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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