Back in the early 1980s I worked as the national service manager for a startup Florida-based company that began manufacturing a new duct-mounted variable-air volume (VAV) system for residential and light-commercial applications.
Of course, VAV systems were nothing new by then because they had been applied in upscale commercial systems for years to provide individual temperature control in offices and other smaller spaces. There have been pneumatically-controlled, electrically-controlled and even balloon-driven systems that worked off pressure in the ducts. However, such systems, though surely the most comfortable, always have been expensive, their operation always has been a mystery to service techs, and there are problems with getting them to switch over properly between heating and cooling modes.
Though it’s a fact that any office space or residential room could be made more comfortable and the whole system would be more energy efficient if it were fitted with VAV boxes, many contractors still prefer to steer clear of offering them because of price and because of liability if the complicated system doesn’t continue to work as promised. So, even in two-story houses where a VAV solution would keep the upstairs and downstairs areas more comfortable, most still prefer to sell two separate HVAC systems, one for each level.
What are the problems with VAV systems? If they are electronically operated, there is all the wiring and what do you do if the electronics fail out of warranty? There also is the problem of teaching the system the difference between heating and cooling modes. Just opening and closing ducts by a thermostat doesn’t work unless the system knows whether hot or cold air is being sent from the HVAC equipment since you want the dampers to open on a call for cooling and to also open on a call for heating during the winter.
Another problem with VAV systems is you can’t send extra cooling to a room — for example if the main system isn’t calling for cooling. Also, you can’t put a thermostatically-operated damper in the room where the main heating/cooling thermostat is located because you don’t want a controller operating the controller. In addition, there is a problem when VAV systems close too many dampers, which can cause the main HVAC system to freeze up or get too hot. In the past, this was handled by installing complicated “bypass” dampers that just short-circuited the conditioned airflow from the discharge to the return.
I just want to mention I noticed a new VAV product at this year’s AHR Expo that properly highlights the types of digital technology advances we are seeing in this field. A company called Titus was featuring a lay-in VAV diffuser that doesn’t require any wiring at all. It is powered by the light in the room. And what about the thermostat? Its Helios thermostat also works off the light in a room and it communicates with one or more of the diffusers wirelessly.
The good news is with the advent and incorporation of digital technology, things are changing and I’m sure we will see greater use of VAV systems in the future. Now, with the advent of variable-refrigerant volume (VRV) commercial systems and continuously variable-speed compressors and fans being used in residential and light-commercial systems, there is no more need for bypass dampers (and the main systems tend to run continuously rather than just cycling on and off).
Also, using Bluetooth and Wi-Fi technology, I’m sure we soon will see the temperatures in individual rooms being set and maintained by cell phones or home computers that are communicating with VAV systems.