Defrost is required for energy recovery ventilators (ERV’s) in cold climates to prevent frost buildup and ensure proper operation. ERVs are designed to recover sensible and latent energy from the exhaust air to pre-condition the incoming outdoor air. In cold weather conditions, moisture in the exhaust air can freeze on the ERV device surfaces. Frost results in the following performance issues:
- Inhibiting airflow: When frost accumulates, it restricts the flow of air through the ERV. Reduced airflow can result in poor ventilation and reduced indoor air quality.
- Reduced energy recovery efficiency: The primary function of an ERV is to recover energy and when frost is present, the heat exchange process becomes less effective, meaning that the ERV’s energy-saving capabilities are compromised.
- Potential damage: If frost continues to accumulate, it can cause physical damage to the ERV’s recovery device, reducing the unit’s lifespan and requiring costly repairs or replacements.
To prevent these issues from occurring, defrost strategies are used to ensure the ERV is operating at its full potential and design. There are a variety of different strategies and each one has its benefits and drawbacks, which must be considered carefully to ensure that overall system performance is optimized.
Now, let’s look at the different options available for defrost on energy recovery cores and energy recovery wheels.
- Exhaust only Defrost: This method shuts down the supply fan to eliminate cold outdoor air from entering the energy recovery device, while exhaust air continues to run. This allows the warm exhaust air to remove frost before re-introducing outdoor air into the unit. This is the most cost-effective method of energy recovery but puts the building into a negative pressure and prevents ventilation air from being introduced.
- Recirculation Defrost: This method shuts down the exhaust fan, closes off the outdoor air, and recirculates return air through the unit. This allows the warm return air to remove frost before returning the unit to normal operation. This stops ventilation air from being introduced but keeps the building at a neutral pressure. The main consideration with this option is what type of return air is being recirculated, i.e you would not want bathroom exhaust recirculated throughout the building.
- Face and Bypass Defrost: This method uses dampers to adjust the percentage of outdoor air that is passed through the recovery device. In defrost mode, a portion of the outdoor air will be by-passed around the recovery device, this allows the higher percentage of return air to defrost. Once the device is defrosted, the by-pass damper will close, and all the outdoor air will return to passing through the recovery device. This method allows for continuous ventilation and will keep the building at a neutral pressure. Since this method utilizes dampers and actuators, it comes at a price premium over exhaust only and recirculation methods.
- Wheel Speed Control: This method uses a variable frequency drive (VFD) which slows down or stops the rotation of the energy recovery wheel. By reducing the rotational speed, the recovery device’s contact with the cold, outdoor air is minimized, spending more time in the exhaust air stream, which allows the frost on the wheel to melt. This method allows for continuous ventilation and keeping the building at a neutral pressure. This is the most common method for energy recovery wheels.
- Outside Air Preheating: This method uses a heating coil (electric, hydronic, etc.) upstream of the recovery device to preheat the outdoor air. This method can be used either as continuous preheat so the recovery device will never enter a defrost state or kick on when defrost is required. Continuous pre-heat will allow for the most efficient recovery operation since no frost will inhibit the performance, but also uses the most external energy for controlling defrost.
A summary of the different strategies is shown below:
Picture Credit: Aldes
Manufacturers use different controls strategies to determine when to activate defrost, which include measuring the exhaust air/outdoor air temperature, energy recovery device pressure drop, and time cycled defrost. It is important to understand the different options available to choose the appropriate one to utilize in your building system. The team at O’Dell Associated can assist you with selecting the best method for your application.
