CAV

 

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  • A Continuous Air Volume system uses fans and ducts to distribute air through a large volume of space.  Return ducts then send “used” air back to the fans and heaters for reconditioning.  As the name implies, this type of system is great for large, continuous spaces, rather than smaller, divided buildings.  One key point of a CAV is that the temperature and air quality can be precisely controlled.  This is important for hospitals or laboratories.

 

  • CAV systems include using a constant air volume reheat system, a multi-zone system, and a furnace.  Please see the graphics below for diagrams of the reheat and multi-zone systems. 

 

  • Single story and smaller buildings are likely to use a furnace due to space constraints.  Ductwork is kept to a minimum here, and as we all know, there is never enough room for ductwork anyway.   

  • Larger spaces tend to use either the multi-zone or reheat systems.  Reheat systems are much like a regular CAV system with an extra heater along the duct path to essentially reheat the air without having to return to the fans.

 

  • The multi-zone system (shown above) separates spaces somewhat like a Variable Air Volume, or VAV, system.  The main large space is controlled exactly like a normal CAV, but the smaller areas can have their own thermostat and ducting from the fan. 

 

  • For example, this would work well for a theatre, which includes the large hall and a few offices or dressing rooms.  The focus is, of course, on the main performance hall, so the chosen system would be a CAV.  Other examples would be gymnasiums, auditoriums and factories.

 

  • As mentioned above, the key point of a CAV is that the temperature and air quality can be precisely controlled.  This does have its limitations though.  In a VAV, different areas of a building or space can be controlled independently, depending on user comfort levels.  The multi-zone system is the most like a VAV of the CAV systems, but not completely.  CAV systems are used in large spaces.  Apartment buildings, schools and individualized offices therefore cannot use the CAV systems due to their lack of large common spaces.

 

  • CAV use the same materials as VAV and most other typical HVAC systems.  A furnace (if you choose to use that subsystem), aluminum ductwork, thermostats, heaters, simple or architecturally-pleasing vent covers and fans are all included as components in Continuous Air Volume Systems.

 

  • CAV systems are relatively expensive to set up, due to the amount of materials needed.  Labor costs will also be high, depending on the size of the space to be conditioned.  In the long run, however, the costs will decrease (excluding electricity to run the system - that would depend on the system size and how long the system runs) as long as the system is set up as efficiently as possible.  Maintenance costs should also be fairly low, as the only real events in the life of a good CAV system would be an annual cleaning.

 

Typical Uses

  • This system is very common in today’s society.  It can be used on larger scale buildings and private residences.  The AHU can be placed in virtually any place in the vicinity of the structure and the duct work required to distribute the load only requires a limited amount of space and comes in many different dimensions. 

 

Parameters

  • Temperature ranges for a CAV system are variable and depend on the users input.  A typical range of temperatures that a CAV system can reach are about 32-120oF.  The humidity can also be controlled, to an extent.  Typical CAV system ranges are from 0-95% humidity.

 

 

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