Airxchange Technology : Frequently Asked Questions

Energy Recovery Ventilation FAQs

General

Technical


What is an Energy Recovery Ventilator (ERV)?

An ERV uses site-recovered energy from building exhaust air (usually wasted to the atmosphere) to pre-condition fresh outdoor air for temperature and humidity, dramatically reducing building energy costs. Pre-conditioning reduces workload on the heating and cooling system, reducing energy demand for treating outdoor air by up to 80%. Reduced workload complements ongoing energy cost savings and allows downsizing of new or retrofit HVAC equipment for immediate return on investment.

What is an Energy Recovery Wheel?

An energy recovery wheel, also referred to as an enthalpy wheel or heat wheel, is a major component in an energy recovery ventilator (ERV). Rotating between two airstreams within the ERV cabinet (exhaust air and supply air), energy recovery wheels transfer heat and humidity from the warm/humid airstream to the cool/dry airstream. Airxchange’s unique, segmented, and sustainable wheel design provides a simple yet effective means of optimizing energy transfer performance for the life of the HVAC system.

What is site-recovered energy?

Commercial building indoor air regulations require frequent exchange of stale indoor air with fresh outdoor air (often multiple changes per hour, depending on building use). Building exhaust air, which has been heated or cooled to room temperature, provides a constant source of recoverable energy from the building site. Without an ERV, the energy in building exhaust air is wasted to the atmosphere while new, additional energy is consumed to heat or cool outdoor air to room conditions. Using an ERV to take advantage of site-recovered exhaust air energy reduces total HVAC energy consumption by up to 40%.

How does an ERV work?

ERVs continually exchange air within a building utilizing fresh air supply and stale air exhaust fans within the ERV cabinet. In winter, as exhaust air passes through the ERV, energy is captured by the wheel. As the wheel rotates into the incoming airstream, energy is released by the wheel to heat and humidify the incoming airstream closer to indoor conditions, reducing unit workload and energy consumed by the heating system.

Under summer conditions, heat and humidity are captured from the fresh outdoor air. Rotation of the wheel allows the heat and humidity to be transferred to the cooler, drier exhaust air as it passes through the wheel. As a result, the outdoor air is conditioned closer to building conditions, reducing unit workload and energy consumed by the cooling system.

In both winter and summer, as the difference between outdoor and indoor conditions (temperature and humidity) increase, the energy reduction and resulting cost savings increase.

How does an ERV affect the size of the HVAC system?

ERVs use site-recovered energy to lessen the load on the HVAC system, effectively reducing the need for additional heating and cooling capacity. Reduced HVAC equipment size means lower first cost and higher performance as the combination of the ERV and smaller-sized HVAC unit provide the same amount of work while using up to 40% less energy.

Can Airxchange Energy Recovery Wheels be integrated into HVAC equipment design?

Airxchange wheels are offered by most major HVAC manufacturers as integrated energy efficiency solutions in packaged rooftop and air handling units.

I already have an HVAC system. Can I use an ERV wheel in a retrofit?

Major HVAC manufacturers offer Airxchange wheels for new and retrofit applications. Wheels can be integrated into replacement units such as packaged rooftop or air handler units or can be provided with a standalone ERV and connected to existing rooftop units and ductwork. Airxchange ERV wheels can also be used to replace wheels manufactured by others. An Airxchange service technician will assist with on-site construction and installation, which typically takes a day and includes a 5-year warranty.

What is the up-front cost of specifying an Energy Recovery Wheel? Is it more expensive?

Including an ERV wheel in an HVAC system provides substantial cost savings, both up front and over time. Because the energy recovery wheel shifts work away from the mechanical system, a smaller HVAC unit with Energy Recovery will provide the same cooling capacity as a larger unit without Energy Recovery. In new construction, a smaller HVAC unit means smaller ductwork and electrical load, reducing cost throughout the system. Bottom line – you can expect minimal or no added first cost to include an energy recovery wheel in a retrofit or new construction. In addition, first cost may be offset with energy recovery incentives offered by many utilities and local/state/federal governments. The largest savings will be realized over time in reduced HVAC energy use – a system with an ERV wheel uses up to 40% less energy than a system without a wheel. Click here to see how the Turtle River Montessori School reduced their outdoor HVAC energy use by 70% with Airxchange energy recovery wheels.

Do Energy Recovery Wheels add maintenance to my HVAC system and how long do they last?

Airxchange energy recovery wheels are designed to last the life of the HVAC system with minimal maintenance required. A unique segmented wheel design allows for easy removal and effective cleaning to regain full performance, something not possible with non-segmented wheel designs.

Depending on the application, the ERV wheels may need to be cleaned once every 10 years (for instance, in an office building or school) or as frequently as every 6 months (for instance, in a smoking or industrial environment). Airxchange energy recovery wheels are constructed with welded stainless steel and a patented energy transfer material that is impervious to corrosion – even in coastal and marine environments. Wheel bearings are permanently lubricated and the drive belt is under constant tension, eliminating the need for annual adjustments.

Is the ERV wheel complicated for my crew to maintain?

Routine maintenance can be performed by non-technical personnel. Energy transfer sections are light and accessible and can be easily removed and cleaned by one person in as little as 15 minutes. 

Is this a new technology? How do I know it's reliable?

Energy recovery wheel technology was first introduced in the U.S. and Canada in the 1970s in response to rising energy costs. Typical wheels were made of metal and used on industrial applications where large amounts of outdoor air provided reasonable paybacks to the owner. Attempts to bring the heavy metal wheels and their inherent manufacturing expense into the commercial market were difficult due to lower outdoor air requirements and longer paybacks.

Airxchange wheels were first developed for the residential market in 1981 and shortly after, a commercial wheel line was designed using light-weight, cost effective, and durable materials enabling attractive paybacks on commercial applications.

As the leading supplier of energy recovery wheels to most HVAC equipment manufacturers, Airxchange has 30 years’ experience manufacturing energy recovery ventilation products and components. Our energy recovery technology provides a stable, robust product proven in hundreds of thousands of installations in almost every climate and facility type. Airxchange wheels are designed to last the life of the HVAC system, and come with a 5-year warranty and an AHRI third party test and certification label verifying published performance.

Learn more about our technology here.

I've heard Energy Recovery Wheels are appropriate in the South, where it is hot and humid, but what about in other climate conditions?

The ability to remove moisture from outdoor air in summer and add moisture in winter makes energy recovery wheels ideal for most climates in the U.S. and Canada. High efficiency moisture exchange minimizes HVAC equipment loads and first cost while maximizing energy savings, providing the lowest life cycle cost as compared to heat transfer devices with little to no moisture exchange capabilities.

For these reasons, building code requirements for energy recovery ventilation require a minimum of 50% total (heat and moisture) effectiveness. Click here to read about how our ERVs have benefited customers in most climate zones across the US and Canada.

If I'm meeting code, why do I need more outdoor air?

Studies show that increased levels of outdoor air provides health and productivity benefits. According to the EPA, “indoor air can be 2 – 5 times more polluted than outdoor air; therefore, most HVAC system designers understand that increased amounts of outdoor air supply is generally better for Indoor Air Quality (IAQ).” An ERV wheel allows you to increase the amount of outdoor air and still reduce operating cost.

What are the code requirements?

Building codes vary by state. Most states recognize the IECC code and adopt it into their state codes. IECC 2012 will include new ERV requirements at significantly lower outside air flow due to the cost/benefit studies performed by the ASHRAE 90.1-2010 mechanical sub-committee. In addition, the "green code" ASHRAE Standard 189 further reduces outdoor air thresholds requiring ERV and increases minimum ERV effectiveness to 60%. Two states, Maryland and Rhode Island, have adopted Standard 189 as their "Stretch Code" for new buildings.

What are the limits to using the wheel?

The wheel should not be used in environments where temperatures exceed 170°F. Precautions for frost control when used in cold climates are described here.

All Airxchange wheels can be mounted in any position. When a wheel is mounted horizontally, each corner must be properly supported to minimize racking. Also, the drive motor should be down so that the drive pulley is up to facilitate segment removal and maintenance.

When the ratio of exhaust to supply is less than 50%, the use of the wheel is generally uneconomical due to lower system effectiveness.

Wheels should not be used with certain exhaust air compositions like paint, noxious fumes/acids/biohazards. Follow the guide of ASHRAE 62-2007 regarding “Classes of Air” for energy recovery. For an Airxchange technical note regarding “Classes of Air”, click here.

How often does the wheel have to be cleaned?

Ordinarily in a non-smoking environment, a yearly inspection and vacuum can be sufficient. In a heavy smoking exhaust air environment (bar, lounge, casino, etc.), the wheel needs an overnight soak in detergent every 3-6 months. Industrial exhaust conditions (weld shop for instance) may have special requirements. For more detailed cleaning procedures, click here.

What desiccant does Airxchange offer?

Airxchange has standardized on Silica Gel (Type A), a substance that has preference for the adsorption of water vapor molecules over other chemicals. Silica Gel was selected for Airxchange comfort ventilation applications because it transfers two to three times as much water by weight as compared to a Molecular Sieve desiccant at comfort conditions.

Does Silica Gel wear out?

No. Silica Gel will transfer water vapor at original design levels as long as its surface is clean and free of oils or other coating agents that can block access to water vapor. It is constantly regenerated by the return air conditions.

Why doesn’t the desiccant come off over time?

Airxchange has a patented and proprietary process that essentially ‘melts’ the desiccant into the plastic energy transfer matrix so it is permanently fixed. Other manufacturer’s use adhesives, which break down over time. The Airxchange bond is permanent and lasts the life of the wheel and matrix.

How long will the plastic matrix last?

The matrix is designed to last the life of the HVAC system, 20 years or more. It is unaffected by salt, oils, or other corrosive elements. It was chosen to be a light and durable energy transfer material that can be easily washed clean.

Do the belts wear out?

Airxchange uses Urethane, a durable material that is impervious to chemicals and aging that typically attack rubber based products. The urethane belt is designed to last 10 years before a replacement may be required.

How long will the wheel drive motor last?

The Airxchange wheel drive motor has been selected very conservatively to last the average life of the HVAC system. 20 years of field service has shown this to be the case. These motors are oversized for the relatively easy application of turning the wheel.

Do bearings need service?

Airxchange wheel shaft bearings are permanently lubricated and sealed at the factory. They have an L10 life of 30 or more years, which means the user will almost never have to replace a bearing.

How corrosion resistant is the cassette and wheel assembly? Can I use it for seacoast applications? How about for swimming pool ventilation?

Airxchange makes one of the most corrosion resistant cassette/wheel combinations available. The cassette frame is made of G90 galvanized material, the wheel components are stainless steel, and the matrix material is polymer film. In addition, especially for pool applications, the cassette frame can be powder coated on special order basis, which adds further corrosion protection. The Airxchange wheel may be applied with confidence in seacoast applications that have proven problematic for aluminum wheels. No special coatings are required to protect the wheel or the matrix.

How does it work? I don’t understand how the wheel controls moisture and humidity.

Any time outside air (OA) supply has a higher absolute humidity than the indoor air being exhausted (EA), the desiccant coating will capture moisture from the OA and transfer it to EA by the process of adsorption and desorption. This process is driven by the partial pressure of the respective water vapor streams with the desiccant acting as the carrier.

When the OA is drier than EA the opposite occurs. OA is then humidified by the transfer of water vapor from the exhaust to supply air stream. In swimming pool applications or any application where it is preferred to exhaust water vapor from a building, a wheel with no desiccant coating, called a sensible wheel, is recommended.

While the desiccant on the surface of the energy transfer matrix reduces the moisture content of the OA with respect to the exhaust, a percentage still enters the building. For good humidity control, it is important that the cooling system be sized so the evaporator coil remains cold enough to remove moisture from the supply air before it enters the building’s ventilation system. In this way, better control of humidity levels, mold, and mildew is assured. The wheel does not replace, but works well with HVAC unit humidity control enhancements.

Why not use a fixed core (fixed plate) exchanger and avoid the moving parts?

While fixed core/plate exchangers can be good heat transfer devices, few are capable of exchanging water. If the application is a heating only application, a properly sized fixed plate exchanger can be successful. One drawback to the fixed plate exchanger in cold climates is the need for frost control any time the OA temp is below 22°F. This lowers the potential recovery efficiency of the exchanger and creates large volumes of condensate. Because of their ability to transfer water, wheels have a much lower frosting threshold (for similar conditions, 0°F). Another drawback to fixed core/plate exchangers is size. Wheels prove to be much more compact for airflows in excess of 1000 CFM.

Why would a wheel be better than a plate in the winter?

The very advantage claimed by most plate manufacturers – no moving parts – is also the limitation of the design. By rotating between a warm and a cold airstream, the energy transfer matrix of the wheel can operate successfully to much lower temperatures. This is because the desiccant coating depresses the frost point of the wheel by lowering the dew point of the leaving air. As long as the water deposited is removed, the wheel stays above the frosting threshold. This allows lower operating temps – usually 0°F – before frost control is required. There is also no condensate to drain.

I am worried about the transfer of air from exhaust to supply. What can I do?

First, if the exhaust air has noxious chemicals, the use of any wheel should be questioned. A run around loop may be the better choice to isolate the exhaust.

If it is a comfort conditioning application with some toilet exhaust, the wheel can safely and successfully be used. For an Airxchange technical note regarding “Classes of Air”, click here.

All wheels have seals that act to prevent the direct transfer of air from one airstream to the other. The two common transfer mechanisms are (1) leakage driven by pressure differences between airstreams which the seals address and (2) the air carried over by the matrix by rotation from supply to exhaust and exhaust to supply. Airxchange wheels are the narrowest offered which minimizes the effect of (2) and limits the transfer in comfort applications where the pressures are balanced between supply and exhaust air streams to 3-5% total, well inside safe limits for comfort applications.

When I apply the ERV to my rooftop unit, what special precautions do I need to observe?

First and foremost, be sure there are no rooftop sources of pollution near the supply air opening. Avoid plumbing vents, exhaust fans, and any other source of external air pollution. Be sure the exhaust from the ERV is at least 90 degrees opposed to the supply air. It may be desirable to connect a short run of duct to either the supply or the exhaust air stream to avoid rooftop problems (this may be required for some applications by local codes). Second, be sure there is easy service access to the ERV. Follow the manufacturer’s instructions to easily change filters, clean the wheel, adjust fan belts, and inspect the unit. Third, be sure the unit is level and any water that might blow or leak into the ERV from a driving rain can be drained. Finally, be sure all unit connections to the HVAC unit are sealed against leakage, the ERV is attached securely to the roof in case of a severe storm, and all filters and hoods are properly installed.