will come from showers, as the hot water is continuously entering the drain for the duration of the shower. The other important feature to note about drain water from showers is that this drain water is produced simultaneously with hot water requirements, since the shower is using hot water at the same time as the DWHR system is reusing the energy from the hot water going down the drain.

The study evaluated two different configurations. In the first configuration (A), the DWHR system was connected to the hot water heater. In the second configuration (B), the DWHR system was connected to the hot water heater and the cold water tap on the shower.

The savings for a typical family of 4 was significant. The following is a subset of the results of the study and shows the savings for the most effective DWHR unit tested (PowerPipe R60) in kWh/year.

	Configuration
	A	B
PowerPipe R60	1,145 kWh/year	1,385 kWh/year

Figure 2 Annual savings in kWh/year

The savings are significant, given the relatively low cost of installing a DWHR, its long useful life, and the fact that it will require no maintenance once installed.

CMHC tests suggested (07-116 Canada Mortgage and Housing Corporation, 2007) that the benefits from delayed hot water requirements was minimal. The study came to several interesting conclusions:

• ‘Although the devices are very similar, the performance of comparable units can vary widely based on the way in which the soft copper tube is shaped and then wrapped around the drainpipe section.

• The efficiency and effectiveness of DWHR units is lifestyle dependent. Households with high shower use will obtain more benefit from installing a DWHR unit than households where baths are more prevalent.

• Households in rural areas without access to a municipal water supply will need to look at units that have designs that minimize reductions in water line pressure.’

A DWHR system is a good investment. The energy savings are significant and the DWHR has a long useful life with little or no maintenance. The system will extend the availability of hot water during periods of prolonged use, allowing the home owner to install a smaller water heater.