Zone Terminal Units

HVAC tab in model data under Mechanical Ventilation header

 

Variable air volume (VAV) systems control the dry bulb temperature inside a zone by varying the supply air volume instead of the air temperature. At full cooling the VAV terminal unit damper is fully open supplying the specified maximum air flow rate. As the cooling load decreases, the damper closes until it reaches the minimum stop specified by the turndown ratio (zone minimum air flow fraction). VAV systems can be used for interior or perimeter zones with a common fan system, air temperature control, and reheating devices. Heating is provided by use of reheat coils.

 

Note: Zone Terminal unit data is used at the zone level.

Turndown ratio (supply air minimum flow fraction)

The fraction that the zone VAV box can reduce to based on the supply air maximum flow rate. The minimum flow rate to the zone while the system is operating is specified as a fraction of the maximum air flow rate. The turndown ratio is normally specified to meet the minimum ventilation requirement for the occupants. The reheat coil operates only when the damper is at this minimum flow rate when Zone damper heating action is set to 1-Normal (the default).

 

A value of 0 means the VAV box can completely shut off flow into the zone and a value of 1.0 means that the VAV box cannot reduce supply air at all (i.e. it is a constant volume system). For CAV systems this data is fixed at 1 and is not available for edit.

 

Turndown ratio is typically in range 0.3 - 0.5. Minimum is 0 and maximum is 1.

 

Note: in EnergyPlus v.2.0 and earlier a turndown ratio of zero (unusual value) has a special meaning as the reheat coils can never operate in this case even if the Zone damper heating action is set to 2-Reverse.

VAV terminal unit type

DesignBuilder provides 3 different VAV zone terminal unit types:

 

1-Standard - The VAV terminal unit contains a damper and optionally a reheat coil. During heating the VAV unit can either work in Normal mode where the supply air flow into the zone is the minimum air flow rate of the VAV system, essentially acting like a constant volume system or in Reverse action mode the air flow rate through the VAV box may increase above the minimum air flow when the heating demand cannot otherwise be satisfied. This is also the terminal type used for CAV systems.

2-Series PIU - The Series Powered Induction Unit is an air system terminal unit that mixes varying amounts of secondary (recirculated) air and primary (conditioned supply) air to produce a fixed flow of air to a zone. The unit contains a small fan that acts to induce the secondary air and a heating coil for heating the mixed secondary and primary air. The fan runs at a constant volume flow rate whenever the unit is on. The fan is downstream of the primary and secondary air inlets. The variable mixing is accomplished by a damper in the unitís primary air supply inlet duct. This damper can move from fully open (100% primary air. 0% secondary air) to a minimum stop that can be specified in the input description. At full cooling the damper will be fully open. At minimum cooling and for heating the damper will be at the minimum stop and the secondary air flow will be at its maximum. It uses the EnergyPlus SINGLE DUCT:SERIES PIU:REHEAT data.

3-Parallel PIU - The Parallel Powered Induction Unit is an air system terminal unit that mixes varying amounts of secondary (recirculated) air and primary (conditioned supply) air to produce a variable total flow of air to a zone. The unit contains a small fan that acts to induce the secondary air and a heating coil for heating the mixed secondary and primary air. The secondary and primary air streams enter the unit in parallel. The fan sits in the secondary air stream and runs only when secondary air is needed. The primary air inlet contains a damper that can move from fully open (maximum primary air) to a minimum stop (minimum primary air). At full cooling load the primary air damper is fully open and the fan is off. The primary air flow is at maximum and there is little or no secondary air flow. As the cooling load decreases, the primary air damper gradually closes and the secondary air flow remains close to zero. At some point, usually when the primary air flow has reached the minimum, the fan switches on and secondary air is induced. The heating coil will switch on as needed to meet any heating demand. It uses the EnergyPlus SINGLE DUCT:PARALLEL PIU:REHEAT data.

 

See VAV for a schematic diagram showing the 3 Zone terminal unit types connected to an air distribution system.

Parallel fan on flow fraction

For parallel PIUs, this data specifies the fraction of the primary air flow at which fan turns on. In the parallel PIU the fan operation is intermittent. If the primary air flow is above this fraction of the maximum, the fan is off. Otherwise it is on.

 

This data is available when using VAV Compact HVAC with the Parallel PIU VAV terminal unit type and the plant sizing options Autosize when not set or Manual.

Terminal Unit Fans

When using fan powered terminal units (PIU) you can specify the characteristics of the terminal unit fans.

Terminal Unit Fan Total Efficiency

Enter the total efficiency of the zone induction fan. This is the ratio of the power delivered to the air to the electrical input power at maximum flow. This is also the product of the fan motor efficiency and fan (impeller) efficiency. The motor efficiency is the power delivered to the shaft divided by the electrical power input to the motor. The fan efficiency is the power delivered to the air divided by the shaft power. The power delivered to the fluid is the mass

flow rate of the air multiplied by the pressure rise divided by the air density. Must be greater than 0 and less than or equal to 1.

Terminal Unit Fan Delta Pressure

Enter the pressure rise of the zone induction fan in Pa at full flow and standard conditions. Standard conditions are considered 20C at sea level, 101325 Pa.