A standard heating or cooling coil modulates the leaving air temperature by varying the flow of hot or chilled water through the coil. The control valve opens and closes to adjust water flow, the coil transfers more or less heat to the airstream, and the leaving air temperature changes accordingly. This control strategy works well across a wide range of commercial HVAC applications and it is the default approach for most air handling unit designs.
It does not work well in every application. When the required leaving air temperature is very close to the coil surface temperature, when the application requires precise temperature control across a wide range of entering air conditions, or when the load varies rapidly and the water-side control valve cannot respond quickly enough to maintain the required leaving condition, a different approach is needed.
The face and bypass coil configuration addresses these situations by controlling leaving air temperature through the proportion of air that passes through the coil versus the proportion that bypasses it entirely. Understanding when face and bypass is the right answer and how the coil selection interacts with the damper configuration is what allows engineers to specify the system correctly for applications where standard coil control is insufficient.
How face and bypass works
In a face and bypass configuration, the air handler includes a damper system that divides the airstream between two paths. The face path directs air through the coil at full coil capacity. The bypass path directs air around the coil without contacting it. The two airstreams recombine downstream of the coil and the damper, and the mixed leaving air temperature is determined by the proportion of air taking each path.
When the load is at maximum, all air passes through the coil face and the leaving air temperature is at the full coil condition. When the load decreases, the bypass damper opens and the face damper closes proportionally, allowing more air to bypass the coil. The mixed leaving air temperature rises as more bypass air is blended with the air that passed through the coil. At minimum load, most air bypasses the coil and only a small fraction passes through the face.
The coil in a face and bypass system operates at full capacity whenever air is passing through it, rather than at a modulated capacity as in a standard water-side control application. This means the coil surface temperature remains consistent regardless of the load condition, which has implications for humidity control in cooling applications and for temperature stability in heating applications.
When face and bypass is the right answer
Precision temperature control applications are the primary use case for face and bypass coil configurations. Operating rooms, clean rooms, pharmaceutical manufacturing spaces, and laboratory environments often require leaving air temperature control to tolerances of plus or minus one degree Fahrenheit or tighter. Water-side control valve modulation in a standard coil configuration produces leaving air temperature variations that exceed those tolerances under rapidly changing load conditions. Face and bypass damper modulation responds faster and more precisely because the control action is on the air side rather than on the water side.
High-humidity applications benefit from face and bypass because the coil operates at a consistent surface temperature regardless of load. In a standard cooling coil application, reducing chilled water flow to reduce cooling capacity also raises the coil surface temperature, which reduces the moisture removal rate. In a face and bypass cooling application, the coil surface temperature remains at full chilled water temperature regardless of load. The coil continues to remove moisture at the full rate from the air that passes through it, and the bypass air, which has not been dehumidified, is blended in. The net moisture removal rate is proportional to the face fraction, which can be designed to maintain the required leaving humidity across the load range.
Reheat applications in variable air volume systems sometimes use face and bypass to avoid the energy penalty of cooling air and then reheating it. In a standard VAV reheat application, the cooling coil cools the air to the minimum supply temperature and the reheat coil raises it to the required supply temperature for the zone. In a face and bypass application, the proportion of air bypassing the cooling coil is adjusted to deliver the required supply temperature without reheating, which eliminates the energy consumed by the reheat coil when the zone load is below the maximum cooling condition.
Coil selection considerations for face and bypass applications
The coil in a face and bypass system is selected for the full face load condition rather than the average or design load condition, because the coil always operates at full capacity when air is passing through it. A coil that is correctly sized for the design load condition in a standard application may be undersized for a face and bypass application where the coil needs to fully condition the face fraction of air at all load conditions.
Face velocity in a face and bypass system varies as the face damper modulates. At maximum load the face velocity is at design. At reduced load the face damper partially closes and the face velocity decreases as less air passes through the coil. The coil selection needs to account for performance at reduced face velocity, where heat transfer characteristics change and where moisture carryover risk in cooling applications decreases as the air velocity across the wet fin surface drops.
The bypass duct configuration affects the mixing of face and bypass air downstream of the coil. Poor mixing produces temperature stratification in the supply duct that delivers air at different temperatures to different zones served by the air handler. The bypass duct needs to be designed to promote mixing downstream of the coil and damper section, and in some applications mixing baffles or a mixing chamber is required to eliminate stratification before the air reaches the supply fan.
Applications in critical environments
Operating rooms and procedure rooms in healthcare facilities use face and bypass heating coils in the preheat position of the air handling unit to maintain precise control of the entering air temperature to the cooling coil. Precise entering air temperature control at the cooling coil improves the consistency of the leaving air condition delivered to the space, which is important for infection control and for patient comfort in critical care environments.
Pharmaceutical manufacturing cleanrooms use face and bypass configurations in both heating and cooling applications to maintain the tight temperature and humidity tolerances required by the manufacturing process validation. A cleanroom that is qualified at specific environmental conditions requires that those conditions be maintained consistently. Face and bypass control provides the precision and response speed that water-side control alone cannot match in these applications.
For the heating source serving face and bypass heating coil applications, the GP Energy Products team handles commercial boiler selection and service across Pennsylvania, New Jersey, Delaware, and Maryland. Visit gpenergyproducts.com for more. For the chilled water pump systems serving face and bypass cooling coil circuits, the Merion Pump Company team handles pump selection and can provide guidance on flow rate and pressure requirements for face and bypass applications. Visit merionpump.com for more. For air handling unit applications where the face and bypass configuration is part of a factory-assembled package, the FabPro Systems team handles packaged AHU design. Visit fabprosystems.com for more.
HX Coils manufactures custom face and bypass coils for commercial and industrial HVAC applications across the Mid-Atlantic region. Reach out before the coil is specified and we will work through the face and bypass configuration requirements for the specific application.
References
1. ASHRAE. HVAC Systems and Equipment Handbook, Chapter on Coils. Covers face and bypass coil configuration, selection methodology, and application guidance for precision temperature control. ashrae.org
2. ASHRAE Standard 170. Ventilation of Health Care Facilities. Covers temperature and humidity control requirements for operating rooms and critical care environments where face and bypass configurations are commonly used. ashrae.org
3. AHRI Standard 410. Forced-Circulation Air-Cooling and Air-Heating Coils. Governs performance testing and certification for HVAC coils including face and bypass configurations. ahrinet.org
4. ISPE. Good Practice Guide: Heating, Ventilation, and Air Conditioning. Covers precision temperature and humidity control requirements for pharmaceutical cleanroom HVAC applications. ispe.org
