As you know, the VAV, or Variable Air Volume, is a type of heating, ventilating, and/or air-conditioning (HVAC) system that varies the airflow at a constant temperature. The advantages you receive are:
- Lower energy consumption of system fans
- Less fan noise
- Reduced compressor wear
- Additional passive dehumidification
- More precise temperature control
You can install either series flow (constant volume) or parallel flow (variable volume) fan-powered terminals in the ceiling plenum. The question is, which model should you install? the series constant flow or the parallel variable flow?
Let’s take a look at the characteristics of each one as each model carries its own characteristics of delivered airflow, energy consumption, and acoustics.
|Fan Operation – Intermittent operation during occupied and unoccupied modes.||Continuous operation during occupied mode. Intermittent operation during unoccupied mode.|
|Operating Sequence – Variable-volume, constant-temperature device during cooling. Constant-volume, variable-temperature during heating.||Constant-volume, variable-temperature at all times. Delivers design airflow regardless of the load.|
|Fan Energization – Based on zone temperature deviation from set point. No interlock with central system fan required.||Interlocked with central system fan to deliver required air to the zone in both heating and cooling modes.|
|Terminal Fan Operating and Size – Fan runs during heating load. Size for design heating load typically for 40% to 60% of design primary cooling airflow.||Fan runs continually. Fan sizing should meet the greater of design cooling or heating airflow to the zone.|
|Air Valve Sizing – Design cooling airflow.||Design cooling airflow.|
|Minimum Inlet Static Pressure Required for Central Fan Sizing – Sufficient to overcome unit, heating coil, downstream duct and diffuser pressure losses.||Sufficient to overcome air valve pressure loss only.|
|Acoustics – When operating under cooling loads the terminal fan does not run, offering superior acoustic performance similar to single-duct VAV. Under heating loads, the fan operates intermittently. Impact can be minimized by use of a ECM.||Produces slightly higher background sound pressure levels in the occupied space.This sound level remains constant and is less noticeable than intermittent fan operation with PSC motors.|
Series and parallel fan-powered terminals offer specific advantages for particular applications:
Series constant fan-powered terminal
The typical application for the series terminal unit are ones requiring constant air movement. Common applications are conference rooms, laboratories and lobbies.
Further, as the series fan also adds to the system external static pressure, office buildings cam take advantage of this design feature and downsize main air handling equipment. Finally, you can use series terminals in low-temperature air systems to temper cold primary air with warm plenum air and deliver it to the zone.
Parallel intermittent fan-powered terminal
Common applications for parallel terminal unit are in perimeter zones or buildings where loads vary during occupied hours. You will find that core zones, which maintain a more constant cooling requirement, are better for variable airflow, single-duct, models. Typically, you combine parallel fan-powered units (exterior) and single-duct units (interior) to provide an efficient system with lowest first cost.
Even though the overall NC of parallel systems is lower than an equivalent series system, you will notice the intermittent fan when it is energized. If you want to minimize the impact of the NC change, then you can use the soft-start technology of an ECM (Electrically Commutated Motor).
Choosing the right system for your facility
HVAC systems play a major role in the successful operation of your facility. Thus, it is crucial that you choose a system that is “right” for your facility. To choose between the series and parallel fan-powered terminals you should also take into account the end user and consider the following:
- A comfortable and productive tenant environment
- Acceptable installed costs
- Low operating costs
Simply put, series fan-powered terminals are appropriate where very high minimum-airflow rates are required. For example, you could use the series terminal in an interior conference room where the minimum ventilation is more than 50% of the design cooling airflow.
Meanwhile, parallel fan-powered terminals are appropriate in zones with relatively high heating requirements such as north-facing, perimeter zones. Energy codes, such as ANSI/ ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, and Title 24, Part 6, of the California Code of Regulations, generally prohibit reheating more than 30% of maximum airflow.
Regardless of whether you choose a series or parallel fan-powered terminal, a VAV system has many benefits:
- Economical operation
- Improved indoor air quality and comfort
- Reliable operation with simple construction
- A flexible and compatible system as it can be integrated with other air conditioning systems
If you would like more information on the right fit for your facility, please contact us. We’ll be happy to provide guidance.