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Heat recovery ventilation

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Ventilation unit with heat pump & ground heat exchanger - cooling

Heat recovery ventilation (HRV), also known as mechanical ventilation heat recovery (MVHR), is an energy recovery ventilation system that operates between two air sources at different temperatures. It's a method that is used to reduce the heating and cooling demands of buildings. By recovering the residual heat in the exhaust gas, the fresh air introduced into the air conditioning system is preheated (or pre-cooled), and the fresh air's enthalpy is reduced before it enters the room, or the air cooler of the air conditioning unit performs heat and moisture treatment.[1] A typical heat recovery system in buildings comprises a core unit, channels for fresh and exhaust air, and blower fans. Building exhaust air is used as either a heat source or heat sink, depending on the climate conditions, time of year, and requirements of the building. Heat recovery systems typically recover about 60–95% of the heat in the exhaust air and have significantly improved the energy efficiency of buildings.[2]

Working principle

A heat recovery system is designed to supply conditioned air to the occupied space to maintain a certain temperature.[3] A heat recovery system helps keep a house ventilated while recovering heat being emitted from the inside environment. The purpose of heat recovery systems is to transfer the thermal energy from one fluid to another fluid, from one fluid to a solid, or from a solid surface to a fluid at different temperatures and in thermal contact. There is no direct interaction between fluid and fluid or fluid and solid in most heat recovery systems. In some heat recovery systems, fluid leakage is observed due to pressure differences between fluids, resulting in a mixture of the two fluids.[4]

Types

Rotary thermal wheels

Rotary thermal wheels are a mechanical means of heat recovery. A rotating porous metallic wheel transfers thermal energy from one air stream to another by passing through each fluid alternately. The system operates by working as a thermal storage mass whereby the heat from the air is temporarily stored within the wheel matrix until it is transferred to the cooler air stream.[2]

Two types of rotary thermal wheels exist: heat wheels and enthalpy (desiccant) wheels. Though there is a geometrical similarity between heat and enthalpy wheels, there are differences that affect the operation of each design. In a system utilizing a desiccant wheel, the moisture in the air stream with the highest relative humidity is transferred to the opposite air stream after flowing through the wheel. This can work in both directions of incoming air to exhaust air and exhaust air to incoming air. The supply air can then be used directly or employed to further cool the air. This is an energy-intensive process.[5]

Fixed plate heat exchangers

Fixed plate heat exchangers are the most commonly used type of heat exchanger and have been developed for 40 years. Thin metal plates are stacked with a small spacing between plates. Two different air streams pass through these spaces, adjacent to each other. Heat transfer occurs as the temperature transfers through the plate from one air stream to the other. The efficiency of these devices has reached of 90% sensible heat efficiency in transferring sensible heat from one air stream to another.[6] The high levels of efficiency are attributed to the high heat transfer coefficients of the materials used, operational pressure and temperature range.[2]

Heat pipes

Heat pipes are a heat recovery device that uses a multi-phase process to transfer heat from one air stream to another.[2] Heat is transferred using an evaporator and condenser within a wicked, sealed pipe containing a fluid which undergoes a constant phase change to transfer heat. The fluid within the pipes changes from a fluid to a gas in the evaporator section, absorbing the thermal energy from the warm air stream. The gas condenses back to a fluid in the condenser section where the thermal energy is dissipated into the cooler air stream raising the temperature. The fluid/gas is transported from one side of the heat pipe to the other through pressure, wick forces or gravity, depending on the arrangement of the heat pipe.

Types of heat exchangers.[citation needed]

Run-around

Run-around systems are hybrid heat recovery system that incorporates characteristics from other heat recovery technology to form a single device, capable of recovering heat from one air stream and delivering to another a significant distance away. The general case of run-around heat recovery, two fixed plate heat exchangers are located in two separate air streams and are linked by a closed loop containing a fluid that is continually pumped between the two heat exchangers. The fluid is heated and cooled constantly as it flows around the loop, providing heat recovery. The constant flow of the fluid through the loop requires pumps to move between the two heat exchangers. Though this is an additional energy demand, using pumps to circulate fluid is less energy intensive than fans to circulate air.[7]

Phase change materials

Phase change materials, or PCMs, are a technology that is used to store sensible and latent heat within a building structure at a higher storage capacity than standard building materials. PCMs have been studied extensively due to their ability to store heat and transfer heating and cooling demands from conventional peak times to off-peak times.

The concept of the thermal mass of a building for heat storage, that the physical structure of the building absorbs heat to help cool the air, has long been understood and investigated. A study of PCMs in comparison to traditional building materials has shown that the thermal storage capacity of PCMs is twelve times higher than standard building materials over the same temperature range.[8] The pressure drop across PCMs has not been investigated to be able to comment on the effect that the material may have on air streams. However, as the PCM can be incorporated directly into the building structure, this would not affect the flow in the same way other heat exchanger technologies do, it can be suggested that there is no pressure loss created by the inclusion of PCMs in the building fabric.[9]

Applications

Heat recovery ventilation with an earth-to-air heat exchanger, which is essential to achieve German Passivhaus standards.

Rotary thermal wheel

O’Connor et al.[10] studied the effect that a rotary thermal wheel has on the supply air flow rates into a building. A computational model was created to simulate the effects of a rotary thermal wheel on air flow rates when incorporated into a commercial wind tower system. The simulation was validated with a scale model experiment in a closed-loop subsonic wind tunnel. The data obtained from both tests were compared in order to analyze the flow rates. Although the flow rates were reduced compared to a wind tower which did not include a rotary thermal wheel, the guideline ventilation rates for occupants in a school or office building were met above an external wind speed of 3 m/s, which is lower than the average wind speed of the UK (4–5 m/s).

No full-scale experimental or field test data was completed in this study, therefore it cannot be conclusively proved that rotary thermal wheels are feasible for integration into a commercial wind tower system. However, despite the air flow rate decrease within the building after the introduction of the rotary thermal wheel, the reduction was not large enough to prevent the ventilation guideline rates from being met. Sufficient research has not yet been conducted to determine the suitability of rotary thermal wheels in natural ventilation, ventilation supply rates can be met but the thermal capabilities of the rotary thermal wheel have not yet been investigated. Further work would beneficial to increase understanding of the system.[9]

Fixed plate heat exchangers

Plate ground heat exchanger inside the foundation walls

Mardiana et al.[11] integrated a fixed plate heat exchanger into a commercial wind tower, highlighting the advantages of this type of system as a means of zero energy ventilation which can be simply modified. Full scale laboratory testing was undertaken in order to determine the effects and efficiency of the combined system. A wind tower was integrated with a fixed plate heat exchanger and was mounted centrally in a sealed test room.

The results from this study indicate that the combination of a wind tower passive ventilation system and a fixed plate heat recovery device could provide an effective combined technology to recover waste heat from exhaust air and cool incoming warm air with zero energy demand. Though no quantitative data for the ventilation rates within the test room was provided, it can be assumed that due to the high-pressure loss across the heat exchanger that these were significantly reduced from the standard operation of a wind tower. Further investigation of this combined technology is essential in understanding the air flow characteristics of the system.[9]

Heat pipes

Due to the low-pressure loss of heat pipe systems, more research has been conducted into the integration of this technology into passive ventilation than other heat recovery systems. Commercial wind towers were again used as the passive ventilation system for integrating this heat recovery technology. This further enhances the suggestion that commercial wind towers provide a worthwhile alternative to mechanical ventilation, capable of supplying and exhausting air at the same time.[9]

Run-around systems

Flaga-Maryanczyk et al.[12] conducted a study in Sweden which examined a passive ventilation system which integrated a run-around system using a ground source heat pump as the heat source to warm incoming air. Experimental measurements and weather data were taken from the passive house used in the study. A CFD model of the passive house was created with the measurements taken from the sensors and weather station used as input data. The model was run to calculate the effectiveness of the run-around system and the capabilities of the ground source heat pump.

Ground source heat pumps provide a reliable source of consistent thermal energy when buried 10–20 m below the ground surface. The ground temperature is warmer than the ambient air in winter and cooler than the ambient air in summer, providing both a heat source and a heat sink. It was found that in February, the coldest month in the climate, the ground source heat pump was capable of delivering almost 25% of the heating needs of the house and occupants.[9]

Phase change materials

The majority of research interest in PCMs is the application of phase change material integration into traditional porous building materials such as concrete and wall boards. Kosny et al.[13] analyzed the thermal performance of buildings that have PCM-enhanced construction materials within the structure. Analysis showed that the addition of PCMs is beneficial in terms of improving thermal performance.

A significant drawback of PCM used in a passive ventilation system for heat recovery is the lack of instantaneous heat transfer across different airstreams. Phase change materials are a heat storage technology, whereby the heat is stored within the PCM until the air temperature has fallen to a significant level where it can be released back into the air stream. No research has been conducted into the use of PCMs between two airstreams of different temperatures where continuous, instantaneous heat transfer can occur. An investigation into this area would be beneficial for passive ventilation heat recovery research.[9]

Advantages and disadvantages[9]

Type of HRV Advantages Disadvantages Performance Parameters Efficiency  % Pressure Drop (Pa) Humidity Control
Rotary thermal wheel High efficiency

Sensible and latent heat recovery

Compact design

Frost control available

Cross contamination possible Requires adjacent airstreams

Mechanically driven, requiring energy input

Rotation speed

Air velocity

Wheel Porosity

80+ 4-45 Yes
Fixed plate No moving parts hence high reliability

High heat transfer coefficient

No cross contamination

Frost control possible

Sensible and latent heat recovery

High pressure loss across exchanger

Limited to two separate air streams

Condensation build up

Frost building up in cold climates

Material type

Operating pressure

Temperature

Flow arrangement

70-90 7-30 Yes
Heat pipes No moving parts, high reliability

No cross contamination

Low pressure loss

Compact design

Heat recovery in two directions possible

Requires close air streams

Internal fluid should match local climate conditions

Fluid type

Contact time

Arrangement/configuration

Structure

80 1-5 No
Run-around Airstreams can be separate

No cross contamination

Low pressure loss

Multiple sources of heat recovery

Multiple pumps required to move fluid

Difficult to integrate into existing structures

Low efficiency

Cost

Exchanger type

Fluid type

Heat source

50-80 ~1 No
Phase change materials Easy incorporation into building materials

Offset peak energy demands

No pressure loss

No cross contamination

No moving parts

Long life cycle

Thermal storage as opposed to instantaneous transfer

Expensive

Not proven technology

Difficulty in selecting appropriate material

Impregnation method ~ 0 No

Environmental impacts[14]

Energy saving is one of the key issues for both fossil fuel consumption and the protection of the global environment. The rising cost of energy and global warming underlined that developing improved energy systems is necessary to increase energy efficiency while reducing greenhouse gas emissions. One of the most effective ways to reduce energy demand is to use energy more efficiently. Therefore, waste heat recovery is becoming popular in recent years since it improves energy efficiency. About 26% of industrial energy is still wasted as hot gas or fluid in many countries.[15] However, during last two decades there has been remarkable attention to recover waste heat from various industries and to optimize the units which are used to absorb heat from waste gases. Thus, these attempts enhance reducing of global warming as well as of energy demand.

Energy consumption

In most industrialized countries, HVAC is responsible for one-third of the total energy consumption. Moreover, cooling and dehumidifying fresh ventilation air compose 20–40% of the total energy load for HVAC in hot and humid climatic regions. However, that percentage can be higher where 100% fresh air ventilation is required. This means more energy is needed to meet the fresh air requirements of the occupants. Heat recovery is becoming more necessary due to an increased energy cost for the treatment of fresh air. The main purpose of heat recovery systems is to mitigate the energy consumption of buildings for heating, cooling, and ventilation by recovering the waste heat. In this regard, stand-alone or combined heat recovery systems can be incorporated into residential or commercial buildings for energy saving. Reduction in energy consumption levels can also notably contribute in reducing greenhouse gas emissions.[citation needed]

Greenhouse gases

CO2, N2O and CH4 are common greenhouse gases and CO2 is the largest contributor to climate change. Therefore, the greenhouse gas emissions are frequently denoted as CO2 equivalent emissions. Total global greenhouse gas emissions increased 12.7% between 2000 and 2005. In 2005, around 8.3 Gt CO2 was released by the building sector. Moreover, buildings are responsible for more than 30% of greenhouse gas emissions each year in most developed countries. According to another study, buildings in European Union countries cause about 50% of the CO2 emissions in the atmosphere. It is possible to mitigate greenhouse gas emissions by 70% compared to the levels expected to be seen in 2030 if the proper measures are taken. The increase in greenhouse gas emissions due to the high demand for energy use concluded as global warming. In this regard, mitigating gas emissions in the atmosphere stands out as one of the most crucial problems of the world today that should be resolved. Heat recovery systems have a remarkable potential to contribute to decreasing greenhouse gas emissions by reducing the energy required to heat and cool buildings. The Scotch Whisky Association has carried out a project at Glenmorangie distillery to recover latent heat from new wash stills to heat other process waters. They have found that 175 t a year of CO2 will be saved with a payback period of under one year. In another report, it is underlined that 10 MW of recovered heat can be utilized for saving €350,000 per year in emission costs. UK Climate Change Act of 2008 is targeting a 34% reduction in greenhouse gas emissions by 2020 compared with 1990 levels and an 80% reduction by 2050. They emphasize the notable potential and importance of heat recovery technologies to achieve this goal.

See also

counter-flow cross-plate energy recovery heat exchanger

Energy recovery ventilation (ERV) is the energy recovery process in residential and commercial HVAC systems that exchanges the energy contained in normally exhausted air of a building or conditioned space, using it to treat (precondition) the incoming outdoor ventilation air. The specific equipment involved may be called an Energy Recovery Ventilator, also commonly referred to simply as an ERV.

During the warmer seasons, an ERV system pre-cools and dehumidifies; during cooler seasons the system humidifies and pre-heats.[16] An ERV system helps HVAC design meet ventilation and energy standards (e.g., ASHRAE), improves indoor air quality and reduces total HVAC equipment capacity, thereby reducing energy consumption.

An ERV system helps HVAC systems design meet ventilation and energy standards (e.g., ASHRAE), improves indoor air quality, and reduces total HVAC equipment loads, thereby reducing energy consumption.

ERV systems enable an HVAC system to maintain a 40-50% indoor relative humidity, essentially in all conditions. ERV's must use power for a blower to overcome the pressure drop in the system, hence incurring a slight energy demand.[16]

Importance

Nearly half of global energy is used in buildings,[17] and half of heating/cooling cost is caused by ventilation when it is done by the "open window" method [definition needed] according to the regulations[citation needed]. Secondly, energy generation and grid is made to meet the peak demand of power. To use proper ventilation; recovery is a cost-efficient, sustainable and quick way to reduce global energy consumption and give better indoor air quality (IAQ) and protect buildings, and environment.

Methods of transfer

An ERV is a type of air-to-air heat exchanger that transfers sensible heat as well as latent heat. Because both temperature and moisture are transferred, ERVs are described as total enthalpic devices. In contrast, a heat recovery ventilator (HRV) can only transfer sensible heat. HRVs can be considered sensible only devices because they only exchange sensible heat. In other words, all ERVs are HRVs, but not all HRVs are ERVs. It is incorrect to use the terms HRV, AAHX (air-to-air heat exchanger), and ERV interchangeably.[18]

During the cooling season, the system works to cool and dehumidify the incoming, outside air. To do this, the system takes the rejected heat and sends it into the exhaust airstream. Subsequently, this air cools the condenser coil at a lower temperature than if the rejected heat had not entered the exhaust airstream. During the heating seasons, the system works in reverse. Instead of discharging the heat into the exhaust airstream, the system draws heat from the exhaust airstream in order to pre-heat the incoming air. At this stage, the air passes through a primary unit and then into the space being conditioned. With this type of system, it is normal during the cooling seasons for the exhaust air to be cooler than the ventilation air and, during the heating seasons, warmer than the ventilation air. It is for this reason the system works efficiently and effectively. The coefficient of performance (COP) will increase as the conditions become more extreme (i.e., more hot and humid for cooling and colder for heating).[19]

Efficiency

The efficiency of an ERV system is the ratio of energy transferred between the two air streams compared with the total energy transported through the heat exchanger.[20][21]

With the variety of products on the market, efficiency will vary as well. Some of these systems have been known to have heat exchange efficiencies as high as 70-80% while others have as low as 50%. Even though this lower figure is preferable to the basic HVAC system, it is not up to par with the rest of its class. Studies are being done to increase the heat transfer efficiency to 90%.[20]

The use of modern low-cost gas-phase heat exchanger technology will allow for significant improvements in efficiency. The use of high conductivity porous material is believed to produce an exchange effectiveness in excess of 90%, producing a five times improvement in energy recovery.[20]

The Home Ventilating Institute (HVI) has developed a standard test for any and all units manufactured within the United States. Regardless, not all have been tested. It is imperative to investigate efficiency claims, comparing data produced by HVI as well as that produced by the manufacturer. (Note: all units sold in Canada are placed through the R-2000 program, a standard test equivalent to the HVI test).[21]

Types of energy recovery devices

Energy recovery device Type of transfer
Rotary enthalpy wheel Total & sensible
Fixed plate Total** & sensible
Heat pipe Sensible
Run around coil Sensible
Thermosiphon Sensible
Twin towers[22] Sensible

**Total energy exchange only available on hygroscopic units and condensate return units

Rotary air-to-air enthalpy wheel

Main article: Thermal wheel

The rotating wheel heat exchanger is a rotating cylinder filled with an air permeable material, typically polymer, aluminum, or synthetic fiber, providing the large surface area required for the sensible enthalpy transfer. (Enthalpy is a measure of heat.) As the wheel rotates between the supply and exhaust air streams it picks up heat energy and releases it into the colder air stream. The driving force behind the exchange is the difference in temperatures between the opposing air streams (the thermal gradient).

The enthalpy exchange is accomplished through the use of desiccants. Desiccants transfer moisture through the process of adsorption which is predominately driven by the difference in the partial pressure of vapor within the opposing air-streams. Typical desiccants consist of silica gel, and molecular sieves.

Enthalpy wheels are the most effective devices to transfer both latent and sensible heat energy. Choice of construction materials for the rotor, most commonly polymer, aluminum, or fiberglass, determines durability.

When using rotary energy recovery devices the two air streams must be adjacent to one another to allow for the local transfer of energy. Also, there should be special considerations paid in colder climates to avoid wheel frosting. Systems can avoid frosting by modulating wheel speed, preheating the air, or stop/jogging the system.

Plate heat exchanger

Main article: Recuperator

Fixed plate heat exchangers have no moving parts, and consist of alternating layers of plates that are separated and sealed. Typical flow is cross current and since the majority of plates are solid and non permeable, sensible only transfer is the result.

The tempering of incoming fresh air is done by a heat or energy recovery core. In this case, the core is made of aluminum or plastic plates. Humidity levels are adjusted through the transferring of water vapor. This is done with a rotating wheel either containing a desiccant material or permeable plates.[23]

Enthalpy plates were introduced in 2006 by Paul, a special company for ventilation systems for passive houses. A crosscurrent countercurrent air-to-air heat exchanger built with a humidity permeable material. Polymer fixed-plate countercurrent energy recovery ventilators were introduced in 1998 by Building Performance Equipment (BPE), a residential, commercial, and industrial air-to-air energy recovery manufacturer. These heat exchangers can be both introduced as a retrofit for increased energy savings and fresh air as well as an alternative to new construction. In new construction situations, energy recovery will effectively reduce the required heating/cooling capacity of the system. The percentage of the total energy saved will depend on the efficiency of the device (up to 90% sensible) and the latitude of the building.

Due to the need to use multiple sections, fixed plate energy exchangers are often associated with high pressure drop and larger footprints. Due to their inability to offer a high amount of latent energy transfer these systems also have a high chance for frosting in colder climates.

The technology patented by Finnish company RecyclingEnergy Int. Corp.[24] is based on a regenerative plate heat exchanger taking advantage of humidity of air by cyclical condensation and evaporation, e.g. latent heat, enabling not only high annual thermal efficiency but also microbe-free plates due to self-cleaning/washing method. Therefore the unit is called an enthalpy recovery ventilator rather than heat or energy recovery ventilator. Company´s patented LatentHeatPump is based on its enthalpy recovery ventilator having COP of 33 in the summer and 15 in the winter.

References

  1. ^ Zhongzheng, Lu; Zunyuan, Xie; Qian, Lu; Zhijin, Zhao (2000). An Encyclopedia of Architecture & Civil Engineering of China. China Architecture & Building Press.
  2. ^ a b c d Mardiana-Idayu, A.; Riffat, S.B. (February 2012). "Review on heat recovery technologies for building applications". Renewable and Sustainable Energy Reviews. 16 (2): 1241–1255. doi:10.1016/j.rser.2011.09.026. ISSN 1364-0321. S2CID 108291190.
  3. ^ S. C. Sugarman (2005). HVAC fundamentals. The Fairmont Press, Inc.
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  6. ^ Nielsen, Toke Rammer; Rose, Jørgen; Kragh, Jesper (February 2009). "Dynamic model of counter flow air to air heat exchanger for comfort ventilation with condensation and frost formation". Applied Thermal Engineering. 29 (2–3): 462–468. doi:10.1016/j.applthermaleng.2008.03.006. ISSN 1359-4311.
  7. ^ Vali, Alireza; Simonson, Carey J.; Besant, Robert W.; Mahmood, Gazi (December 2009). "Numerical model and effectiveness correlations for a run-around heat recovery system with combined counter and cross flow exchangers". International Journal of Heat and Mass Transfer. 52 (25–26): 5827–5840. doi:10.1016/j.ijheatmasstransfer.2009.07.020. ISSN 0017-9310.
  8. ^ Feldman, D.; Banu, D.; Hawes, D.W. (February 1995). "Development and application of organic phase change mixtures in thermal storage gypsum wallboard". Solar Energy Materials and Solar Cells. 36 (2): 147–157. doi:10.1016/0927-0248(94)00168-r. ISSN 0927-0248.
  9. ^ a b c d e f g O’Connor, Dominic; Calautit, John Kaiser S.; Hughes, Ben Richard (February 2016). "A review of heat recovery technology for passive ventilation applications" (PDF). Renewable and Sustainable Energy Reviews. 54: 1481–1493. doi:10.1016/j.rser.2015.10.039. ISSN 1364-0321.
  10. ^ O’Connor, Dominic; Calautit, John Kaiser; Hughes, Ben Richard (October 2014). "A study of passive ventilation integrated with heat recovery" (PDF). Energy and Buildings. 82: 799–811. doi:10.1016/j.enbuild.2014.05.050. ISSN 0378-7788.
  11. ^ Mardiana A, Riffat SB, Worall M. Integrated heat recovery system with windcatcher for building applications: towards energy-efficient technologies. In: Mendez-Vilas A, editor. Materials and processes for energy: communicating current research and technological developments. Badajoz: Formatex Research Center; 2013.
  12. ^ Flaga-Maryanczyk, Agnieszka; Schnotale, Jacek; Radon, Jan; Was, Krzysztof (January 2014). "Experimental measurements and CFD simulation of a ground source heat exchanger operating at a cold climate for a passive house ventilation system". Energy and Buildings. 68: 562–570. doi:10.1016/j.enbuild.2013.09.008. ISSN 0378-7788.
  13. ^ Kosny J, Yarbrough D, Miller W, Petrie T, Childs P, Syed AM, Leuthold D. Thermal performance of PCM-enhanced building envelope systems. In: Proceedings of the ASHRAE/DOE/BTECC conference on the thermal performance of the exterior envelopes of whole buildings X. Clear Water Beach, FL; 2–7 December 2007. p. 1–8.
  14. ^ Cuce, Pinar Mert; Riffat, Saffa (July 2015). "A comprehensive review of heat recovery systems for building applications". Renewable and Sustainable Energy Reviews. 47: 665–682. doi:10.1016/j.rser.2015.03.087. ISSN 1364-0321.
  15. ^ Teke, İsmail; Ağra, Özden; Atayılmaz, Ş. Özgür; Demir, Hakan (May 2010). "Determining the best type of heat exchangers for heat recovery". Applied Thermal Engineering. 30 (6–7): 577–583. doi:10.1016/j.applthermaleng.2009.10.021. ISSN 1359-4311.
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  17. ^ "2.3 The buildings sector - InterAcademy Council". www.interacademycouncil.net. Archived from the original on 2008-06-01.
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  19. ^ Braun, James E, Kevin B Mercer. "Symposium Papers - OR-05-11 - Energy Recovery Ventilation: Energy, Humidity, and Economic Implications - Evaluation of a Ventilation Heat Pump for Small Commercial Buildings." ASHRAE Transactions. 111, no. 1, (2005)
  20. ^ a b c Pulsifer, J. E., A. R. Raffray, and M. S. Tillack. "Improved Performance of Energy Recovery Ventilators Using Advanced Porous Heat Transfer Media." UCSD-ENG-089. December 2001.
  21. ^ a b Christensen, Bill. “Sustainable Building Sourcebook.” City of Austin’s Green Building Program. Guidelines 3.0. 1994.
  22. ^ "Chapter 44: Air-Air Energy Recovery" (PDF). ASHRAE Systems and Equipment Handbook. American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). July 2000. p. 44.17. ISBN 978-1883413804.
  23. ^ Huelman, Pat, Wanda Olson. Common Questions about Heating and Energy Recovery Ventilators Archived 2010-12-30 at the Wayback Machine University of Minnesota Extension. 1999. 2010.
  24. ^ Recycling Energy
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