HVAC Systems in Schools &
Commercial Buildings and
• US EPA Radon Prevention in the Design and
Construction of Schools and Other Large Buildings
– http://www.epa.gov/ordntrnt/ORD/NRMRL/Pubs/1993/air/ EPA625r-92016.htm
• US EPA HVAC Systems in the Current Stock of
U.S. K-12 Schools (PB92-218338)
Relationship between HVAC
System and Radon
…pressure-driven transport is by far the most
common way radon enters a building…
• HVAC systems in schools and other large
buildings play a significant role in building
• HVAC systems which provide outside air
make-up will reduce the radon concentration
in the areas they serve.
What is an HVAC system?
• A system that provides,
• All schools and commercial buildings in Illinois
must be heated and nearly all local building codes
have ventilation air requirements.
• The focus of this seminar will be all air HVAC
systems. This type of system uses air as the means
of both heating and cooling
• Heating systems systems that don t have a fan that
supplies air to the the rooms of the building are not
covered by this presentation.
• Basic Central System and Functions
– Basic system
– Two types built-up or package units – Single zone
– Pressurization – Economizer
• Most Common Types of Central HVAC Systems
– Multizone – VAV
Basic HVAC System
• Features include
– Outside Air (O.A.), exhaust and return dampers & Filter – Heating coil (may also have a cooling coil)
– Supply fan (constant flow rate and may also have a return fan) – Temperature sensor in room
Built-up HVAC Systems
• Built up units use the
buildings structure to form some parts of the HVAC system
• These systems may use 100 to 400 sqft or more of below grade floor and wall as part of the pressure
• The walls and the floor on the suction side of the fan can be –1.0 to –2.0 w.g. relative to atmosphere
Package HVAC Systems
• Package units are built with a sheetmetal exterior and are not directly connected to the building structure
• Unit can still affect the
mechanical room pressure if :
– Doors are open on fan suction – Mis-positioned outside air or
return air damper
• Mechanical room pressure can be –0.05 to –0.15 w.g. relative to atmosphere
• Ventilation is the introduction of outside air
to maintain odors and internal pollutants at
• Assume the 15% min. outside air (O.A.)
• Must have ventilation or O.A. for the
HVAC system to provide dilution.
Ventilation effect on Radon
• Doubling the ventilation rate or %O.A.
makeup will result in a 50% reduction in
Radon concentration in the space served.
• Increased ventilation will also significantly
increase the cost to heat and or cool the space.
• The goal is to have the system operate with
Quick measurement of % O.A.
• Measure the temperature in the return, O.A. and the mixed air.
• Using the equation below calculate the % O.A.
• This method works best when O.A. temp below 40F and becomes less
accurate the closer the the temperature get to each other.
• Also if the mixed air is not well mixed it will
• HVAC system with supply fan (no return)
– Achieve pressurization with simple controls
• HVAC system with supply and return fan
– Need much more complicated control system to
consistently maintain pressure
Pressure Profile Supply Fan only
• Room is always
• Entire return duct
has to be at a
than atmosphere to
• If the outside air
damper opens the
pressure in the
space will continue
Pressure Profile Supply& Return
• Room can be either
positive or negative
• To ensure the room
is positive the flow
of the two fans must
• With fan flow
control the OA
damper can open
fully without over
• Provides free cooling when OA is colder than about 55F.
• In this mode the RA, EA & OA dampers
modulate to maintain a 55F to 60F setpoint. • In the economizer
mode OA makup will vary from min. to
Economizer Effect on Radon
• May cause significant reductions in concentration as system modulates from 15% OA to 100% OA.
• If the concentration were 20 pCi/L at 15% the
concentration at 100% OA could be less than 4 pCi/L • To ensure consistent readings the HVAC system should
not be operating in in the economizer mode while measurements are being made.
• The potential exists for a system to be operating in the
economizer mode whenever the temperature is below 55F but most systems go to minimum O.A. when heating.
Types of HVAC Systems
• Most common types of HVAC Systems
– Variable Air Volume (VAV) – Unit ventilator
– A separate duct is routed to each zone.
– Simple ventilation and pressure control
– Constant supply flow
– Temperature sensor in each zone – Temperature control dmprs located
• Variable supply flow
• Temperature sensor in each zone • Room VAV box damper varies flow
to control room temp.
• As VAV box dampers close, supply flow is reduced & vice versa when the boxes open
• Pressurization and ventilation controls more complicated
• Very common in schools
• May or may not have outside air make up
• May pressurize room if O.A. makeup is
• If ventilation is
provided may have exhaust fan
• Main difference between a roof top unit (RTU) and other HVAC systems is its location. Instead of the
mechanical room it is on the roof
• Roof top units can be
single zone, multizone or VAV systems with all the same capabilities
Exhaust Air Systems
• Spaces served by exhaust systems will operate at a
slightly negative pressure to atmosphere.
• These systems are used to remove odors or fumes
from the conditioned space (restrooms & kitchens)
and this air must be exhausted to outside
• Ventilation air from the associated HVAC system is
provide to prevent the room from getting too negative
• In most cases exhaust fans should be shutdown when
main HVAC system is shutdown because these spaces
usually operate at less than atmospheric pressure and
the make up air is not being provided
Combustion air supply duct. Combustion air Damper in duct interlocked to boiler
• Boiler rooms will operate at negative pressure relative to outside. The causes include:
• Combustion air drawn from outside or other parts of building
• Stack effect created by the boiler
• Domestic hot water heaters and gas fired unit heaters have a similar effect.
• The negative pressure will exist when ever the heating equipment runs.
HVAC System Operation
• Basic system has constant air volume (CAV)
• Smaller systems (particularly single zone)
– Cycle on/off like residential systems
– May not have outside air or exhaust openings
• Large systems
– Run continuously during occupied hours
– Provide outside air makeup
• Illinois regulations Section 422.130 Measurement Protocol, f.4 School and Commercial Buildings Measurements states that during testing ..the HVAC system shall be operated normally.
• The greatest challenge in evaluating an HVAC system is
that you can t see the air and rarely can you feel the pressure • So how can you tell if it is operating normally?
– Unless you have a lot of experience with HVAC system you can only ask someone who is familiar with the system
– If there is no one familiar with the operation of the system contact an an experienced mechanical contractor or engineer
– The one condition I would not consider normal is if the system is operating in economizer. In this mode both building pressure and ventilation can be dramatically higher than normal.
• HVAC systems can effect Radon concentrations by
changing the pressure of a space or by dilution
– Maintaining a slight positive pressure in the grade or below grade spaces can reduce or eliminates Radon transport into building while the system is operating
– Built-up systems can create negative pressures increasing radon transport
– Ventilation will reduce the radon concentration in the spaces it serves