It’s 6:30 am on a sunny Monday morning in January. One of your techs called in sick, there is ice on the sidewalks, and security emailed about the employee entrance door sticking. As you drive to work, you think of all the tasks you need to accomplish. You enter the main lobby and realize you are standing in a puddle of water, the ceiling tile is soaked and on the verge of collapse. Welcome to Monday morning.

Now is the time to ensure your system's HVAC coils are freeze protected and working properly. A frozen coil not only causes water damage, there is also the replacement cost and downtime for repairs when it is least convenient. In addition, a closed off lobby is certain to get the attention of the boys upstairs. 

A Misconception

Some believe steam coils, closed air dampers and recirculating water will prevent coils from freezing. Think Again. It is important to note that if a steam trap fails, condensate can back up and freeze coils, air dampers can stick, and recirculating systems can fail or contain dead spots. 

Below are some devices and practices that can help ensure your system is protected when you need it most.

Common Freeze Protection Methods

DRAINING THE COILS. Draining and valving off the chilled water coil at the end of each season will prevent freezing. If there is no water, there is no risk of freezing.

Tips: 

•  Old valves may not hold. Be prepared to install new valves where needed.
•  Not all coils are accessible or have a properly pitched drain. 
•  Vent all air properly when refilling in the spring.

USING A GLYCOL MIXTURE. Isolate, drain, then fill the coil with a mixture of propylene glycol and water that has a freeze point lower than the coldest outdoor temperature for your region.

Tips: 

• Be sure to use inhibited glycol and check for proper percentage of concentration or freeze point.
• Make sure you flush 100% of the glycol out of the coil in the spring, before introducing chilled water back into the coil. Small amounts of glycol left in the chilled water system can lead to microbiological issues.
• Vent all air properly when refilling in the spring.

FACE AND BYPASS DAMPERS. The damper is connected to a temperature sensor that adjusts the volume of air entering the system. This allows more cold air over the coils, forcing the steam valve to fully open, increasing flow of steam and condensate. 

STEAM DISTRIBUTION COILS. These coils contain a unique design with an inner tube that distributes steam to an outer tube containing the condensate. These coils are designed for lower temperature applications and reduce the likelihood of freezing.

FREEZESTAT SENSORS. This sensor should be installed on all chilled water systems where temperatures drop below freezing. It will sense when below freezing air is coming in contact with unprotected coils and bring in heat if possible or shut down the damper to prevent the coil from freezing. 

Tips: 

• Make sure the automation is working properly and sensors are reading accurately. 

BYPASS PUMPS. The bypass pump will turn on when the temperature drops close to freezing. This separate pump keeps the water moving fast enough in the coil so that the potential for freezing is reduced.

Tips: 

• Be sure there is good circulation in all areas of the coil to prevent dead zones leading to freeze ups.

COIL LOCATION. Coils that do not have freeze protection should be located downstream of heating coils.

Tips: 

• This is not an option if you use the heating coils for reheat/dehumidification during winter months.

AIR BLENDER LOCATION. Air blenders should be installed upstream of heating coils mixing outdoor air with return air, to minimize cold spots on the coil.

CLOSING FRESH AIR DAMPERS. Closing off the fresh air will reduce or prevent cold air from contacting the coils. This is only an option where the AHU contains only chilled water coils or where there is a good return air blending system.

Tips: 

• Buildings require a certain CFM of fresh air for ventilation and for exhausted air. Restricting fresh air will have a significant negative impact on the quality of air in your building.
• Restricting air intake will force air to be drawn from other areas of the building including doors, windows or poorly sealed seams. This can cause cold air in undesired locations.

The best time to check your coils for freeze protection is now. Having a system that is ready to accept whatever mother nature has in store, will provide peace of mind as we move into winter.

For the Water Side of Things

To learn more about winter preparedness for both chillers/towers and boiler systems, please reference our Cooling Layup Procedure or Boiler Startup Checklist available for download in our Knowledge Center.

Jeff Freitag – Director of Sales, Watertech of America, Inc. Jeff has been in the water treatment industry for almost 30 years and has a diverse background in water chemistry, pretreatment equipment, filtration, chemical feed and control, testing and monitoring, training, and consulting. He joined the Watertech team in 2000 and has held several positions before moving into his current role as Director of Sales. Some call him “Jeff.” Most who have worked with him simply refer to him as “The Water Guy!”