Heating Ventilation Air Conditioning
(HVAC) systems
Prof. Ing. Piercarlo Romagnoni
Dipartimento di Progettazione e Pianificazione in Ambienti
Complessi
Università IUAV di Venezia
Dorsoduro 2206 – 30123 Venezia
pierca@iuav.it
HVAC Energy-Efficiency Principles
• Circulate only the amount of air needed for
ventilation, and only when needed, while circulating
hot or cold water for most of the heating and cooling
(recall: energy required to move air or water varies
with flow rate cubed, and ~ 25-100 times less energy
is required to deliver heat via water than via air)
• In other words, separate, if possible, the
heating/cooling and ventilation functions
• Separate cooling from dehumidification functions
using solid or liquid desiccants, with the desiccant
regenerated using either waste heat from
cogeneration (entailing ~ 0 sacrificed electricity
because temperatures of only 50-65ºC are needed)
or using solar thermal energy
• Distribute heat at the coolest possible temperature
and coldness at the warmest possible temperature –
in both cases by using large radiators (such as radiant
ceiling or floors)
• Allow the temperature maintained by the HVAC
system to vary seasonally (allowing temperatures of
up to 28-30ºC on the hottest days)
HVAC systems in residential buildings
• If super-insulated, heat from the airflow at the rates
required for ventilation only is often sufficient (with
perhaps supplemental radiant heating of floors or
towel racks in bathrooms)
• Otherwise, use radiant floor heating or large wall
radiators (water at 30ºC will be plenty warm enough
in super-insulated buildings)
• Mechanical ventilation with heat recovery via a heat
exchanger when windows need to be closed
• Variable-speed drives on ventilation fans
Large wall-mounted radiator in a daycare centre in Frankfurt – an
inexpensive alternative to radiant floor heating
Source: Danny Harvey
Heat Exchangers
• Transfer heat from a warm air or water flow to a cold
air or water flow
• Do so by maximizing the surface area between the
two fluid flows
• This can be done either with one tube inside
another, or through a series of plates
Counterflow flat plate heat exchanger
Source: Bower (1995, Understanding Ventilation: How to design, select, and install residential
ventilation systems, Healthy House Institute, Bloomington, Indiana)
Crossflow flat plate heat exchanger
Source: Bower (1995, Understanding Ventilation: How to design, select, and install residential
ventilation systems, Healthy House Institute, Bloomington, Indiana)
Residential heat exchanger (as part of a
mechanical (fan-driven) ventilation system)
Source: Danny Harvey
How to control the mechanical air flow (residential) :
Single air flow: variable or fixed flow rate
Single flow
Mechanical ventilation
Double flow
How to control the mechanical air flow (residential) :
Duoble flow with heat recovery
How to control the mechanical air flow (residential) :
Double flow with heat recovery system
HEAT RECOVERY SYSTEMS
- Crossed flows
- Rotative
Heat recovery
The performance of a heat exchanger is measured by its “effectiveness”,
which is defined as
(Tsupply-Tincoming)/(Toutgoing-Tincoming)
Heat exchangers for commercial buildings have an effectiveness of 60-80%,
meaning that 60-80% of the temperature difference and hence heat content
difference between the incoming and outgoing air can be added to the
incoming air rather than sent outside.
Residential heat exchangers have an effectiveness as high as 95%
However, adding a heat exchanger increases the fan energy required to move
air (since it adds resistance to air motion) – so it should be bypassed when
there is little difference in the temperature of inside and outside air
Heat recovered from exhaust air
Climatic zone E, DD = 2400, the system operates for 14 h/day with
efficiency 60%, it is possible to evaluate the energy saved from
R = 7 · V
kWh/year
whereas the energy requested for fans is:
M = 0,2 · V kWh/year
(V
in m3/h)
(V
in m3/h)
Fans
Do not cool the air, they only make the air feel cooler
They in fact add heat to the air
Thus, they should be turned off when not in use
They only save energy if people set the thermostat
on the air conditioner to a higher temperature (or
dispense with the AC altogether)
• Most are incredibly inefficient (only 4-12% of
electrical power input ends up moving air)
•
•
•
•
Fans: performance curves
An aerodynamic ceiling fan (36% efficiency at high speed vs 12% for typical
fans, where efficiency = power imparted to air flow divided by electrical
power)
Source: Florida Solar Energy Center
Recent conventional
HVAC heating systems
• The fans operate at a fixed speed, tending to
circulate a fixed quantity of air all the time
• The air is either overcooled centrally, then reheated
electrically by the required amount just before
entering a room, or
• The air flow is throttled to prevent overcooling (but
usually some rooms end up too warm and others too
cold)
New HVAC systems
• Will use variable speed fans – with the airflow rate
varying according to a fixed schedule. Savings of 5060% in overall HVAC energy use have been achieved
from this alone
• As the airflow is still much more than required for
ventilation purposes, 80% or so of the air will be
recirculated on each circuit and blended with 20%
outside air,
• This saves energy compared to venting 100% of the
air to the outside and completely replacing it with
fresh air that needs to be cooled and dehumidified
or heated and humidified
However, we can still do much better
• If heating and cooling are largely provided through radiant
floor or ceiling panels, then the airflow can be reduced to
just that needed for ventilation (fresh-air purposes)
• Having reduced the airflow to that level, it can be entirely
vented to the outside and replaced with 100% fresh air on
each circuit (this is called a Dedicated outdoor air supply, or
DOAS, system) without wasting energy
• This gives better indoor air quality and saves energy
- reduced fan energy use
- heat picked up from lights at the ceilings is directly vented
to the outside rather than having to be removed by the
chillers before 80% of the air is sent through the building
again
We can also do much better in the way that the ventilation
enters
in
and
passes
through
a
room.
The ventilation air typically enters a room from some outlet in
the ceiling or in a wall and mixes turbulently with the room
air, relying on dilution to remove air contaminants. This
requires greater air flow (and recall, required fan power
increases with air flow rate almost to the third power) but is
not very effective in providing good air quality.
Two essential elements of highly-efficient HVAC
systems in commercial buildings can be:
• Displacement ventilation
• Chilled ceiling cooling
Chilled ceiling cooling
• Our perception of temperature depends roughly
50:50 on the air temperature and on the radiant
temperature (the temperature of the surroundings,
which are a source of infrared radiation on our
bodies)
• A nice sensation of coolness is achieved if the ceiling
is cooled to 16-20ºC by circulating water at this
temperature through panels attached to the ceiling
• The result is a much higher chiller COP than
conventional cooling systems (which use water at 68ºC) and warmer permitted air temperature
Chilled Ceiling cooling panels
Source: www.advancedbuildings.org
Because the ceiling panels need water cooled down to only
16-20ºC, and the cooling tower almost always produces water
at this temperature, the cooling tower water can be directly
used in a chilled ceiling cooling system most of the time –
providing yet further energy savings
Displacement ventilation
• Ventilation air is introduced from vents in the floor at
a temperature slightly below the desired room
temperature
• The air is heated from internal heat sources and rises
in a laminar manner, displacing the pre-existing air,
and exiting through vents in the ceiling
• 40-60% less airflow is required than in a conventional
ventilation system (which we assume to be already
reduced to the flow required for air quality purposes
only)
Displacement ventilation floor diffuser
Source: Danny Harvey
Displacement ventilation
It’s really true? Only if the heat source is active
Because the airflow has been reduced to that needed for
ventilation purposes only (with most of the cooling done with
chilled ceilings), 100% of the (much reduced) airflow must be
vented to the outside and replaced with fresh outside air on
each circuit. As previously noted, this forms a dedicated
outdoor air supply (DOAS) system.
It is healthier because air is not recirculated from one part of
the building to another, and saves energy because internal
heat that is transferred to the air is directly vented to the
outside, rather than passing through the chiller when the air is
recirculated
Displacement ventilation: limits
from REHVA guidebook n.1 “Displacement ventilation”
Displacement ventilation: limits
DR  34  t a   v  0,05 
0 , 62
 0,37  v  Tu  3,14 
From REHVA guidebook n.1 “Displacement ventilation”
Displacement ventilation: limits
From REHVA guidebook n.1 “Displacement ventilation”
A further efficiency measure is to vary the airflow based on
human occupancy (as determined by CO2 sensors). This gives
a demand-controlled ventilation (DCV) system (this is now
required by the California building code for high-density
buildings). DCV alone can save 20-30% of total HVAC energy
use.
Underfloor ventilation
Comfort
UNI EN 15251 requires that the ventilation flow rate can be
calcluated by means of two different contributions:
- Ventilation flow rate required by occupants (bio effluents) qP;
- Ventilation flow rate ventilation for the pollution from the
building and systems qb.
The whole flow rate is calculated as flollows:
qtot = n qP + A qb
A = superface of the building [m2];
n = nr people [-]
qP as in UNI EN15251
qb as in UNI EN15251 for different categories
UNI EN 15251
Alternatively, tables list for different categories the
recommended ventilation rates expressed either per person
or per square meter floor area.
To summarize, the most energy-efficient building will have
• Optimal orientation and form
• A high performance envelope
• Capacity to use passive ventilation and cooling whenever
outdoor conditions permit
• Demand-controlled, displacement ventilation that, of
necessity, will be a DOAS system
• Chilled ceiling cooling
• Desiccant dehumidification using either waste heat from
cogeneration (ideally supplied by a district heating system) or
using solar thermal energy
• Heat exchangers to transfer heat or coldness from the
outgoing to the incoming air
• High efficiency equipment, correctly sized and commissioned
Exhaust air is overheated by passing through a sort of
solarium, then passes through a heat exchanger to heat the
incoming fresh air to a greater extent than would be possible
with a conventional heat exchanger system. And unlike
systems for passive solar preheating of ventilation air, we still
get the benefit of heat recovery on the exhaust air at night
Air temperatures during flow through solarium
and heat exchanger
Source: Ursula Schneider, Pos Architekten, Vienna