Reheat systems for commercial buildings
What is a reheat system?
To help keep people productive and comfortable,
commercial buildings need heating, cooling,
outdoor air for ventilation, and air circulation.
A reheat system is one of several types of Heating,
Ventilating and Air-Conditioning (HVAC) systems
found in commercial buildings. The primary
advantage of a reheat system is that it can provide
relatively precise control of space temperature
and humidity. The main disadvantage is that a
reheat system is energy intensive, especially if
not optimized. Reheat systems are often found
in larger buildings such as offices, institutional
buildings, medical facilities and laboratories.
Reheat system air flow types
Reheat systems can be either Constant Volume (CV) or Variable Air Volume (VAV) and use either electric or hot
water reheat coils. As the terms suggest, VAV systems vary the air flow rate in response to the changing heating and cooling load of the space, while a CV system maintains a constant air flow rate. A VAV reheat system is
expected to be more energy efficient than a constant volume system. However, if the VAV system is running as a
constant volume system because of its design, construction or maintenance, it will use more energy.
Numerous ways to optimize a reheat system can be found in the Advanced Variable Air Volume Systems Design
Guide at http://energydesignresources.com/resources/publications/design-guidelines/design-guidelinesadvanced-variable-air-volume-(vav)-systems.aspx.
How a reheat system works
A reheat system works by cooling air to a temperature low enough to condense or remove moisture (for humidity
control) and to offset the largest heat gain generated anywhere in the building. The cold (~50˚F - 55˚F) air is
pushed out everywhere in the building and then warmed up throughout the reheat system, depending on the
space cooling needs. This system provides the flexibility of delivering different amounts of cooling to different
zones at different times.
In many large commercial buildings, different parts of the building will have significantly different cooling needs
at the same time of day. For example, during a summer morning after a cool night, the perimeter of a building
(the part exposed to the outside walls) may only need a small amount of cooling, while the interior zone
containing many people and lots of office equipment and computers may need considerably more cooling.
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Sample VAV reheat system schematic
AHU or RTU
Damper
Return Air/Return Duct
Relief
Air
( 72 - 76˚F)
AHU or RTU
Damper
Relief
Air Outside
Air
˜
Supply Fan
W/VFD
Return Air/Return Duct
Return Fan
W/VFD
( 72 - 76˚F)
˜
A
Continued
Below
Return Fan
W/VFD
Supply Air ( 55˚F)
Supply Duct
˜
A
B
AHU - Air Handling Unit
Continued
RTU - Rooftop Unit
VAV - Variable AirBelow
Volume
Filter
Supply Air ( 55˚F)
VFD -˜Variable Frequency Drive
Supply Duct
Outside
T - Thermostat to VAV Box
Air
AHU - Air Handling Unit
(Not to scale.)
RTU - Rooftop Unit
- Variable
Air Volume
A reheat system consists of ductwork
an air handling unitVAV
(AHU)
or rooftop
unit (RTU) to reheat coils
Preheat Cooling
Filter connecting
Coil
Coil
VFD - Variable Frequency Drive
dispersed throughout the building. The (optional)
RTU or AHU houses dampers for
mixing
air,
filters,
perhaps a preheating
T - Thermostat to VAV Box
Supply Fan
W/VFD
Preheat Cooling
Coil
Coil
(optional)
B
or Plenum
( 72 - 76˚F)
coil, a cooling coil, aReturn
fan,Duct
variable
frequency
drives and controls in a common housing. Ductwork leaves
the
From
˜
A
Building
RTU/AHU and is branched and routed to all parts of the building. At each room or zone (a group of rooms) a VAV
Supply Air ( 55˚F)
To
box and reheat
B coil deliver the˜ needed temperature and air flow for that space.
Building
A
B
Reheat
Coil
Return Duct or Plenum ( 72 - 76˚F)
˜
> 55˚F
VAV Box
Supply
Air
> 55˚F
From
Building
( 55˚F)
˜
To
Building
Reheat
Coil
Conference
Room
> 55˚F
> 55˚F
T
Office
T
Office
Office
VAV Box
Conference
Room
T
Office
Office
T
Office
Reheat system life cycle cost considerations
Hot water reheat systems typically have lower operating costs than electric reheat systems, but are a bit more
expensive to install. However, costs can vary over time. A hot water reheat system includes water piping, valves,
coils and pumps. An electric system includes circuit breakers, wire and coils.
Example based on the following assumptions:
• 20 reheat coils (3.1 kW and 10,500 Btu/hr)
Electric
coils
Hot Water
Difference
Coils
• 10,000-square-foot to 20,000-square-foot building
Purchase cost
$20,500
$28,800
$8,300
• Running at full load of 500 hours per year
Operating cost $4,300/yr
$ 680/yr
$3,680/yr
• Natural gas cost $0.60/therm; electric cost $0.14/kWh
In new construction, the payback is 2.3 years. Using net present values, this hot water system saves $36,500
compared to an electric reheat system over 15 years.