Heat Ventilation and Air Conditioning system

This contains many example of HVAC system

1. Lecture 14a: Introduction to
Secondary Systems
Material prepared by GARD Analytics, Inc. and University of Illinois
at Urbana-Champaign under contract to the National Renewable Energy
Laboratory. All material Copyright 2002-2003 U.S.D.O.E. - All rights reserved

2. 2
Importance of this Lecture to the
Simulation of Buildings
Simulation is important, but
understanding what one is simulating
is equally critical
Lack of understanding (treating an
HVAC system like a black box) can
lead to errors in input

3. 3
Purpose of this Lecture
Gain an understanding of:

Different types of space conditioning
systems

General characteristics about how they
operate and what some of their
advantages/disadvantages might be

Prepare for upcoming lectures on
EnergyPlus system input

4. 4
All-Air Systems: Overview
Provides all sensible and latent cooling, preheating,
and humidification required by the zone(s)
Additional cooling or humidification at the zone
rare (industrial systems)
Heating is either provided by the main air stream
by the central system or locally at a specific zone
Classified into single- and dual-duct categories as
well as constant and variable volume categories
Conditioning depends on air mass flow rate and
temperature (enthalpy) difference between supply
and room air

5. 5
Single-Duct Systems
Main heating and cooling coils in series arrangement
Ducts supply air to all terminals at a common temperature
Capacity varied by varying temperature or flow rate
Types of single-duct systems

Constant Volume
Single zone
Multiple-zone reheat
Bypass VAV

Variable Air Volume (VAV)
Throttling
Fan-powered
Reheat
Induction
Variable diffusers

6. 6
Dual-Duct Systems
Main heating and cooling coils in parallel
May use separate warm and cold air duct distribution systems,
blending air at the terminal device
May blend air near the main unit and have separate duct for each
zone
Most vary supply temperature, limited number (around 1% of all
installed systems?) vary flow rate
Types of dual-duct systems

Single zone (“dual duct”)
Constant volume
Variable air volume
Dual conduit

Multizone
Constant volume
Variable air volume
Three-deck multizone

7. 7
Variable Volume vs. Variable
Temperature
Variable Volume

Throttling back flow when less
heating/cooling required

Reduced flow results in reduced fan energy

Potential concerns about outdoor air
quantities for IAQ and humidity control
Variable Temperature

Temperature of supply air changes as
thermal loading conditions change

Variation in temperature may require
additional energy (or use of more outside air)

8. 8
Advantages vs. Other System
Types
Flexible: high degree of flexibility on how to
meet loads, distribute air, etc.
Low noise: most equipment kept away
from occupied spaces
Control: probably the best control of both
temperature and humidity can be achieved
with air systems (precise control situations)
Most easily understood and popular
Indoor air quality: IAQ is part of the system
(not an “afterthought”)

9. 9
Disadvantages vs. Other System
Types
Space: require additional space for air
distribution ductwork (vertically and
horizontally) adds to building size in all
directions
Concerns about access to terminal devices
Requires air balancing which can be
difficult on large systems
Perimeter heating not always available
during construction

10. 10
Single-Zone Constant Volume
Also known as single zone
draw through system
Simplest all-air system, meets
all conditioning needs of
space
System can be in zone or at a
remote location, with or
without distribution ductwork
Little or no ductwork means
low pressure drop and lower
fan energy
Systems can be turned off
without affecting adjacent
systems

11. 11
Multiple-Zone Reheat (Constant
Volume)
Also known as terminal reheat in
some circles
Single air stream at a fixed
temperature delivered to various
spaces
Local supply temperature is varied
by the use of a terminal reheat
coil
Main duct temperature typically
cooled to cold deck temperature
(all year)large amount of energy
consumption
When main duct air temperature
varies, could have humidity
problems

12. 12
Constant Volume Bypass
Bypass replaces reheat coil
Total flow of system remains constant but flow of
air to space is varied
Excess flow is bypassed around zone and directly
into return duct
Saves the energy that would be used by the reheat
coil
Requires a return fan to avoid air short circuiting
back into the room from the bypass; still have
contact supply fan energy
Usually only used in smaller applications where
humidity control is not as important

13. 13
Variable Air Volume Systems
Objective: reduce flow rate when
loads are not as high to save fan
energy (fan energy proportional to
flow rate cubed)
Especially effective for perimeter
zones that may receive solar heating
Temperature is maintain same for all
zones, flow rate varied to each
Flow rate bounded at lower end by a
minimum air fraction
Concern 1: indoor air quality—
outside air may limit lower bound of
VAV flow rate
Concern 2: humidity—it may also
limit lower bound of VAV flow rate
Terminal devices used to further
reduce cooling or to provide heating
From maximum cooling point, VAV
first throttles back flow and then
adds reheat (or uses terminal device)

14. 14
Dual-Duct Systems: Overview
Two air streams are conditioned at a central
location: one warm, one cold
Air distributed to spaces through:

Two ducts (one warm, one cold) and mixed locally,
disadvantage of two ducts (both must be sized to handle
their maximum flow rate though this may be different for
heating and cooling, cost, space)

A single duct per zone after mixed centrally,
disadvantage(?) of multiple ducts
Tends to require more energy than a VAV system
Dual-duct can serve one or more thermal zones
May have more than one fan (dual fan—one for
each duct)

15. 15
Dual-Duct Constant Volume
Total air flow rate remains fixed,
but volume through each duct
varies with heating/cooling load
Single fan with reheat
Similar to the terminal reheat
system
Reheat applied at a central
location rather than at each
individual zone
Air is not cooled and then
reheated as in terminal reheat
Uses less energy than terminal
reheat because some air is
heated and other air is cooled
Constant volume system—air
flow is constant and thus fan
energy is always same (high)

16. 16
Dual-Duct Constant Volume
(cont’d)
Single fan, no reheat

Similar to a single-duct system

Simply has a cooling coil bypass and is very
similar to a single duct system

Does not expend energy to reheat air, simply
uses a mixture of return and outdoor air

Less air is sent through cooling coil, may result
in less dehumidification and thus moisture
problems during parts of the year (spring/fall)

17. 17
Dual-Duct Variable Air Volume
(DDVAV)
Blends warm and cold air in
variable volume combinations
Flow reduced below maximum
load and flows mixed at minimum
flow rate (which might be limited
by outside air or humidity
concerns)
Can be combined with single duct
VAV systems for zones that are
cooling only (interior spaces)
Dual-duct terminal units can also
serve as separate VAV systems
(one warm, one cold) which can
reduce fan energy and heating
and cooling energy

18. 18
Single Fan DDVAV
Fan sized for the “coincident peak” of the hot and
cold deck volumes
Control via two static pressure controllers (one in
each deck)
Cold deck typically kept at same temperature
though it may be varied
Hot deck sometimes adjusted up when
temperature outside is low or when humidity is
high (to force more air through the cold deck)
Some systems may use a precooling coil for the
mixed or outside air

19. 19
Dual Fan DDVAV
Volume of each fan independently controlled
Each fan sized for anticipated maximum
coincident hot or cold volume (not sum of
instantaneous peaks)
Cold deck maintained with either outside air
(free cooling) or mechanical refrigeration
Hot deck can use return air, heating coil, or
outside air (rare, humidity concerns) to
provide heating

20. 20
Dual Conduit
Really a “dual system” configuration
Primary air (constant flow) system:

Used to meet exterior transmission (perimeter) loads

Run only during peak periods

Runs without outside air (can be fairly local)

Must be sized to account for action of secondary system at
minimum air fraction
Secondary air (variable volume) system:

Runs year-round

Serves both interior and perimeter spaces

Used to meet the loads from people, lights, equipment, and
solar heating

Used to bring in outside (fresh) air

Variation on this system mixes the two air streams using a dual
duct terminal box—in this case the primary system is heating
only and the secondary system must meet the entire cooling
load

21. 21
Multizone Systems: Overview
Similar to a dual-duct system (though
only for more than one zone)
Multizone systems share same
advantages and disadvantages as
dual-duct systems
Multizone systems can be obtained as
“packaged” units (lower first costs) of
up to approximately 12 zones

22. 22
Three-Deck Multizone
Has an additional duct for
“unconditioned” air
This duct is in addition to the
warm and cold air ducts (hence
the name)
Only two of the ducts
unconditioned and either
warm or cold used at any given
time (seasonal switchover)
Generally not used due to the
extra initial expenses
Can be mimicked by a
standard dual-duct by
seasonally scheduling coils

23. 23
Summary
Single-duct systems:
Main heating and cooling coils in series arrangement
Ducts supply air to all terminals at a common
temperature
Capacity varied by varying temperature or flow rate
Dual-duct systems
Main heating and cooling coils in parallel
May use separate warm and cold air duct distribution
systems, blending air at the terminal device
May blend air near the main unit; separate duct for each
zone
Most vary supply temperature

24. Lecture 14b: Introduction to
Secondary Systems
Material prepared by GARD Analytics, Inc. and University of Illinois
at Urbana-Champaign under contract to the National Renewable Energy
Laboratory. All material Copyright 2002-2003 U.S.D.O.E. - All rights reserved

25. 25
Special Systems: Dual Air Stream
System
Two interconnected all air systems (diagram)
Primary air system used for outside air
Secondary air system used to condition
space
Used when there are high space gains or
high air flow rates required
Both systems might have heating and
cooling coils

26. 26
Special Systems: Under-floor Air
Distribution (UFAD)
Also called “up air system”
Air distributed from floor rather than
overhead
Air enters space through floor registers or
through furniture
Individual control and high degree of
flexibility
Floor surface is also “conditioned” (pseudo-
radiant system)

27. 27
Special Systems: Evaporative
Cooling
Water vapor added to air stream to reduce
the dry bulb temperature
Raises the humidity level so this tends to
only have application in drier climates
Cooling can either be direct (of supply or
outside air stream) or indirect (return air
with heat exchanger)
Can offset cooling needs, requires water as
a resource

28. 28
Special Systems: Low-
Temperature Systems
Concept: lower the supply air temperature for
cooling so that we can reduce the flow rate (less
flow means less fan energy)
Typically only used in conjunction with ice storage
systems
Ice can be used to produce lower supply air
temperatures
Cannot produce colder air directly using chiller
because lower evaporator temperatures will make
chiller less efficient
Terminal units may be required to maintain a
sufficient amount of air movement within the
space

29. 29
Terminal Units: Overview
Terminal unit is the last (hence terminal)
device on the air distribution system
(between the duct and the conditioned
space)
Two types:
Supply outlet—register or diffuser: goal is to
supply air to the space without causing drafts or
excessive noise
Terminal unit: controls the quantity and
temperature of the supply air (by further
conditioning the air)

30. 30
Constant Volume Reheat
Terminal Units
Used mainly in terminal reheat systems
Reheat coil adds heat to avoid
overcooling (supply air temperature
generally cooled down to around 13C
first)
Can lead to excessive energy
consumption (frowned upon by
ASHRAE Standard 90.1)

31. 31
Variable Air Volume Terminal
Units
Terminal unit reduces the flow rate in
an attempt to provide only enough
cooling to match the actual cooling
load
Care must be taken to avoid too
much flow in areas close to the air
handler

32. 32
Throttling Units
Throttling Unit Without Reheat

Air flow rate can be throttled down to meet the cooling
load

Sometimes flow can be completely shutoff (air quality
concerns)

Concerns about noise from throttling (typically have some
sort of sound attenuation)
Throttling Unit With Reheat

Air flow rate can be throttled down to some minimum

Below the minimum, reheat energy would be required

Can also be used in cases where there are actual heating
loads

Flow rate reduced first, then reheat added

Some systems will ramp up flow rate again once reheat
turned on to avoid excessive reheat energy

Reheat is sometimes replace with a baseboard unit

33. 33
Induction Unit
Flow from room or ceiling induced (Bernoulli
effect) and mixed with primary air stream
Primary air flow rate reduced while keeping
actual flow relatively constant (avoids
stagnant air concerns)
Requires higher static pressures to achieve
induction effectthis leads to higher fan
energy (though overall it should be reduced
since this is VAV)

34. 34
Fan-Powered Terminal Unit
Also referred to as “fan-powered VAV box”
Basic idea is to reuse heat from space, lights, etc. to provide the
“reheating” (similar to induction except we are now actively
moving air with a fan)
Better circulation due to increased air movement (over
throttling unit)
Parallel arrangement—local fan is outside primary air supply
stream
Series arrangement—local fan is inside primary air stream and
runs when space is occupied
Various heater options available to provide perimeter heating
(coil, baseboard, radiant system)
During unoccupied hours, main system can be shut off and
local fans can be run to meet loads as needed
Increase in fan energy over throttling units possibly offset by
reuse of heat from space
Noise a potential problem since fan is so close to occupied
space

35. 35
In-Room Terminal Systems: Main
Characteristics
Combined air and water system
components:

Central air-conditioning equipment

Duct and water distribution systems

Room terminal units

36. 36
In-Room Terminal Systems: Main
Characteristics
“Primary” or ventilation air provided either centrally or locally

Central air: handles indoor air quality requirements and latent
loads (humidity)

Centralized maintenance and lack of moisture condensation in
the terminal units

Requires ductwork but typically smaller than all-air systems

Air can be supplied through the terminal unit or separately from
it

Moisture addition/removal can be accomplished centrally
Local air: handles all conditioning requirements locally
through building apertures

Eliminates requirements for ducts since outside air is introduced
through terminal unit

Increase in maintenance costs due to outside air handling
“distribution”

Generally lower first costs

37. 37
In-Room Terminal Systems: Main
Characteristics
Water supplied to local coils (“secondary water”) used to
provide additional conditioning beyond that which is done
by the central air

Usually sensible only—latent cooling at the terminal unit would
significantly decrease the life of the unit and could lead to
odors/bacterial growth

Any condensate is typically left in space to get reabsorbed by
room air at some later time (drain usually recommended)

Can be either heating or cooling
Applications

Primarily exterior spaces with mainly sensible loads and no
strict humidity control requirements

Spaces where individual control is important or preferable

Common installations: hospitals, hotels, schools, apartment
buildings, office buildings, research laboratories

38. 38
In-Room Terminal Systems: Main
Characteristics
Potential for heating without the
circulation of air (unoccupied periods,
similar to baseboard heating)
Main categories/arrangements
Induction units
Fan-coil units
Two-, Three-, and Four-Pipe
arrangements
Packaged units

39. 39
Induction Units: Characteristics
(Requires) centrally
conditioned “primary” air
supplied to unit
Uses Bernoulli effect to draws
secondary air through
secondary coil
Requires medium to high
pressure to achieve secondary
air flow
Secondary air flow is simply
room air drawn into unit
(through filter/screen and coil)
Units typically installed
around perimeter of building
near windows
Mixing Box
Zone
r
e
t
u
r
n
a
ir
primary air
induced air
in
d
u
c
t
io
n
u
n
it
humidifier
supply fan
cooling coil
heat
recovery
outside
air
relief air
reheat/recool
coil

40. 40
Induction Units: Advantage
Individual temperature control on separate
thermostats at fairly low cost (biggest advantage)
Ducts and air handling units can be reduced in
size since much of conditioning can be done
locally
Moisture addition/removal, filtering, and outside
air can be done centrally
Space heating during unoccupied hours does not
require fan operation
Components last relatively long (15-25 years) with
limited maintenance (cleaning filters and nozzles)

41. 41
Induction Units: Disadvantages
Usually limited to perimeter spaces, requiring another
system for interior spaces
Primary air flow is constant and flow to units cannot be
shutoff individually (local coils can be shutoff)
Secondary air flow dependent on condition of lint screen
and nozzle
Potentially lower chilled water temperature needed due to
reduce air flow to zones and desire to avoid local cooling coil
condensation
Not appropriate for spaces with high ventilation
requirements since primary flow is reduced
Local moisture sources (open windows, etc.) can cause
unexpected condensation
Higher initial costs and higher operating costs (due to high
pressure requirements) than most air systems

42. 42
Fan-Coil Units: Characteristics
Components: coils, fan/fan motor, filter, insulated
condensate pan/drain, controls/valves, return and supply air
openings
Heating and cooling (though maybe only one at a time)
Forced convection from coil to air using local fan
Outside air locally or (usually) separate from the unit via a
central source (example: high-rise hotels)

Local outside air eliminates balancing problem if windows are
opened

Local outside air not allowed in commercial buildings because
wind pressure changes outside reduce control over outside air

Local outside air systems might require coil freeze protection
Water (hot and/or cold) supplied from central source
Electric heater might be needed for a two-pipe system for
shoulder seasons

43. 43
Fan-Coil Units: Types/Locations
Vertical units

Floor mounted, can also be low profile style

Usually installed at the perimeter under window sill

Low profile units can present maintenance problems
Horizontal units

Ceiling mounted, saving floor space

May use ductwork to distribute air (greater pressure
required at fan)\

Can also combine air with central (outside) air

Maintenance and condensate handling more difficult
though initial costs lower
Chase-enclosed units

Unit is typically floor to ceiling configuration

More likely to see these in hotels and residential buildings

Back-to-back placement with sound treatment

44. 44
Fan-Coil Units: Selection and
Capacity/Control Issues
Common technique is to select a unit that can
meet the cooling needs of a space at the medium
speed setting of a three-speed unit (safety factor
and less noise during most operation conditions)
Sizes can be reduced if outside air handled
separately and introduced at a temperature close
to room air conditions (~21C)/reasonable humidity
Automated control on water flow rate, manual or
auto control on air flow rate
On-off control not recommended (noise/circulation
issues)

45. 45
Fan-Coil Units: Water Distribution
Schemes
General considerations

Pipes refer to fluid pipes entering and
leaving the unit

More pipes increases initial costs

Less pipes requires “changeover”
strategies and may result in lack of
heating or cooling when needed

All can be used with central ventilation

46. 46
Fan-Coil Units: Water Distribution
Schemes
Two-Pipe
One inlet pipe, one outlet pipe (single coil)
Heating and cooling happen seasonally with changeover
from one to the other (problems during intermediate
seasons)
Changeover typically determined by outdoor air
temperature, all zones changeover at same time
Can result in frequent changeovers
Two-pipe changeover with partial or full electric strip
heat can help avoid frequent changeovers by meeting
smaller heating loads
Changeovers not necessary if building is dominated by
either heating or cooling

47. 47
Fan-Coil Units: Water Distribution
Schemes
Three-Pipe
Two inlet pipes, common return pipe (two coils)
Generally not recommended since mixing the
hot and cold water return is a waste of energy
Four-Pipe
Two inlet pipes, two outlet pipes (two coils)
Highest initial cost but best performance of fan
coil units
May include a deadband between heating and
cooling to avoid simultaneous heating/cooling

48. 48
Fan-Coil Units: Advantages
Individual control of spaces/central
water production
Pipes require less space than ducts
(easier for retrofits?)
Potential lack of central AHU may also
save space

49. 49
Fan-Coil Units: Disadvantages
Greater maintenance costs,
maintenance in occupied areas
Multiple condensate drains
problematic
Ventilation may not be uniform or
guaranteed if outside air is provided
locally

50. 50
Packaged Terminal Systems
Similar in concept to fan-coil units
except that cooling provided
locally by “window AC” type unit
Heating can either be through
central water/steam source or
local (electric coil/heat pump)
Advantages/disadvantages
similar to other air/water systems
with additional concerns:
Units have relatively short
(appliance) life
Concerns about outside air and
water leaks into building through
unit
Noise from compressor can be
excessive and variable

51. 51
Summary
Induction Units:

Centrally conditioned “primary” air

Bernoulli effect to draws secondary air through
secondary coil
Fan-Coil Units:

Forced convection from coil to air using local fan

Outside air locally or separate from the unit via a
central source

Water (hot and/or cold) supplied from central
source
Many other system types possible: packaged,
low temperature, evaporative, etc.