![Hot water heat recycling unit](https://upload.wikimedia.org/wikipedia/commons/3/37/Hot_water_heat_recycling_unit.jpg)
Water heat recycling (also known as drain water heat recovery, waste water heat recovery, greywater heat recovery, or sometimes shower water heat recovery) is the use of a
heat exchanger
A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...
to
recover energy and reuse heat from drain water from various activities such as dish-washing, clothes washing and especially showers. The technology is used to reduce
primary energy
Primary energy (PE) is an energy form found in nature that has not been subjected to any human engineered conversion process. It is energy contained in raw fuels, and other forms of energy, including waste, received as input to a system. Pr ...
consumption for
water heating
Water heating is a heat transfer process that uses an energy source to heat water above its initial temperature. Typical domestic uses of hot water include cooking, cleaning, bathing, and space heating. In industry, hot water and water heated t ...
.
How it works
![Greywater Recycling Heat Recovery System 1](https://upload.wikimedia.org/wikipedia/commons/2/2e/Greywater_Recycling_Heat_Recovery_System_1.jpg)
The cold water that is put into a water heating device can be preheated using the reclaimed thermal energy from a shower so that the input water does not need as much energy to be heated before being used in a shower, dishwasher, or sink. The water entering a storage tank is usually close to 11 °C but by recovering the energy in the hot water from a bath or dishwasher, the temperature of the water entering the holding tank can be elevated to 25 °C, saving energy required to increase the temperature of a given amount of water by 14 °C. This water is then heated up a little further to 37 °C before leaving the tank and going to the average shower.
When recycling water from a bath (100-150 litres) or shower (50-80 litres) the waste water temperature is circa 20-25 °C. An in-house
greywater
Greywater (or grey water, sullage, also spelled gray water in the United States) refers to domestic wastewater generated in households or office buildings from streams without fecal contamination, i.e., all streams except for the wastewater from ...
recycling tank holds 150-175 litres allowing for the majority of waste water to be stored. Utilizing a built in copper heat exchange with circulation pump the residual heat is recovered and transferred to the cold feed of a combi-boiler or hot-water cylinder, reducing the energy used by the existing central heating system to heat water.
Impact and cost
Heating water accounts for 18% of the average household utility bill. Standard units save up to 60% of the heat energy that is otherwise lost down the drain when using the shower.
Installing a water heat recycler reduces energy consumption and thus greenhouse gas emissions and the overall energy dependency of the household.
Typical retail price for a domestic drain water heat recovery unit ranges from around $400 to $1,000 Canadian. For a regular household, water heating is usually about 20% of overall energy demand. The energy savings can result in an average payback time for the initial investment of 2–10 years.
A 2-year independent study of waste water heat recovery systems installed into residential houses in the UK found savings of 380kWh and 500kWh per person per year.
AIMC4 study by Barratt Developments PLC, Stewart Milne Homes, and Crest Nicholson.
Industrial scale and HVAC
A
heat pump
A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using a refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing h ...
can be combined with municipal sewage lines to allow a large building's
HVAC
Heating, ventilation, and air conditioning (HVAC) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. HV ...
system recycle the winter heat or summer cool (compared to the outside air) of water flowing out of many homes and businesses.
The reverse is also possible: heat from air conditioning and industrial
chiller
A chiller is a machine that removes heat from a liquid coolant via a vapor-compression refrigeration, vapor-compression, adsorption refrigeration, or absorption refrigerator, absorption refrigeration cycles. This liquid can then be circulated th ...
s can be used to pre-heat water.
Heat rejected by a chiller system for providing air-conditioning to larger buildings can be recovered by installing a heat-exchanger between the incoming domestic cold water, and condenser water return.
A conventional chilled water system rejects heat gathered by the condenser water loop from the refrigerant to a cooling tower.
By diverting a fraction of mass flow rate of condenser water away from the cooling tower, and circulating it through a heat-exchanger (usually a plate-and-frame configuration), incoming domestic cold water can be pre-heated before reaching the boiler. This reduces the required increase in temperature of the water before it can be supplied to the end user, and therefore lowering boiler fuel burn.
See also
{{Portal, Energy
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Heat recovery ventilation
Heat recovery ventilation (HRV), also known as mechanical ventilation heat recovery (MVHR), is an energy recovery ventilation system which works between two air sources at different temperatures. Heat recovery is a method which is used to reduce ...
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Green building
Green building (also known as green construction or sustainable building) refers to both a structure and the application of processes that are environmentally responsible and resource-efficient throughout a building's life-cycle: from planni ...
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Renewable heat Renewable heat is an application of renewable energy referring to the generation of heat from renewable sources; for example, feeding radiators with water warmed by focused solar radiation rather than by a fossil fuel boiler. Renewable heat technol ...
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Zero energy building
A Zero Energy Building (ZEB), also known as a Net Zero Energy (NZE) building, is a building with net zero energy consumption, meaning the total amount of energy used by the building on an annual basis is equal to the amount of renewable energy c ...
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Low-energy building
A low-energy house is characterized by an energy-efficient design and technical features which enable it to provide high living standards and comfort with low energy consumption and carbon emissions. Traditional heating and active cooling systems ...
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Pinch analysis
Pinch analysis is a methodology for minimising energy consumption of process (engineering), chemical processes by calculating thermodynamically feasible ''energy targets'' (or minimum energy consumption) and achieving them by optimising heat reco ...
References
Building
Low-energy building
Energy harvesting
Heating, ventilation, and air conditioning