Drain Water Heat Recovery (DWHR)

Benefits

Reduced Energy Consumption

  • Captures some of the heat from wastewater primarily from showers.
  • Reduces demand on energy infrastructure.
  • Provides more hot water for same amount of energy purchased.

Reduced Energy Costs

  • Reduces utility bills.

Reduced Environmental Impact

  • Reduces pollutant emissions associated with fuel-fired water heaters or electricity generation for electric water heaters.
  • Reduces depletion of non-renewable fuel resources.
  • Extends the life of the water heating elements or burner.

Description

  • A Drain Water Heat Recovery System recovers heat from water that would otherwise be lost down the drain. It works best with simultaneous flows like showers, where there is a demand for hot water at the same time as hot water is going down the drain. The recovered heat can be used to preheat cold water going to the hot water tank or for other purposes permitted by codes and regulations.
  • A DWHR unit consists of copper pipe tightly wound around a vertical section of copper drainpipe (Figure 1). As water flows down the drainpipe, it clings to the inside surface of the drainpipe. The heat from the drain water is transferred through the copper drainpipe to fresh cold water flowing in the outer copper coil.
  • The warmed water is then sent either to the hot water tank or other permitted end-use. In either case, the amount of energy needed to provide hot water is reduced. DWHR systems provide greater potential for energy savings as the number of simultaneous flows increases.
  • DWHR units vary in terms of pipe size, orientation of the drain line, heat exchanger design, cost and energy savings achieved.
  • Water heating accounts for 20 – 25% of the total energy consumption in a typical home, DWHR units can reduce hot water requirements for showering by 40 to 60%.

Figure 1 — DWHR Schematic from Technical Research Highlight 07-116 (OPIMS 65680)

The cold water enters the copper coil around the copper drainpipe from the bottom and, as it rises through the coil, it draws heat from the hot water draining from the showers and sinks. The heated water in the coil is directed to the bathroom or kitchen fixtures or to the hot water tank while cooled wastewater drains out to the sewer.

Design/Installation/Operation/Maintenance Considerations

  • DWHR units are available in lengths from 900 to 3050 mm (36 to 120 inches) and two diameters: 75 or 100 mm (3 or 4 inch).
  • A DHWR unit can easily be retrofitted into existing vertical drains or included in new construction projects (Figure 2).
  • According to Natural Resources Canada-sponsored research, one of the most effective models is the 75 mm (3 inch) diameter, 1500 mm (60 inch) long unit.
  • There are generally two ways to plumb the units: directly to the hot water tank, or — where permitted by building codes and regulations — to the shower feed (to preheat the cold water).
  • If showers all drain to one drain pipe, the savings will be more substantial. If showers do not drain to one drain pipe, each downpipe that carries wastewater from a shower could have a DWHR unit installed. Alternately, one unit can be installed on the drain pipe that serves the shower that is used the most.
  • Most cost-effective for households with three or more occupants who use the shower more often than the bathtub.
  • Does not require special equipment or tools for installation.
  • Has no moving parts and no special maintenance requirements.

What Does it Save?

Actual cost savings are dependent on energy costs, location, system configuration, hot water heater type and water usage.

Here is an example of the possible savings a family of four could see with a DWHR unit. The example family lives in a 2-storey house built in 1973. They take four 7 minute showers a day at 41°C. The temperature of the hot water supply is set at 55°C (130°F). The existing hot water tank is in the basement. A 75 mm diameter, 1500 mm long DWHR unit is installed.

If the hot water is supplied by a 78% efficient natural gas hot water tank, this DHWR unit could recover enough heat to provide up to 15% of the overall hot water needs (about 130 m3 of gas).

If the hot water is supplied by a conventional electric hot water tank, this DWHR unit could recover enough heat to provide up to 20% of the overall hot water needs for the household (about 1100 kWh).

The charts below show the annual expected savings in purchased energy for water heating with a DHWR unit plumbed into the cold water feed and the hot water tank, based on 2012 rates for gas (for Vancouver, Calgary, Toronto) and electricity ( for Montreal, Halifax, Whitehorse). The first bar darker bar is the water heating costs without the DWHR unit and the lighter bar is the water heating costs with the DWHR. The estimated cost savings are shown in each case immediately above the bars for each city.

DWHR Annual Energy Cost Saving — Electric Water Heater

DWHR Annual Energy Cost Saving — Electric Water Heater
  Vancouver Calgary Toronto Montreal Halifax Whitehorse
Annual Energy Cost — Pre-Retrofit 395 508 590 317 644 536
Annual Energy Cost — With DWHR 307 376 457 243 487 365
Annual Energy Cost Savings 88 132 132 74 158 171

DWHR Annual Energy Saving — Gas-fired Water Heater

DWHR Annual Energy Cost Saving — Electric Water Heater
  Vancouver Calgary Toronto Montreal
Annual Energy Cost — Pre-Retrofit 302 186 254 370
Annual Energy Cost — With DWHR 246 145 206 289
Annual Energy Cost Savings 56 41 48 81

Figure 2 — Drain Water Heat Recovery System

A drain water heat recovery system.

The information contained in this publication represents current research results available to CMHC. Readers are advised to evaluate the information, materials and techniques cautiously for themselves and to consult appropriate professional resources to determine whether information, materials and techniques are suitable in their case. The text is intended as general information only and project and site-specific factors of climate, cost, aesthetics, practicality, utility and compliance with applicable building codes and standards must be taken into consideration. A number of assumptions were applied with respect to fuel prices, water rates, costs of materials, equipment and labour, planning horizons, etc. Actual reductions in energy consumption and fuel savings will vary. Any reliance or action taken based on the information, materials and techniques described are the responsibility of the user. CMHC accepts no responsibility for consequences arising from the reader’s use of the information, materials and techniques herein.

Last revised: 2013

Canada

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