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How To Optimize Seasonal Ventilation: Economizers vs. Enthalpy Wheels

Reading Time: 3 minutes

Mechanical engineers and facility owners alike encounter the same frustration every spring and fall: costly HVAC inefficiencies.

During these “shoulder seasons,” fluctuations in temperature and moisture content within the air pose unique challenges, pushing the limits of the building’s mechanical systems. Below, our ventilation experts explore the science and challenges of seasonal commercial ventilation, along with common solutions.

Autumn Tree

The Challenge of Seasonal Commercial Ventilation

When the seasons change, the outdoor levels of moisture content can vary greatly between daytime and nighttime. Often, the nighttime moisture content is quite high in cold temperatures and causes a large amount of moisture to enter the building. As a result, the cooling system has to deal with this excess moisture during startup in the daytime.

Since there’s not a large cooling load in the night time, AC systems often short cycle, causing less moisture to be caught in AC coils to remove it. When the AC system reaches its limit, the compressor stops working effectively and is unable to remove moisture in the air.

To reduce humidification, heating and cooling systems may begin to operate simultaneously. This not only causes significant energy waste, but also hinders the performance and efficiency of the building’s ventilation system.

Although this topic is being researched quite heavily, there are varying perspectives on the most effective method to optimize performance and efficiency. Two of the most common solutions include the use of economizers and enthalpic devices.

Common Solutions for Improving Efficiency

Image of night sky


When outside temperatures drop, economizers turn the compressor off and draw in the cool air. This process, called free cooling, offers an alternative to using the HVAC unit to produce cooler air and vent mechanically into the building.

While relying on an economizer to provide free cooling during the nighttime can lessen the cooling load during the daytime and reduce energy costs, there are some drawbacks to this method.

Economizers do not have any way to deal with the fluctuations in moisture levels in the air. By pulling in outside air, economizers cannot control the quality or humidity of the air being introduced to the building.

In fact, some research has shown economizers actually bring in more moisture than originally outlined in simulation models, further adding to the challenge of moisture control. Economizers may be an effective solution if the outdoor climate offers the ideal temperature and humidity, but may present additional issues otherwise.

Enthalpy Wheels in ERVs

Enthalpy wheels can be included in the design of the ERV to address moisture issues. While most ERV devices transfer heat (sensible) or moisture (latent) energy, enthalpy wheels allow for the exchange of both.

1. The enthalpy device features a sponge-like material within the heat exchanger to assist in energy transfer.
2. Water vapor goes from high to low through the latent material in the exchanger.
3. Its rotary wheel rotates in response to the air stream.
4. As the air stream crosses the enthalpy wheel, it captures some of that moisture and transfers it to the outgoing air stream.
5. As a result, the moisture load is reduced, ensuring the indoor environment is drier than outside air at nighttime.

However, this process only works if the indoor environment is at a lower vapor pressure than the outdoor air. If moisture levels creep up and the AC system is unable to reduce humidity levels, the enthalpy wheel is unlikely to operate in the right direction. When the AC system operates properly and is able to control moisture, however, enthalpy wheels can effectively assist in reducing the load.

Many designers are unaware of the advantages enthalpy wheels offer during shoulder seasons. Economizers are often used because they cost less but, in some situations, they may actually be a less effective solution.

All of Systemair ERV solutions have enthalpic options available to address the challenges of seasonal ventilation. Contact our ventilation experts to learn more about Systemair’s ERV performance and solutions.

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