This unit has a bypass duct in which the hot water heating coil is located, and utilizes face and bypass damper control of cooling and valve control of heating.

When the room thermostat senses a drop in temperature during cooling, the face dampers close (interrupting cooling) and the bypass damper opens. In the "dead band" range of the room thermostat, neither cooling nor heating takes place. On a further drop in room temperature, the hot water valve opens to the hot water heating coil. (Hot water shutoff valves are shown in the closed position. Hot water risers are not shown.)

Advantages For The Buiding Owner
• Small Footprint: Smallest on the market
• Lower pumping costs: Elimination of system resistance caused by valve trains, coils, and run-out piping
• Humidity Control: Face and bypass dampers
• Quiet Operation
• Aesthetics: Aluminum Grilles - No unsightly steel panels or access openingsAdvantages For The Contractor
• No unit balancing of cooling coil or reverse return piping
• No venting of individual units
• Reduced number of solder joints
• Small riser/drain grouping (8 to 10 inch core drill)
• Ideal for retrofit projects
• Galvanized cabinet is ideal for direct drywall application (No framing)
• Small units are easier to handle
• Units can be installed early in the construction cycleSequence of Operation
The Four-Pipe unit employs continuous chilled water circulation through the riser heat-exchanger, intermittent warm water circulation through the heating coil, and continuous fan operation. Independent hot water risers for the heating coil are required, which can either be factory-installed or field-installed. The thermostat cycles an internal face and bypass damper which directs air either over the riser heat-exchanger when cooling is required or through the bypass section which contains the hot water coil.

A decrease in room temperature from full cooling will first cause the face dampers to close against the cold riser heat-exchanger, while simultaneously opening the bypass damper to bypass air through the inactive hot water coil. A further decrease in room temperature will open the electric valve on the hot water coil. The thermostat cycles the dampers and hot water valve as required to give each individual unit the capability of providing either heating or cooling. Heating/cooling changeover of the thermostat is automatic, and the user has manual fan speed selection of HI, LO, or OFF.