Most engineers who specify HVAC coils work with standard heating and cooling coils their entire careers and rarely encounter a face and bypass coil application. When they do encounter one, the specification requirements are different enough from standard coil practice that the differences are worth understanding before the drawings are finalized.
Face and bypass coils are a specific coil configuration used in applications where precise discharge air temperature control is required across a wide range of load conditions. They are not the right answer for every application, but for the applications where they are the right answer, nothing else does the job as well. This article covers what face and bypass coils are, how the control strategy works, and when to specify them.
What a face and bypass coil is
A standard heating or cooling coil occupies the full face area of the air handler or duct section it is installed in. All of the air passing through the unit passes through the coil. The leaving air temperature is controlled by varying the flow rate or temperature of the fluid in the coil. This works well for most applications and is the correct specification for the majority of commercial HVAC coil applications.
A face and bypass coil system divides the air handler face into two sections. One section is occupied by the coil. The adjacent section is an open bypass passage with a damper. The two sections are controlled by a set of opposed blade dampers that work in opposition: as the coil damper opens, the bypass damper closes, and vice versa. By modulating the proportion of supply air that passes through the coil versus the proportion that bypasses the coil entirely, the system controls the mixed leaving air temperature without varying the fluid flow or temperature in the coil itself.
The coil in a face and bypass system operates at full design conditions whenever it is active. The fluid temperature and flow rate through the coil remain constant. The discharge air temperature is controlled entirely by the ratio of conditioned air to bypassed air at the mixing point downstream of the coil and bypass dampers.
Why the control strategy matters
The face and bypass control strategy produces a characteristic that is not achievable with a standard coil and valve arrangement in the same way: precise discharge air temperature control at very low load conditions without requiring the coil to operate at very low fluid temperatures or very low fluid flow rates.
In a standard cooling coil application controlled by a two-way valve, maintaining a low discharge air temperature setpoint at low load requires reducing the chilled water flow rate through the coil. At very low flow rates, the coil performance becomes less predictable and the control loop becomes less stable. In applications where the discharge air temperature setpoint needs to be maintained very precisely across a wide range of load conditions, the valve control approach has practical limitations.
In a face and bypass system, the chilled water flow rate through the coil remains constant regardless of the load. The coil is always producing fully conditioned air at the design leaving conditions. The discharge air temperature is controlled by mixing that fully conditioned air with unconditioned bypass air. This mixing control is inherently stable and precise because it does not depend on the coil’s hydraulic behavior at low flow conditions.
The same principle applies to heating applications. A face and bypass heating coil system can maintain a precise discharge air temperature setpoint across a wide range of load conditions without requiring the heating coil to operate at reduced flow or reduced temperature.
When to specify a face and bypass coil
Face and bypass coil systems are the right specification in three types of applications.
Laboratory and research facility HVAC. Laboratory spaces frequently require precise discharge air temperature control to maintain the environmental conditions required for experimental work. Variable air volume systems in laboratory applications often require the supply air temperature to be maintained within tight tolerances regardless of the load, which is a face and bypass application.
Reheat applications in humidity-controlled spaces. In spaces where both temperature and humidity are controlled independently, the supply air is often cooled to a low dewpoint to remove moisture and then reheated to the required supply air temperature. A face and bypass heating coil downstream of the cooling coil provides precise reheat control without the energy waste of a full reheat coil operating at reduced capacity.
Applications with high minimum airflow requirements. In spaces where the minimum air change rate is high relative to the peak cooling or heating load, a standard coil controlled by a valve may need to operate at very low capacity for extended periods. Face and bypass control is more stable and energy efficient in this operating regime.
Healthcare facilities with precise temperature requirements. Operating rooms, procedure rooms, and other critical healthcare spaces require supply air temperature control within tight tolerances. Face and bypass coil systems are frequently specified in these applications for the same reason they are specified in laboratory applications: precise control at all load conditions.
Coil design requirements for face and bypass applications
The coil in a face and bypass system is designed differently from a standard HVAC coil in several respects.
The coil face area is only a portion of the total air handler face area. The damper section occupies the remainder. The coil needs to be fabricated to the specific dimensions of the coil section, not the full air handler face. Getting these dimensions right requires coordination between the coil specification, the damper specification, and the air handler casing design. A coil that does not precisely fit the coil section of the face and bypass assembly will allow air to bypass the coil through gaps around the coil perimeter rather than through the controlled bypass passage.
The coil is designed for the full design airflow through the coil section, not the total supply airflow. When the system is at full cooling or heating demand, all of the supply air passes through the coil. The coil selection needs to be based on the full supply airflow at that condition, with the coil face velocity and row count selected accordingly.
The coil connections need to be located and oriented to accommodate the damper assembly and the air handler casing. In face and bypass configurations the coil is typically installed vertically with the connections on the side of the coil facing away from the bypass section. Connection location and orientation need to be confirmed against the specific air handler configuration before the coil is fabricated.
HX Coils reviews every face and bypass coil application before fabrication begins. The review confirms the coil dimensions against the air handler casing, the connection configuration against the available clearances, and the coil selection against the full load airflow conditions. For face and bypass applications in healthcare facilities, the material specification and documentation requirements discussed in our medical facility coil article apply. For applications where the face and bypass coil is a heating coil supplied by a commercial steam or hot water system, the GP Energy Products team handles the boiler side of that conversation. Visit gpenergyproducts.com for more on GP Energy’s commercial boiler capabilities.
HX Coils manufactures custom face and bypass coils for commercial and industrial HVAC applications across the Mid-Atlantic region. If you have a face and bypass application in development or a replacement face and bypass coil to specify, reach out before the order is placed and we will review the application and confirm the specification is right before fabrication begins.
References
1. ASHRAE. HVAC Systems and Equipment Handbook, Chapter on Coils. Covers coil types, face and bypass coil design, and application guidance. ashrae.org
2. ASHRAE Standard 170. Ventilation of Health Care Facilities. Covers temperature control requirements for healthcare HVAC applications including face and bypass coil specifications. ashrae.org
3. AHRI Standard 410. Forced-Circulation Air-Cooling and Air-Heating Coils. Governs performance testing and certification requirements for HVAC coils including face and bypass configurations. ahrinet.org
4. ASHRAE. Laboratory Design Guide. Covers HVAC system requirements for laboratory applications including face and bypass coil selection. ashrae.org
All technical claims are consistent with the standards listed above. Confirm face and bypass coil dimensions and connection configuration with the air handler manufacturer before finalizing the coil specification.
