Modern heating, ventilating and air conditioning (HVAC) systems move energy around the building from where it is not wanted (too hot) to where it is needed (too cold).
When there is a surplus of energy (summer), the HVAC discards the extra energy. When there is a deficit (winter) then energy is added. Energy represents a cost to operate the building (second to the cost of the occupants) and since most energy is still drawn from fossil fuels, it has a significant carbon impact (carbon dioxide is a greenhouse gas).
Energy recovery is a technology that “reuses” energy to minimize the need for more (primary) energy to be consumed by the building. The technology comes in many forms; heat pump chillers, geothermal systems and air to air energy recovery which will be discussed here. It is impossible to deliver a high performance building that does not use one or more energy recovery technologies.
Ventilation system with Air to Air Energy Recovery
Ventilation is necessary in buildings to maintain good indoor air quality (IAQ), offset exhaust air from bathrooms, kitchen hood and other exhaust systems and building pressurization. For many climate zones, the ambient air conditions are much different than the inside air conditions. Significant work must be done to heat, cool, humidify and/or dehumidify the outdoor air prior to delivering to the occupied space.
Air to Air energy recovery allows the heat that is in the exhaust air be used to heat the incoming ventilation air in winter. In summer, the reverse happens and the cool, dry exhaust air is used as a heat sink to absorb the heat and humidity from the incoming air. For HVAC systems that require a dedicated outdoor air system (DOAS) such as Variable Refrigerant Volume (VRF), watersource heatpumps (WSHP), fancoils and chilled beams, the ventilation unit can have a heat recovery device included in the unit. For all air HVAC systems (i.e. variable air volume (VAV)), air to air energy recovery can be installed in the ventilation air stream.
There are several common technologies used for air to air energy recovery. Most require the supply and exhaust air stream to be connected to the device so the energy transfer can be achieved. For this article the air streams will be labeled as;
Outdoor Air: the airstream from the outdoors to the energy recovery device
Supply Air: the air stream from the energy recovery device to the building
Return Air: the airstream from the building to the energy recovery device
Exhaust air: the air stream from the energy recovery device to the outdoors.
Ventilation Unit With Energy Recovery
The figure shows a typical ventilation unit with an energy recovery device bridging the supply and exhaust air streams. This allows the energy recovery device to move energy from one air stream to the other.
The ideal air to air energy recovery device can:
Sensible heat is energy that changes the temperature of air. A sensible energy recovery device recovers heat energy and changes the temperature of the two airstreams.
Latent energy is the energy associated with condensing water from a gas (moisture) to a liquid (condensation). To dehumidify air the moisture must be removed so the latent energy of the moisture must be absorbed. As this is 1000 Btu/lb (2326 kJ/kg) of water, it is a significant load when air conditioning air.
Enthalpy is a the sum of both latent and sensible heat. An enthalpy recovery device can recover both sensible and latent energy.
Heat Recovery Ventilator is a common industry name for a ventilation unit that can only recover sensible heat.
Energy Recovery Ventilator is a common industry name for a ventilation unit that can recover both sensible and latent energy.
Energy Recovery Rotor
A rotor or wheel energy recovery device has a drive system that rotates the rotor between the two airstreams at around 20 rpm. The mass of the rotor absorbs heat in the warm air stream and then rotates into the cold air stream where the heat is released. The rotors can be wrapped (as shown in the Figure) or sectionalized. They are often made of aluminum but can also be composite. Generally the thicker the rotor, the better the energy recovery as there is more thermal mass and surface area to absorb the heat energy. The rotor thickness can range from 4 to 10 inches (100 to 2500 mm). Even with a thick rotor, the whole section is typically less than 2 ft (60 cm) in direction of airflow so rotors offer the smallest footprint of the common devices. Since this device only absorbs heat it is known as a sensible rotor.
Rotor Energy Recovery Device
Rotors can have their surface area coated with a desiccant. The desiccant enhances the rotor so it can now transfer moisture from one airstream to another. This makes the rotor an enthalpy or total energy recover device.
Desiccants
A desiccant is a hydroscopic substance that will bond with water vapor (moisture) in a process known as adsorption. An common example is silica gel. Table salt is another desiccant.
Desiccants used in air to air energy recovery are selected so they will adsorb moisture in the air stream with a higher water vapor partial pressure (the more humid airstream) and release the moisture when exposed to the airstream with the lower partial pressure (the dry airstream).
Since no extra energy was required to “dry” the desiccant they are referred to as passive desiccants.
In some industrial air drying processes where very dry air (less than 10 gr/lb) is required, active desiccants are used. These require the drying airstream to be heated to get the desiccant to release the moisture. In this case, the desiccants are known as active desiccants.
Sensible and Latent Energy Recovery vs. Rotor Speed
The latent and sensible energy recovery processes behave differently in an enthalpy rotor. The Latent performance is more linear with rotor speed while the sensible rotor capacity behaves more like a water control valve. Sensible capacity control only occurs in the last few rpms so a wide rotor speed range is critical for temperature control. Accurate control will only occur in rotor speed can be reduced to 1 rpm.
It is also possible for latent energy to move in one direction while sensible energy moves in the other. This can occur in locations such as the Pacific northwest.
Energy transfer capacity control with rotors is achieved either by varying the rotor speed or bypassing air around the rotor. Rotor speed is very common but requires a very high turn down on rotor speed (less than 1 rpm) to be effective. Bypassing air requires a large enough cabinet (as much as 20% larger) for the bypass. The bypass can be used to reduce air pressure drop if energy recovery is not required.
Rotor Stepper Motor with 20:1 Turndown
Air leakage with rotors will happen but with careful design minimal impact to IAQ and energy performance can be achieved. One transfer air path is exhaust air crossing into the supply air stream causing recirculation. Another air path is outdoor air passing through the rotor and back out the exhaust air path. This does not cause cross contamination but adds to the fan energy use (the exhaust air fan consumes energy due to the short circuit of air). Managing air leakage from one air stream to another requires several different approaches working in unison.
Swegon brush seals are used on rotor face
Swegon lip seals are used at rotor perimeter
To evaluate the level of air transfer, AHRI 1060 has two tests as part of the performance rating program.
Exhaust Air Transfer Ratio (EATR) compares a tracer gas concentration difference between the supply air and the outdoor air and the difference between the return air and the supply air. It is expressed as a percentage and can be used to evaluate the possibility of cross contamination to the supply air stream.
Outdoor Air Correction Factor (OACF) is the supply airflow divided by the outdoor airflow. OACF will indicate any short cycling of outdoor air back the exhaust air stream (if OACF is less tan 1.0) or short cycling of return air to the supply air (if OACF is greater than 1.)
Supply and exhaust fan position makes a big difference on air leakage. The fan arrangement shown in Air to Air Energy Recovery Units is known as drawthrough – drawthrough. It minimizes outdoor air recycling with reasonable exhaust air transfer. If the exhaust air fan was relocated to blowthrough (on top of the supply air fan) there would be significant cross contamination but the unit foot print would be smaller (less expensive.)
Rotor seals are used at the rim of the rotor on both sides to minimize air transfer. Brush or lip seals are used where the rotor passes from one airstream to another.
Rotor Seals (Blue Lip Seals)
Another common approach is a Purge. As the rotor passes from one airstream to another, any air that is in the rotor itself will pass out of the rotor in the opposite airstream. To avoid this, a purge is used. While purges work very well with constant flow ventilation units, variable airflow units introduce a challenge. As the airflow slows down, the air spends longer in the rotor. At half the airflow, the air is in the rotor twice as long, and can be carried pass the purge device and released into the opposite airstream. The best solution is to vary the rotor speed in conjunction with the varying airflow.
Air to Air Plate Heat Exchanger
Plate heat exchangers have separate passages that allow the two airstreams to pass by each other without mixing. They are typically made of aluminum or composite. A cross flow plate heat exchanger is a cube with the two airstreams at right angles to each other. The unit is compact but less efficient. A cross flow heat exchanger allows the two airstreams to move in opposing directions requiring a larger footprint but with improved efficiency. The sealed nature of the two airflows means air leakage is a very small portion. Plates are good when return air stream has contaminants that should not mix with the supply air. It should be noted that plates are not perfectly sealed, care should be taken if 100% separation is required.
Crossflow vs. Counterflow Heat Exchanger
Most plate heat exchangers only have sensible heat transfer. Some plate heat exchangers utilize a membrane to separate the airstreams that is permeable to moisture. An Enthalpy plate can allow moisture movement from the humid airstream to the dry airstream. It generally requires a larger core to provide enough surface area for moisture transfer
Plate Face and Bypass Damper Control
Heatpipe Principle
A heat pipe is a tube partially filled with liquid refrigerant (A). Heat is added (warm air flows over the outside of the tube) which boils the refrigerant and cools the air. Since this is a change of state, the refrigerant temperature remains constant helping maintain a large temperature difference and improving heat transfer. It also means a small amount of refrigerant can hold a lot of energy.
The heatpipe tube is positioned so that as the refrigerant boils to a gas, it floats to the top of the tube (B). The top of the tube is in a separate cold air stream. The refrigerant condenses (C) releasing its latent energy to the cold air stream and warming the air. Gravity returns the liquid refrigerant back to the bottom of the tube to repeat the process (D).
Common Heatpipe Configurations
The images show common heatpipe configurations for side by side air streams (left) and over/under airstream (right). The heatpipes are pitched so the liquid refrigerant pools in the hot air stream and the gas refrigerant condenses in the cold air stream. This means the heatpipe can generally only move heat in one direction (i.e. it is configured to recover heat in the winter and will not operate in the summer). Some manufacturers mount the heatpipe on a pivot with an actuator so the tilt can be reversed offering bidirectional energy flow. Heatpipes only move sensible energy. They offer the same level of air path separation as plates.
One version of a heatpipe is the wrap around. It is in a U shape with the evaporator on the upstream side and the condenser on the downstream side. In between a cooling coil is installed. The air enters the evaporator boiling the refrigerant and cooling the entering air. The supply air then enters the main cooling coil where it is cooled and dehumidified. Finally the supply air passes through the condenser part of the heatpipe where it is reheated delivering dehumidified but neutral temperature supply air to the building.
Wrap Around Heatpipe
Many heatpipe applications do not have capacity control. This may not be a large concern. A common application is using a vertical stacked heatpipe for sensible reheat down stream of an enthalpy rotor and main cooling coil. The return air temperature is constant (building condition) and the supply air temperature off the cooling coil is constant. In this application capacity control is not critical.
When capacity control is required, a tilt mechanism can be used. Bypass dampers can also be applied but will increase cabinet size. In some applications it is possible to add solenoid valves in the refrigerant tubes. The solenoid valves can be closed reducing the heat transfer.
Many heatpipe applications do not have capacity control. This may not be a large concern. A common application is using a vertical stacked heatpipe for sensible reheat down stream of an enthalpy rotor and main cooling coil. The return air temperature is constant (building condition) and the supply air temperature off the cooling coil is constant. In this application capacity control is not critical.
When capacity control is required, a tilt mechanism can be used. Bypass dampers can also be applied but will increase cabinet size. In some applications it is possible to add solenoid valves in the refrigerant tubes. The solenoid valves can be closed reducing the heat transfer.
Solenoid Control Valves for Heatpipe Control
Run Around Loop
The image shows a run around loop. A fluid (typically a water-glycol solution) is pumped from one coil to another. The first coil is in the exhaust air path where heat leaves the exhaust air and warms the fluid. The second coil is in the supply air path when the heat leaves the fluid to warm the supply air.
Run around loops offer only sensible energy recovery and have the lowest efficiency. Generally it is not worth operating them in the summer as the pump work will exceed the energy recovered. They do have a couple of key advantages. They guarantee full separation of supply and exhaust air. In some applications such as labs, this may be a critical requirement. The supply and exhaust air paths do not have to be in the same area. The heat energy can travel between the air paths in the fluid. Finally, multiple exhaust and supply coils can be on a common circuit.
Capacity control can be accomplished either with control valves (two way or three way) on the supply air unit or variable flow pumps. A variable flow pump works best if there is one supply air coil.
Rotor | Sensible Plate | Enthalpy Plate | Heatpipe | Run Around Loop | |
---|---|---|---|---|---|
Energy Recovered | Sensible or Enthalpy | Sensible | Enthalpy | Sensible | Sensible |
Efficiency | 50 to 80% | 50 to 75% | 55 to 75% | 40 to 60% | 45 to 65% |
EATR | 0.5 to 10% | 0 to 2% | 0 to 5% | 0 to 1% | 0% |
OACF | 0.99 to 1.10 | 0.97 to 1.06 | 0.97 to 1.06 | 0.99 to 1.01 | 1.0 |
Note: There are multiple Efficiency Standards in use so it is important to know which one is being used when comparing technologies.
Air to Air Energy Recovery Device Comparison
The table above shows common performance metrics of air to air energy recovery technologies. Where enthalpy recovery is important (cooling and dehumidifying) rotors are most common. Rotor EATR and OACF values can range widely so care in application and commissioning is required to offer the same levels seen with plate heat exchangers. Heatpipes can be used successfully as a second component such as reheat or evaporative cooling. Run around loops have no air transfer (EATR = 0) and are the right choice for areas such as labs where no crossover can be accepted. While the run around loop efficiency may be lower, these spaces tend to operate 24-7, so the annual energy savings can deliver a payback.
Classification of Exhaust air and dilution limits (Refer to ASHRAE Standard 62.1-2019) | Recommendations |
---|---|
Class 1 – Air with low contaminant concentration, low sensory-irritation intensity, and inoffensive odor. Recirculation or transfer of class 1 air to any space shall be permitted. | Use EATR and OACF to calculate adjusted intake rates and insure that proper outside air ventilation is provided. |
Class 2 – Air with moderate contaminant concentration, mild sensory- irritation intensity, or mildly offensive odors. (Class 2 air also includes air that is not necessarily harmful or objectionable but that is inappropriate for transfer or recirculation to spaces used for different purposes.)
Recirculated Class 2 air shall not exceed 10% of the outdoor air intake flow. |
Minimize EATR to reduce re-circulation of exhaust air. Most devices will require no special measures to achieve this level of dilution. System design, including multiple exhaust points from a variety of spaces can increase dilution performance. |
Class 3 – Air with significant contaminant concentration, significant sensory-irritation intensity, or offensive odor.
Recirculated Class 3 air shall not exceed 5% of the outdoor air intake flow. |
Minimize EATR to reduce re-circulation of exhaust air. System design, including separate exhaust air duct systems for class 3 exhaust, multiple exhaust points including class 1 and 2 air, purge, etc. will influence dilution performance. |
Class 4 – Air with highly objectionable fumes or gases or with potentially dangerous particles, bioaerosols, or gases, at concentrations high enough to be considered as harmful.
Class 4 air shall not be recirculated or transferred to any space or recirculated within the space of origin. |
AAERVE may not be an acceptable technology. Only specific designs with zero EATR and not susceptible to failure should be used in this application |
ASHRAE Std 62.1 Air Classification Recommendations
Comparing the various air to air energy recovery technologies can be difficult as they all have advantages and disadvantages. How they deliver as part of the complete energy recovery ventilation system is the real test.
To appreciate how climate can impact air to air energy recovery and influence technology choice, three locations will reviewed; Miami (hot and humid), Montréal, Canada (cold) and Phoenix (hot and dry). The design conditions weather data comes form ASHRAE fundamentals handbook. Note there is summer design drybulb (hot day) weather and the mean coincident wetbulb (MCWB). This condition is only exceeded by 0.4% or 35 hours. There is also dehumidification design conditions. This is a different day that is not the hottest but has the largest humidity load. In this case, the humidity ratio (amount of water in a cubic foot of air) and the mean coincident drybulb temperature (MCDB) are shown.
Location | Summer | Cooling | Summer | Dehumidification | Winter | HDD | CDD | |
DB °F | MCDB °F | HR Gr/lb | MCDB °F | db °F | wb °F | |||
Miami, FL | 91.9 | 77.6 | 147.6 | 83.5 | 48.8 | 113 | 4578 | |
Montreal, QC | 86.0 | 71.7 | 114.2 | 78.8 | -9.5 | 7771 | 490 | |
Phoenix, AZ | 110.3 | 69.5 | 120.2 | 82 | 39.2 | 912 | 4636 |
ASHRAE Fundamentals, 0.4% data
ASHRAE Design Weather Conditions
The examples will consider a sensible rotor with 80% efficiency and an enthalpy rotor with 80% sensible efficiency and 77% latent efficiency (note latent and sensible efficiencies are rarely the same for any given component).
Miami Energy Recovery Psych Chart
The Miami Psychrometric chart shows the design day cooling and heating recovery of the rotor. The horizontal blue line shows a sensible rotor can deliver (14.7 Btu/h-cfm) and is based on the design drybulb day (best day for a sensible only device). The more vertical blue line shows the performance (63.8 Btu/h-cfm) for an enthalpy rotor and is based on the most humid day. It is clear that in a location like Miami where dehumidification is key, an enthalpy recovery device is critical. The red line shows the rotor on a design winter day.
Montreal Energy Recovery Psych Chart
The psychrometric chart shows the results for Montreal. Montreal is not as hot or humid as Miami so the energy savings are smaller but still significant. Montreal still benefits from enthalpy energy recovery which delivers almost 4 times the energy savings as just a sensible device.
In winter, the energy recovery is significant. The bulk of the energy recovery is sensible heat so the enthalpy and sensible performances are almost the same (84.5 Btu/h-cfm vs. 70.7 Btu/h-cfm for just sensible). Being able to recover humidity in the winter has some key advantages. First, a building in this climate can be very dry in the winter (less than 10% RH). An enthalpy recovery device can return the humidity to the building and raise the relative humidity. Operating humidifiers requires a high energy budget (1000 Btu/lb of water). The second advantage of enthalpy recovery in winter is the significant improvement in defrost.
Phoenix Energy Recovery Psych Chart
The psychrometric chart shows Phoenix, a hot a dry climate. It benefits with almost twice the sensible energy recovery of the other two locations (30.6 Btu/h-cfm). Phoenix also benefits the least from enthalpy recovery.
[make same chart but add the color blocks for hours by psych condition]
While this analysis is good for recognizing the downsizing of supplemental heating and cooling equipment its weakness with the previous analysis is it does not consider the amount of time and thus the amount of energy savings that will occur. Phoenix Energy Recovery with Hours shows Phoenix again but with hours the outdoor weather spends at different psychrometric conditions. This makes it clear that while enthalpy may help a little, the total number of dry hours limits the savings.
Frost Condition in Montreal
Go back to Montreal, and consider what happens in winter. The return air is an energy source that can be used to warm the outdoor air. As the energy is transferred, the return air temperature drops. At some point the return air temperature can reach freezing conditions (32°F (0°C)) If the return air is saturated (at its dewpoint) then frost will start to form on the recovery device rendering it useless and possibly damaging it.
In the psychrometric chart, the red line Ssa shows the sensible heat gain in the supply air stream – it is a 65 °F (36°C) temperature rise. Assuming the supply and exhaust airflow rates are equal, then the return air temperature will drop from 72 °F (22°C) to 6.8 °F (-14°C). The green line Sra shows the sensible heat loss in the return air stream. In this example, the return air has 20% relative humidity (RH). This means the return air will reach saturation as it reaches freezing temperatures and frost will occur. Had the return air relative humidity been below 5% frosting would not have occurred. It is very difficult to know when and if a frosting condition will happen because it is very sensitive to the building humidity level.
The red Esa and green Era lines show the same process but with an enthalpy recovery device. The key difference here is while the return air temperature will drops as the energy is transferred just like a sensible device, humidity is also being transferred which lowers the saturation point (the lines have slope on psychrometric chart). Depending on the climate, this can increase the annual energy savings by 5 to 15% because you can extract more sensible heat from the return air stream without frosting.
Technically the rotor is capable of delivering the winter heat recovery but it cannot sustain it (it will frost). Most equipment selection programs will show the full energy recovery capability but provide a warning that frost may occur. In a properly designed ventilation unit, the recovery capacity will be modulated so the unit does not frost. This does not mean the recovery stops, it means no matter how much colder it gets outside, a fixed amount of energy will be extracted from the return air and used to heat the outdoor air. As the outdoor air temperature continues to drop, so will the supply air temperature but the heat transfer will stay constant.
A ventilating unit used in cold weather climates must have a strategy for dealing with frost. Preheat or return air reheat are possibilities but should carefully considered as they can waste energy. More common is using controls to either avoid a frost condition or recognize it and go into a defrost mode.
Basic Concept of Air to Air Energy Recovery Efficiency
The heat recovery device efficiency is calculated based on some version of the formula shown in the above figure. The details change based on the test standard. The difference in energy between the return air and the outdoor air represents to total energy available for recovery. It makes up the denominator. The difference between the supply air and the outdoor air is what is actually transferred. It makes up the numerator. The result is expressed as a percent. A perfect device would recover all the difference between the outdoor air and the return air and have 100 percent efficiency.
Efficiency can be measured for:
Note that in the real world all three efficiencies will almost certainly be different. This formula works for both heating and cooling (dehumidification).
Sensible Efficiency example showing an example for a sensible recovery device in a cold weather application.
Typical Printout for Enthalpy Rotor
The above performance printout is for an enthalpy rotor. Note the three efficiencies for sensible, latent and total energy recovery. Notice below the efficiencies, effectiveness is shown. This rotor selection in winter will cool the return air down to 24.9°F (-1.4°C) but also lower the humidity ratio so the exhaust air relative humidity is only 69.8%. The rotor will not frost. The efficiency and the effectiveness have the same values. Had the rotor reached a frosting condition, the effectiveness would be less because the energy transfer would have to be less to avoid frosting.
AHRI Standard 1060 and the accompanying ASHRAE method of Test Standard 84 are the most popular tests stands for energy recovery devices in North America. The standards only apply to the recovery device, not the whole recovery unit (does not consider fans for example).
AHRI 1060 Effectiveness Equation
The AHRI 1060 equation is very close to the basic efficiency equation except the ratio of supply airflow/ minimum airflow has been added. In a practical application there is no guarantee the two airflows will be the same. If for example, the return air was only half the supply airflow, then there would only be half the energy available for transfer.
Consider the airflow rate ratio carefully. If the supply air and return airflow are equal, then the term will be 1 and the Std 1060 formula will match the basic formula. However, if the return air is only half the supply airflow then 1/0.5 = 2 and the AHRI Std 1060 formula will increase the efficiency value. Two things to consider, first the measured supply air temperature will change as the airflow ratio changes (in this example it will be lower because there is only half the return airflow and thus half the energy source). The second issue is the energy transfer is not linear to changes in the airflow ratio. Cutting the airflow ratio in half does not cut the recovery in half. The device will actually do slightly better. The recovery device will do a better job of transferring energy when airflow ratio is less than 1, there is just less energy to transfer. The AHRI formula reflects this improvement even if less energy is transferred. Figure 25 shows how the efficiency can go from 82.7 % to 90% while the actual heat transferred drops from 153 kBtu/h to 139 kBtu/h. The device did a better job of transferring energy from a smaller source.
ASHRAE Standard 90.1 – Energy Standard for Buildings Except Low-Rise Residential Buildings is an energy standard. It covers under what conditions air to air energy recovery is required based on system size, percentage of outdoor air and climate zone.
Climate Zone | % Outdoor Air at Full Design Airflow Rate | |||||||
---|---|---|---|---|---|---|---|---|
≥10% and <20% | ≥20% and <30% | ≥30% and <40% | ≥40% and <50% | ≥50% and <60% | ≥60% and <70% | ≥70% and <80% | ≥80% | |
Design Supply Fan Airflow Rate, cfm | ||||||||
3B, 3C, 4B, 4C, 5B | NR | NR | NR | NR | NR | NR | NR | NR |
0B, 1B, 2B, 5C | NR | NR | NR | NR | ≥26000 | ≥12000 | ≥5000 | ≥4000 |
6B | ≥28,000 | ≥26,500 | ≥11000 | ≥5500 | ≥4500 | ≥3500 | ≥2500 | ≥1500 |
0A, 1A, 2A, 3A, 4A, 5A, 6A | ≥26,000 | ≥16,000 | ≥5500 | ≥4500 | ≥3500 | ≥2000 | ≥1000 | ≥120 |
7, 8 | ≥4500 | ≥4000 | ≥2500 | ≥1000 | ≥140 | ≥120 | ≥100 | ≥80 |
NR-Not Required
Climate Zone | % Outdoor Air at Full Design Airflow Rate | |||||||
---|---|---|---|---|---|---|---|---|
≥10% and <20% | ≥20% and <30% | ≥30% and <40% | ≥40% and <50% | ≥50% and <60% | ≥60% and <70% | ≥70% and <80% | ≥80% | |
Design Supply Fan Airflow Rate, cfm | ||||||||
3C | NR | NR | NR | NR | NR | NR | NR | NR |
0B, 1B, 2B, 3B, 4C, 5C | NR | ≥19,500 | ≥9000 | ≥5000 | ≥4000 | ≥3000 | ≥1500 | ≥120 |
0A, 1A, 2A, 3A, 4B, 5B | ≥2500 | ≥2000 | ≥1000 | ≥500 | ≥140 | ≥120 | ≥100 | ≥8 |
4A, 5A, 6A, 6B, 7, 8 | ≥200 | ≥130 | ≥100 | ≥80 | ≥70 | ≥60 | ≥50 | ≥40 |
NR-Not Required
Standard 90.1 requires minimum 50% energy recovery effectiveness using equation 2. Note it is based on enthalpy energy recovery. As well there is no airflow ratio term. Standard 90.1 is solely focused on reducing the energy cost to precondition outdoor air. If there is less return air to draw energy from, then a higher efficiency device will be necessary to achieve the requirement.
AHSRAE Standard 90.1 Equation
The example shows the impact of sensible vs latent recovery when applying Standard 90.1. The example on the left has enthalpy recovery and delivers 82.6% effectiveness easily meeting the standard’s requirement. The example on the right shows sensible recovery and Standard 90.1 effectiveness drops below the requirement.
Standard 90.1 does not explicitly require enthalpy energy devices but the calculation will be based on the total energy recovered. In locations such as Vancouver and Seattle, sensible devices can meet the requirement. Generally, where there is high humidity in summer, even a high-efficiency sensible device cannot meet the standard.
Passive House Institute has rating programs for commercial and residential energy recovery ventilators. This includes the energy recovery device efficiency, the fan energy use, leakage, sound etc. so it is a full unit test standard. The Passive House equation includes a term for fan energy. The same unit tested to Passive House, AHRI and ASHRAE Standard 90.1 will all yield different values because the equations are different.
Passive House
The ASHRAE handbook defines efficiency as “the ratio of output of the device to its input” and effectiveness as “in energy recover ventilators, effectiveness refers to the ratio of actual energy or moisture recovered to the maximum possible amount of energy and/or moisture that can be recovered.”
ASHRAE Standard 84 defines effectiveness as “Actual transfer of moisture or energy / Maximum possible transfer between streams”
AHRI Standard 1060 defines effectiveness as “a ratio of the actual energy transfer (sensible, latent or total) to the product of the minimum energy capacity rate and the maximum difference in temperature, humidity ratio or enthalpy.”
ASHRAE Standard 90.1 defines energy recovery as “50% energy recovery ratio shall mean a change in enthalpy of the outdoor air equal to 50% difference between outdoor air and entering exhaust air enthalpies at design conditions”
Efficiency and effectiveness are not consistently applied in the HVAC industry. The best way to avoid a misunderstanding is to refer (specify) in the entering and leaving air conditions for the device. It is the result that is important.
Eurovent is common in Europe and is the equivalent of AHRI in North America. It is also a full product standard covering the energy recovery device, cabinetry, fans etc.
ASHRAE Standard 189.1 – Standard for the Design of High Performance Green Buildings uses the same approach as ASHRAE Standard 90.1 but requires 60% efficiency.