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1C) Local Zone with Terminal Reheat or without Terminal Reheat
Including or excluding use of terminal reheat is dictated by design criteria’s of the installer. The use of
terminal reheat in a VAV system will always result in a more comfortable set-up for the occupants of the
space. However this may not be practical from a cost standpoint or regional requirements. System designs
will vary from Northern to Southern and Eastern to Western geographical locations because of the specific
regions peak load requirements.
In colder climates, VAV system heating operation without the use of terminal reheat typically always results
in colder outside walls. Although the zone dry-bulb temperature may be well maintained, it may be possible
for occupants not to be comfortable simply because of the low outside wall temperate.
Also, in the perimeter zones, the delivery process of the heating capacity from the ceiling is not as efficient
as when delivering the heating load directly where the losses occur such as in the case of a perimeter
electric baseboard or perimeter hydronic baseboard.
In regions where the heating load is small and required for only a small portion of the year, a properly sized
up zone VAV can deliverer the required heating demand and insure comfort without the use or terminal
reheat. However it is important to design the zone ductwork and area diffusers to be the most efficient with
air delivery close to the outside walls.
In certain problematic cases where air delivery may be an issue, the use of fan powered VAV units may
reduce the occupant discomfort by providing constant airflow to the zone and maximizing the air delivery
process.
1 D) Special Considerations
A typical office installation may require that a single unit service areas being used for different applications.
These areas will commonly be a combination of external and internal zones.
It is always good to verify the intended use of all areas knowing their true peak loads before committing to
its final design and sizing.
It may be necessary to oversize or undersize the design to meet their daily demands. The following are
examples of when over sizing of a zone damper may be needed:
•
Areas with oversized windows that are exposed to the sun longer
•
Conference
rooms
•
Cafeterias
•
Areas with vending machines
•
Areas with extra lighting
•
Areas with computers, photocopier, etc…..
Areas such as computer rooms, kitchens and certain types of conference rooms may warrant a totally
separate system of their own and should not be part of the zones attached to an RTU. Certain critical areas
may call for cooling all year long and based on system settings could only guarantee occupant comfort a
portion of the year.
Knowing the critical areas of a building in advance and designing for them specifically will always result in a
more comfortable occupant. And it can be as simple as adding terminal reheat, radiant floor heating, a fan
powered VAV or even a separate small water source heat pump to critical area.
