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Drain-water heat recovery


After space heating, water heating is the second most costly energy demand in homes, accounting for 20-30% of energy consumption. Ninety percent (90%) of the energy used to heat water in a home is normally wasted out to the sewer. Unfortunately, drain-water (or waste water) has largely been ignored as a valuable resource.

Drain-water heat recovery systems can re-capture some, or most, of this valuable energy and use it to preheat cold, fresh water. By preheating cold water, drain-water heat recovery systems help increase water heating capacity by up to 3 times. This energy greatly reduces the cost of heating water.  Most units available consist of a single tube at a time wrapped around a portion of the ABS drainpipe replaced by copper. To comply with ENERGY STAR®, the units must have less than a 2psi pressure drop with one shower running and must be constructed of at least two parallel coils.


Drain water heat recovery explained

As warm water flows down the drainpipe, incoming cold water flows through the outer copper tubes. Heat transfer is very good between the tubes and the pipe because the contact is very tight.  Because copper is extremely conductive, the heat energy readily passes from the warm inner drain-water to the cold, outer, fresh water.


As water falls vertically it swirls down the pipe, a continuation of the spiral you see as water goes down the drain. The  drain-water clings to the inside pipe wall as it spirals down creating a thin “falling-film” which readily transfers its heat to the pipe wall.  Because copper is used for both the inner pipe and the outer tube, and because the contact between the two is tight, heat transfer rate that is 4-6 times greater than other classes of heat exchanger on a surface area basis.

A 60 inch unit can bring the cold water temperature up from 10 degrees C to as much as 24 degrees C, under equal flow conditions. .

Cost and Savings

Drain water heat recovery technologies work well with all types of primary water heaters and typically save 25%-40% on water heating in homes, depending upon the efficiency of the unit. The most common units are very efficient with your biggest hot water load: the shower and dishwasher.


The total installed cost for a Drain water heat recovery unit is typically between $600 and $1,000 CDN. Factors affecting the efficiency of Drain water heat recovery include the efficiency of the unit, how the freshwater is plumbed, and water use habits.

Correctly installed drain water heat recovery units with a high efficiency typically pay for themselves in 2 to 6 years. The payback is usually the same regardless of length of the heat capture unit.  i.e. cost and savings are proportional however results depend upon the amount and habits of hot water use.

Drain water heat recovery completely passive; there are no moving parts or heating elements, there's nothing to wear out. The unit should typically last for 50 years or more. Its self-cleaning design assures maintenance-free operation. It will fit into almost any house, replacing a section of drainpipe usually in the basement.

Environmental benefits

The Drain water heat recovery unit will reduce Green House Gas emissions by approximately 200kg/person/year when displacing Natural Gas water heating. In a 4 person home, this calculates to one-tonne annually.  Green House Gas emission savings impact can be much more with electrically heated hot water because of peak load reductions. Home owners with time-of-use electricity metering may see significant reductions in their cost to heat water.

Across all sectors in the Canadian Economy it has been conservatively estimated that Drain water heat recovery can reduce Green House Gas emissions by 10 Mega Tonnes per year.



 

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Last modified: November 13, 2018