The HVAC Multi-Zone System Details screen is accessed from the Multi-Zone Details column. The screen will only be available if you have selected Multiple Zone for the Zoning Category on the HVAC System screen.
Select the Distribution Type, Terminal Unit Type (if applicable), and Reheat Type (if applicable).
Note that some of the input fields are interdependent. If you select Single Duct as the Distribution Type, you will not be able to select VAV Mixing Box or CV Mixing Box for Terminal Unit Type, because they are only applicable to dual-duct systems. If you select Three Duct as the Distribution Type, Terminal Unit Type options and Reheat Type options will be disabled, because they are not applicable to this distribution type.
Single Duct - Single-duct systems are the most common types of multiple-zone air distribution systems. A single-duct system uses a single fan to supply cool or warm air to zone terminal units. The zone terminal units will then modulate the amount of supply air and sometimes reheat the supply air to meet the specific heating or cooling requirements of the zone. Some systems will recool the supply air, but reheating is more standard practice.
Single Fan, Dual Duct - A single-fan, dual-duct system uses a single fan to supply air through two primary air ducts, supplying cool air and warm air. Air from each of these ducts is then mixed in a terminal unit (VAV mixing box or CV mixing box) and supplied to the zone as a single air stream.
Dual Fan, Dual Duct - A dual-fan, dual-duct system uses two fans to supply air through two primary air ducts, supplying cool air and warm air. One fan serves the warm air duct, and the other serves the cool air duct. Air from each of these ducts is then mixed in a terminal unit (VAV mixing box or CV mixing box) and supplied to the zone as a single air stream.
Three Duct - A three-duct system uses a single fan to supply three primary air ducts, supplying cool air, warm air and return (or neutral) air. A three-duct system is typically constant volume and uses CV mixing boxes to mix warm air and return air or cool air and return air (but never warm and cool air) to meet individual zone loads.
Duct Design Static Pressure (in.) - Static pressure is the total amount of pressure exerted by the supply or return air stream on the ductwork. In air distribution and return systems, this pressure is measured in inches of water column (in.). Most large supply air distribution systems and multiple-zone systems operate at static pressures exceeding 3 in. water.
VAV Box - A standard VAV box includes a damper, a connection to the supply air duct, and sometimes a hydronic or electric resistance reheat coil. Because the zone thermostat calls for less cooling, the damper begins to close and reduce the amount of supply air to the zone. The reheat coil turns on and reheats the supply air only when the damper is at its minimum position. Standard VAV boxes are used in both single-duct and dual-duct systems.
Fan Powered VAV Box - A fan-powered VAV box typically consists of a fan configured to draw air from a return air plenum , a connection to the supply air duct, dampers to regulate the flow of air from the supply air duct and the flow of air from the plenum to the fan, and a hydronic or electric resistance reheating coil. There are many configurations of fan-powered VAV boxes, but there are only two typical operation sequences: series and parallel.
In series operation, the fan operates continuously, resulting in a constant air supply to the zone. As the zone thermostat calls for less cooling, the dampers modulate to reduce the amount of supply air and increase the amount of plenum air delivered to the zone. The reheat coil turns on and reheats the supply air only when the damper is at its minimum position.
In parallel operation, the fan does not operate as long as the zone calls for cooling. If the main supply air is reduced to a minimum and the zone still requires heating, the fan will come on, supplying additional warmer plenum air to the zone. If the zone requires additional heating after the fan has come on, the reheat coil is activated.
VAV Mixing Box - A VAV mixing box mixes air from the warm and cool air ducts of a dual-duct system. The operation sequence of a mixing box terminal depends on several other conditions such as if the air economizer is operating or if the building is primarily requiring cooling or heating.
If an air economizer or the mechanical cooling plant is operating, the warm air supply to the zones is shut off by the mixing boxes. The mixing box modulates the flow of cool air to the zone to meet the required space conditioning. When the cool air damper has closed to its minimum position, the mixing box will then modulate open to allow warm air to be mixed with the cool air and simultaneous heating and cooling begins. At some point, the mixing box may completely close the cool air supply to the zone. Care must be taken when controlling mixing boxes to ensure that supply air is never completely shut off to a zone. This would prevent adequate outdoor ventilation air from being supplied to the space.
In some cases an entire building may be in heating mode, beyond what can be met by unheated return air alone. In this case, the mixing boxes will shut off the supply of cool air to the zones. The mixing box modulates the flow of warm air to the zone to meet the required space conditioning. When the warm air damper has shut to its minimum position, the mixing box will modulate open to allow cool air to be mixed with the warm air. Like in the primary cooling mode, the warm air damper may totally close at some point, but supply air to the zone must never be completely shut off.
CV Mixing Box - A CV mixing box (or constant-volume mixing box) mixes necessary amounts of cool air and warm air to maintain space conditions. The amount of cool air or warm air will vary, but the total flow of cool and warm air through a CV mixing box will remain constant. Except under special conditions, the code does not allow the use of CV mixing boxes.
Reheat/Recool Coil - A reheat/recool coil is placed in a branch line to a zone of a building and is usually controlled from a thermostat located in the space the branch is serving. Reheat/recool coils are usually found only in single-duct VAV systems. For example, if a reheat coil is installed in a duct, it would provide heating to a zone if the system were in cooling mode and supplying cool air through the distribution system. Reheat coils can be electric resistance elements, hydronic coils, or steam coils. Recool coils are not very common and are almost always hydronic. (The software assumes recool coils are hydronic.)
Electricity - When Electricity is used as the reheat type, electric resistance heating elements are installed in the terminal units and are turned on and off by zone thermostatic controls.
Hydronic - Hot water coils are hydronic coils installed in the terminal units. They are equipped with valves, which modulate the flow of hot water, and are controlled by zone thermostatic controls.
Steam - Steam coils, used as a reheat source, are installed in terminal units. They are equipped with valves, which modulate the flow of steam, and are controlled by zone thermostatic controls.
Enter the relevant fan hp inputs and/or inch water of supply static pressure. If either Supply or Return Fan hp are entered, Fan Variable Flow Control will be enabled.
The supply fan hp is the horsepower listed on the supply fan motor nameplate. The supply fan is the fan that provides air to the distribution ducts of the mechanical system.
The return fan hp is the horsepower listed on the return fan motor nameplate.
Vane-Axial Fan w/Variable Pitch Blades - A vane axial fan resembles a propeller. When equipped with variable pitch blades, the blades are rotated or twisted (thus varying the pitch of the blades) to modulate the air flow from the fan.
Mechanical Adjustable-Speed Drive - A mechanical adjustable-speed drive uses mechanical devices to vary the speed of the fan-and therefore vary the flow of air-without varying the speed of the fan motor. One of the most common approaches is to vary the diameter of the shieves in the belt-drive.
Electrical Adjustable-Speed Drive - An electrical adjustable-speed drive is an electrical control system that will vary the speed of the fan motor, which will vary the flow of air. The most common type of electrical adjustable speed drive is the variable-frequency drive (VFD). A VFD is usually the most efficient approach to varying air flow from a fan.
Other Control - Many different types of fan air flow controls are available. Designers must use a combination of calculations, manufacturer's literature, and actual design conditions to determine the energy performance of different fan air flow controls.