HVAC Systems in Schools & Commercial Buildings and Radon

HVAC Systems in Schools & Commercial Buildings and Radon

References 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 pressure. 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, Heat and Ventilation and some times Air Conditioning 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.

HVAC Systems 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 Unit ventilator

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 boundary 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 Ventilation is the introduction of outside air to maintain odors and internal pollutants at acceptable levels. Assume the 15% min. outside air (O.A.) requirement Must have ventilation or O.A. for the HVAC system to provide dilution.

Ventilation effect on Radon Levels 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 the code required ventilation.

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 decrease the accuracy.

Space Pressurization 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 positive Entire return duct has to be at a higher pressure than atmosphere to exhaust air. If the outside air damper opens the pressure in the space will continue to increase

Pressure Profile Supply& Return Room can be either positive or negative To ensure the room is positive the flow of the two fans must be balanced With fan flow control the OA damper can open fully without over pressurizing the room.

Economizer Operation 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 100%

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 Multizone Variable Air Volume (VAV) Unit ventilator Exhaust and heating systems

Multizone System Constant supply flow Temperature sensor in each zone Temperature control dmprs located at main unit A separate duct is routed to each zone. Simple ventilation and pressure control

VAV System 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

Unit Ventilators Very common in schools May or may not have outside air make up May pressurize room if O.A. makeup is operating If ventilation is provided may have exhaust fan

Rooftop units 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

Heating Systems Combustion air supply duct. Combustion air Damper in duct interlocked to boiler Boiler combustion air intake

Heating System 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 Temperature control dampers (multizone)

HVAC System operated normally 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.

Summary 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