MSC recently signed on a new customer to perform preventive maintenance at a new manufacturing facility. Early the very next morning, when outdoor temperatures had dipped into the teens for the first time that winter, our service department received an emergency call from the facilities director reporting that the 100,000 sq. ft. building was very cold inside with extremely negative air pressure. So much so, in fact, that exterior doors wouldn’t close and a large volume of air was being pulled in around closed loading dock doors.
When MSC’s service technician arrived at the building shortly after 6 AM, the outdoor temperature was 17°F. Interior temperatures hovered in the high 50s, and it was even colder on the loading dock and near exterior doors. There were serious concerns that water and sprinkler lines could freeze without immediate action. Quickly assessing the situation, our service tech determined that all six make-up air units had tripped on freeze-stat while process exhaust fans had continued running, pulling in approximately 50,000 CFM of the frigid air, so our first action was to stop the exhaust fans.
An attempt was made to restart the make-up air units, but the freeze-stats tripped again almost immediately. Our technician detected a cold discharge temperature below setpoint and theorized that the problem was a lack of hot water, but he found that 140°F hot water was circulating and available to all of the unit heaters which were running and attempting to satisfy temperature.
Next, our service technician looked at the setup of the make-up air units, which were on an elevated steel platform about five feet above the roof. A check of the air vents and low-point drains indicated that the system was air-bound and circulating poorly. A low-point drain had been left slightly open, allowing water to escape, and as a result, the water/glycol reservoir tank was empty.
Now we moved quickly to refill the system with water and food-grade glycol until all the air had been vented and circulation to the six make-up units was restored. This process took two days and approximately 425 gallons of a 20% glycol/water mix. We also raised the circulating temperature of the hot water boiler loop to 160°F to provide more heat to the unit heaters and reheat coils in the offices. This, along with turning the exhaust off, allowed the building to gain heat long enough for MSC technicians to make the needed repairs. Once the make-up air units were running without tripping on freeze-stat, we verified proper water circulation throughout the system. Flow was back to normal and the system was restored.