When it comes to central air system applications, the majority of system selections will require terminal unit applications that are in sync with the design intent.
This month, I’m going to examine some of the issues and concerns with the design, installation, commissioning, and operation of one such terminal unit application: VAV terminals. You might want to stop reading this column and pick up your 2019 ASHRAE Handbook, HVAC Applications, to refresh your knowledge of VAV design engineering by reading Chapter 48, Design and Application of Controls, as well as the 2020 Handbook, HVAC Systems, and Equipment, Chapter 4, Air Handling and Distribution.
Let me share with you a few considerations when designing VAV terminals for your HVAC project application:
- Do you want all the air terminals to be VAV, or should you consider constant volume (CV) terminals that will most likely require a heating coil downstream of the CV device to provide constant or minimum air ventilation, specific room air changes, and/or space pressure control? This CV application will require an electric or hot water heating coil. Also under consideration may be a fan-powered box (FPB) air terminal that can be VAV reduced to a minimum room air change, or it, too, can be a CV terminal box with the FPB varying the outdoor supply air volume from the central air system while the terminal’s fan provides constant ventilation, specific room air changes, and/or space pressure control.
- In double duct-dual fan applications, which are frequently used in hospital and pharmaceutical applications, parallel/dual (one hot and one cold) VAV terminals provide space temperature control, constant volume, and space temperature control.
- A VAV terminal concern can be the space thermostat reducing the supply air to prevent overcooling of the occupied space. This may result in no or minimum flow that will be below the adequate air changes for the space application.
- Coordinate with the team. These air terminals require access for testing, adjusting, and balancing, so the decision needs to be made to locate the VAVs above an accessible lay-in-tile ceiling or provide access panels in hard ceilings, e.g., gypsum. If the VAV terminal includes a heating coil downstream to provide maximum-to-minimum supply air, then access to the heating coil will also be required.
- Remember the operation and maintenance (O&M) staff. Those assigned to operate and maintain the VAV terminals will also need to lift ceiling tiles and/or open access panels to inspect the device, its automatic controls, and heating coil on a set preventive maintenance schedule as well as troubleshoot the device when design criteria is not being met.
- VAV terminals are engineered to reduce the upstream supply air pressure to a low-velocity supply air pressure. When designing a VAV terminal system, consideration needs to take into account the noise generated at these devices, which can become objectionable, requiring a downstream acoustic sound attenuator installed in the low-velocity duct to reduce the sound level entering the occupied space.
This list is not comprehensive. VAV terminals may have other issues and concerns too, so design engineers need to be cognizant of these as well.