FREEZE PROTECTION OF HVAC COILS By: Thomas W. Dombrowski, P.E., CFEI
Many HVAC air-handling units contain heating and cooling coils that contain steam or water. Since most air-handling units introduce outdoor air into the building, these coils that will be exposed to this outdoor air must be protected from conditions in which water within a coil will freeze. Figure 1 shows typical ruptures of copper HVAC coil tubes as the result of water within the coil freezing.
the outdoor air damper should be in a full-closed position and vice-versa. However, there are at least three scenarios that can cause part or all of the heating coil to be exposed to freezing conditions. 1. The improper mixing of the return and outdoor air in the mixing box causes stratification of the air as its passes across the heating and cooling coils. Stratification is the layering of the two air streams. Using Figure 2 as an example, improper mixing of the return and outdoor air in the mixing box could produce a warmer layer (return air) at the top of the air-handling unit and a colder layer (outdoor air) at the bottom of the airhandling unit.2. A stuck open outdoor air damper as the result of a malfunction of the damper or actuator. Or the outdoor air damper may be locked in an “open” position by maintenance. 3. A malfunction of the air-handling unit controls.
Figure 1
A typical mixed air temperature senor as part of the air-handling unit controls system will not recognize stratification because it reports an average temperature as measured across the airhandling unit (thus averaging the cold and warm sections of airflow). Therefore, to protect the heating coil a low temperature control (or freezestat) is placed on the discharge side of the heating coil. A freezestat has a vapor-charged sensing element (or capillary) that is intended to open an electrical circuit upon sensing a drop in temperature. The function of a freezestat is to protect coils from exposure to freezing air temperatures if the air at the discharge of the heating coil falls below a predetermined setpoint by:
Figure 2
Figure 2 shows a schematic of a typical air-handling unit. When outdoor air conditions are right, the return air and outdoor air dampers modulate in unison to maintain the desired supply air temperature (typically, in the winter months this would be 55 to 65 °F). When the return air damper is in a full-open position APRIL 2021
1. Shutting down the air-handling unit supply fan. 2. Closing the outdoor air dampers. 3. Opening the heating coil control valve. This will protect the heating coil if an outdoor air damper is stuck open. www.edtengineers.com | The Stress Point TM | 11
