Heat recovery ventilation
Encyclopedia
Heat recovery ventilation, also known as HRV, mechanical ventilation heat recovery, or MVHR, is an energy recovery
ventilation
system using equipment known as a heat recovery ventilator, heat exchanger
, air exchanger, or air-to-air heat exchanger which employs a counter-flow
heat exchanger
(countercurrent heat exchange) between the inbound and outbound air flow. HRV provides fresh air and improved climate control, while also saving energy by reducing heating (and cooling) requirements.
Energy recovery ventilators (ERVs) are closely related, however ERVs also transfer the humidity
level of the exhaust air to the intake air.
and weatherstripping
, buildings are intentionally made more airtight, and consequently less well ventilated. Since all buildings require a source of fresh air, the need for HRVs has become obvious. While opening a window does provide ventilation, the building's heat and humidity will then be lost in the winter and gained in the summer, both of which are undesirable for the indoor climate and for energy efficiency, since the building's HVAC
systems must compensate. HRV technology offers an optimal solution: fresh air, better climate control, and Energy efficiency
- Sustainability
.
There are a number of types of heat exchanger that can be used in Heat recovery ventilation-HRV devices:
A high enough incoming air temperature can also be achieved by
This can be done by an earth warming pipe
("ground-coupled heat exchanger"), usually about 30 m to 40 m long and 20 cm in diameter, typically buried about 1.5 m below ground level. In Germany
and Austria
this is a common configuration for earth to air heat exchangers.
In high humidity
areas where internal condensation could lead to fungal / mould growth in the tube leading to contamination
of the air, several measures exist to prevent this.
The pipes may be either corrugated/slotted to enhance heat transfer and provide condensate drainage or smooth/solid to prevent gas/liquid transfer.
which emit radon
. In these situations the tube needs to be airtight from the surrounding soils, or an Air to Water heat exchanger be used.
Likewise, Flueckiger (1999) in a study of twelve Earth-Air Heat Exchangers varying in design, pipe material, size and age, stated, “This study was performed because of concerns of potential microbial growth in the buried pipes of 'ground-coupled' air systems. The results however demonstrate, that no harmful growth occurs and that the airborne concentrations of viable spore
s and bacteria
, with few exceptions, even decreases after passage through the pipe-system”, and further stated, “Based on these investigations the operation of ground-coupled earth-to-air heat exchangers is acceptable as long as regular controls are undertaken and if appropriate cleaning facilities are available”.
In temperate climates in an energy efficient building, such as a passivhaus, this is more than sufficient for comfort cooling during summer without resorting to an airconditioning system. In more extreme hot climates a very small air to air micro-heat pump in reverse (an air conditioner) with the evaporator (giving heat) on the air inlet after the HRV heat exchanger and the condensor (taking heat) from the air outlet after the heat exchanger will suffice.
For example, during the winter, the earth at the depth of the earth to air heat exchanger is ordinarily much warmer than the air temperature. The air becomes warmed by the earth before reaching the air heat exchanger.
In the summer, the opposite is true. The air becomes cooled in the earth to air exchanger. But after passing through the EAHX, the air is warmed by the heat recovery ventilator using the warmth of the outgoing air. In this case, the HRV can have an internal bypass such that the inflowing air bypasses the heat exchanger maximising the cooling potential of the earth.
In autumn and spring there may be no thermal benefit from the EAHX—it may heat/cool the air too much and it will be better to use external air directly. In this case it is helpful to have a bypass such that the EAHX is disconnected and air taken directly from outside. A differential temperature sensor with a motorized valve can control the bypass function.
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Energy recovery
Energy recovery includes any technique or method of minimizing the input of energy to an overall system by the exchange of energy from one sub-system of the overall system with another...
ventilation
Ventilation (architecture)
Ventilating is the process of "changing" or replacing air in any space to provide high indoor air quality...
system using equipment known as a heat recovery ventilator, heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
, air exchanger, or air-to-air heat exchanger which employs a counter-flow
Countercurrent exchange
Countercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or some component, between two flowing bodies flowing in opposite directions to each other. The flowing bodies can be liquids, gases, or...
heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
(countercurrent heat exchange) between the inbound and outbound air flow. HRV provides fresh air and improved climate control, while also saving energy by reducing heating (and cooling) requirements.
Energy recovery ventilators (ERVs) are closely related, however ERVs also transfer the humidity
Humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
level of the exhaust air to the intake air.
Benefits
As building efficiency is improved with insulationBuilding insulation
building insulation refers broadly to any object in a building used as insulation for any purpose. While the majority of insulation in buildings is for thermal purposes, the term also applies to acoustic insulation, fire insulation, and impact insulation...
and weatherstripping
Weatherstripping
Weatherstripping is the process of sealing openings such as doors, windows, and trunks from the elements, or the materials used to carry out such sealing process. The goal of weatherstripping is to prevent rain and water from entering by either blocking it outright or by blocking most of it and...
, buildings are intentionally made more airtight, and consequently less well ventilated. Since all buildings require a source of fresh air, the need for HRVs has become obvious. While opening a window does provide ventilation, the building's heat and humidity will then be lost in the winter and gained in the summer, both of which are undesirable for the indoor climate and for energy efficiency, since the building's HVAC
HVAC
HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...
systems must compensate. HRV technology offers an optimal solution: fresh air, better climate control, and Energy efficiency
Efficient energy use
Efficient energy use, sometimes simply called energy efficiency, is the goal of efforts to reduce the amount of energy required to provide products and services. For example, insulating a home allows a building to use less heating and cooling energy to achieve and maintain a comfortable temperature...
- Sustainability
Sustainability
Sustainability is the capacity to endure. For humans, sustainability is the long-term maintenance of well being, which has environmental, economic, and social dimensions, and encompasses the concept of union, an interdependent relationship and mutual responsible position with all living and non...
.
Technology
Heat recovery ventilation-HRVs and ERVs can be stand-alone devices that operate independently, or they can be built-in, or added to existing HVAC systems. For a small building in which nearly every room has an exterior wall, then the HRV/ERV device can be small and provide ventilation for a single room. A larger building would require either many small units, or a large central unit. The only requirements for the building are an air supply, either directly from an exterior wall or ducted to one, and an energy supply for air circulation, such as wind energy or electricity for a fan. When used with 'central' HVAC systems, then the system would be of the 'forced-air' type.Air to air heat exchanger
- cross flow heat exchanger up to 60% efficient (passive)
- RecuperatorRecuperatorA recuperator is a special purpose counter-flow energy recovery heat exchanger positioned within the supply and exhaust air streams of an air handling system, or in the exhaust gases of an industrial process, in order to recover the waste heat...
, or cross plate heat exchanger, a countercurrentCountercurrent exchangeCountercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or some component, between two flowing bodies flowing in opposite directions to each other. The flowing bodies can be liquids, gases, or...
heat exchanger, as diagrammed to the right - Thermal WheelThermal wheelA Thermal Wheel, also known as a Rotary heat exchanger, or Rotary air-to-air enthalpy wheel, or Heat recovery wheel, is a type of energy recovery heat exchanger positioned within the supply and exhaust air streams of an air handling system, or in the exhaust gases of an industrial process, in order...
, or rotary heat exchanger (requires motor to turn wheel) - Heat pipeHeat pipeA heat pipe or heat pin is a heat-transfer device that combines the principles of both thermal conductivity and phase transition to efficiently manage the transfer of heat between two solid interfaces....
- thin multiple heat wires (Fine wire heat exchanger)
- See also:
- Shell and tube heat exchangerShell and tube heat exchangerA shell and tube heat exchanger is a class of heat exchanger designs. It is the most common type of heat exchanger in oil refineries and other large chemical processes, and is suited for higher-pressure applications. As its name implies, this type of heat exchanger consists of a shell with a...
- Plate heat exchangerPlate heat exchangerA plate heat exchanger is a type of heat exchanger that uses metal plates to transfer heat between two fluids. This has a major advantage over a conventional heat exchanger in that the fluids are exposed to a much larger surface area because the fluids spread out over the plates. This facilitates...
- Plate fin heat exchangerPlate fin heat exchangerA plate-fin heat exchanger is a type of heat exchanger design that uses plates and finned chambers to transfer heat between fluids. It is often categorized as a compact heat exchanger to emphasise its relatively high heat transfer surface area to volume ratio....
- Ground-coupled heat exchangerGround-coupled heat exchangerA ground-coupled heat exchanger is an underground heat exchanger loop that can capture or dissipate heat to or from the ground. They use the Earth's near constant subterranean temperature to warm or cool air or other fluids for residential, agricultural or industrial uses...
- Dynamic scraped surface heat exchanger
- Waste Heat Recovery Unit
- Micro heat exchangerMicro heat exchangerMicro heat exchangers, Micro-scale heat exchangers, or microstructured heat exchangers are heat exchangers in which fluid flows in lateral confinements with typical dimensions below 1 mm...
- Moving bed heat exchangerMoving bed heat exchangerMoving bed heat exchangers are stationary heat exchangers for bulk materials for continuous processes in chemical engineering.-Construction:...
- Shell and tube heat exchanger
Incoming air
The air coming into the heat exchanger should be at least 0°C. Otherwise humidity in the outgoing air may condense, freeze and block the heat exchanger.A high enough incoming air temperature can also be achieved by
- recirculating some of the exhaust air (causing loss of air quality) when required,
- by using a very small (1 kW) heat pumpHeat pumpA heat pump is a machine or device that effectively "moves" thermal energy from one location called the "source," which is at a lower temperature, to another location called the "sink" or "heat sink", which is at a higher temperature. An air conditioner is a particular type of heat pump, but the...
to warm the inlet air above freezing before it enters the HRV. (The 'cold' side of this heatpump is situated in the warm air outlet.) - using a heating "battery" supplied by heat from a heat source eg hot water circuit from a wood fired boiler, etc.
Earth-to-air heat exchanger
Ground-coupled heat exchanger
A ground-coupled heat exchanger is an underground heat exchanger loop that can capture or dissipate heat to or from the ground. They use the Earth's near constant subterranean temperature to warm or cool air or other fluids for residential, agricultural or industrial uses...
("ground-coupled heat exchanger"), usually about 30 m to 40 m long and 20 cm in diameter, typically buried about 1.5 m below ground level. In Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
and Austria
Austria
Austria , officially the Republic of Austria , is a landlocked country of roughly 8.4 million people in Central Europe. It is bordered by the Czech Republic and Germany to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the...
this is a common configuration for earth to air heat exchangers.
In high humidity
Humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
areas where internal condensation could lead to fungal / mould growth in the tube leading to contamination
Contamination
Contamination is the presence of a minor and unwanted constituent in material, physical body, natural environment, at a workplace, etc.-Specifics:"Contamination" also has more specific meanings in science:...
of the air, several measures exist to prevent this.
- Ensuring the tube drains of water.
- Regular cleaning
- Tubes with an imbedded bactericide coating such as silver ions (non-toxic for humans)
- Air filters F7 / EU7 (>0,4 micrometres) to traps mould (of a size between 2 & 20 micrometres).
- UV air purification
- Use a earth to "water" heat exchanger, see below.
The pipes may be either corrugated/slotted to enhance heat transfer and provide condensate drainage or smooth/solid to prevent gas/liquid transfer.
Radon
One critical problem being located in soils with underlying rock strataStratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
which emit radon
Radon
Radon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as the decay product of uranium or thorium. Its most stable isotope, 222Rn, has a half-life of 3.8 days...
. In these situations the tube needs to be airtight from the surrounding soils, or an Air to Water heat exchanger be used.
Bacteria and fungi
Formal research indicates that Earth-Air Heat Exchangers reduce building ventilation air pollution. Rabindra (2004) states, “The Earth-Air Tunnel is found not to support the growth of bacteria and fungi; rather it is found to reduce the quantity of bacteria and fungi thus making the air safer for humans to inhale. It is therefore clear that the use of EAT (Earth-Air Tunnel) not only helps save the energy but also helps reduce the air pollution by reducing bacteria and fungi.”Likewise, Flueckiger (1999) in a study of twelve Earth-Air Heat Exchangers varying in design, pipe material, size and age, stated, “This study was performed because of concerns of potential microbial growth in the buried pipes of 'ground-coupled' air systems. The results however demonstrate, that no harmful growth occurs and that the airborne concentrations of viable spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s and bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, with few exceptions, even decreases after passage through the pipe-system”, and further stated, “Based on these investigations the operation of ground-coupled earth-to-air heat exchangers is acceptable as long as regular controls are undertaken and if appropriate cleaning facilities are available”.
Earth-to-Water heat exchanger
An alternative to the earth to air heat exchanger is the earth to "water" heat exchanger. This is typically similar to a geothermal heat pump tubing embedded horizontally in the soil (or could be a vertical pipe/sonde) to a similar depth of the EAHX. It uses approximately double the length of pipe Ø 35 mm ie around 80 metres compared to an EAHX. A heat exchanger coil is placed before the air inlet of the HRV. Typically a brine liquid (heavily salted water) is used as the heat exchange fluid which is slightly more efficient and environmentally friendly than polypropylene heat transfer liquids.In temperate climates in an energy efficient building, such as a passivhaus, this is more than sufficient for comfort cooling during summer without resorting to an airconditioning system. In more extreme hot climates a very small air to air micro-heat pump in reverse (an air conditioner) with the evaporator (giving heat) on the air inlet after the HRV heat exchanger and the condensor (taking heat) from the air outlet after the heat exchanger will suffice.
Seasonal bypassing
At certain times of the year it is more thermally efficient to bypass the Heat recovery ventilation-HRV heat exchanger or the earth to air heat exchanger (EAHX).For example, during the winter, the earth at the depth of the earth to air heat exchanger is ordinarily much warmer than the air temperature. The air becomes warmed by the earth before reaching the air heat exchanger.
In the summer, the opposite is true. The air becomes cooled in the earth to air exchanger. But after passing through the EAHX, the air is warmed by the heat recovery ventilator using the warmth of the outgoing air. In this case, the HRV can have an internal bypass such that the inflowing air bypasses the heat exchanger maximising the cooling potential of the earth.
In autumn and spring there may be no thermal benefit from the EAHX—it may heat/cool the air too much and it will be better to use external air directly. In this case it is helpful to have a bypass such that the EAHX is disconnected and air taken directly from outside. A differential temperature sensor with a motorized valve can control the bypass function.
See also
- Ground-coupled heat exchangerGround-coupled heat exchangerA ground-coupled heat exchanger is an underground heat exchanger loop that can capture or dissipate heat to or from the ground. They use the Earth's near constant subterranean temperature to warm or cool air or other fluids for residential, agricultural or industrial uses...
- Solar air heatSolar air heatSolar air heating is a solar thermal technology in which the energy from the sun, solar insolation, is captured by an absorbing medium and used to heat air. Solar air heating is a renewable energy heating technology used to heat or condition air for buildings or process heat applications...
- Renewable heatRenewable heatRenewable heat is an application of renewable energy and it refers to the renewable generation of heat, rather than electrical power ....
- Energy recovery ventilation
- Hot water heat recyclingHot water heat recyclingWater heat recycling is the use of a heat exchanger to recover energy and reuse heat from drain water from various activities such as dish-washing, clothes washing and especially showers...
- Seasonal thermal storage
- Passive coolingPassive coolingPassive cooling refers to technologies or design features used to cool buildings without power consumption, such as those technologies discussed in the Passive house project.-Passive cooling:...
- Solar air conditioningSolar air conditioningSolar air conditioning refers to any air conditioning system that uses solar power.This can be done through passive solar, solar thermal energy conversion and photovoltaic conversion . The U.S...
- Air conditioning - Health implications
- HVACHVACHVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...
- Passive housePassive houseThe term passive house refers to the rigorous, voluntary, Passivhaus standard for energy efficiency in a building, reducing its ecological footprint. It results in ultra-low energy buildings that require little energy for space heating or cooling. A similar standard, MINERGIE-P, is used in...
- "Passivhaus" - Low-energy houseLow-energy houseA low-energy house is any type of house that from design, technologies and building products uses less energy, from any source, than a traditional or average contemporary house...
- Low energy building
- List of low-energy building techniques
- Green buildingGreen buildingGreen building refers to a structure and using process that is environmentally responsible and resource-efficient throughout a building's life-cycle: from siting to design, construction, operation, maintenance, renovation, and demolition...
- Zero energy buildingZero energy buildingA zero-energy building, also known as a zero net energy building, Net-Zero Energy Building , or Net Zero Building, is a popular term to describe a building with zero net energy consumption and zero carbon emissions annually. Zero energy buildings can be independent from the energy grid supply...
- SustainabilitySustainabilitySustainability is the capacity to endure. For humans, sustainability is the long-term maintenance of well being, which has environmental, economic, and social dimensions, and encompasses the concept of union, an interdependent relationship and mutual responsible position with all living and non...
- Sustainable architectureSustainable architectureSustainable architecture is a general term that describes environmentally conscious design techniques in the field of architecture. Sustainable architecture is framed by the larger discussion of sustainability and the pressing economic and political issues of our world...
- Sustainable designSustainable designSustainable design is the philosophy of designing physical objects, the built environment, and services to comply with the principles of economic, social, and ecological sustainability.-Intentions:The intention of sustainable design is to "eliminate negative environmental...
Sources
- 1. Rabindra Nath B, Shailendra Kumar M, and Pawan Basnyat; Use of Earth Air Tunnel HVAC system in minimizing indoor air pollution; Air Quality Monitoring and Management, proceedings of Better Air Quality 2004.
- 2. Flueckiger B, Monn C; Microbial investigations and allergen measurements in ground-coupled earth-to-air heat exchangers; AIVC 20th Conference and Indoor Air 99, the 8th International Conference on Indoor Air Quality and Climate, UK, Garston, BRE, 1999, proceedings, Edinburgh, Scotland, 8–13 August 1999, Volume 5.
Non-commercial links
- AIVC Information Paper VIP6 "Air-to-Air Heat Recovery in Ventilation Systems"
- Heat recovery in Industry
- Energy and Heat Recovery Ventilators (ERV/HRV)
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