Demand Control Ventilation for Modular Classrooms in Arizona
By Kevin Estepp
Mesa Public School District #4 is the largest school district in the state of Arizona. It boasts over 70,000 students in over 100 campuses scattered over 200 square miles. Because of rapid growth, Mesa Schools, like many other districts across the nation, were forced to utilize modular classrooms for two reasons. First they could be built faster than brick and mortar buildings. Secondly, since the population growth outpaced the funding for new buildings, less expensive modular classrooms became the only viable choice.
The use of modular classrooms, however, came with another price—indoor air quality (IAQ). It seems that during two decades of growth, the modular classrooms were provided with little, if any ventilation. Ventilation provides the necessary fresh air to dilute the indoor air pollutants to an acceptable level for but modular classrooms were initially purchased before “Indoor Air Quality” was even a phrase. As complaints mounted, David Peterson, director of facilities at Mesa Public School District #4, looked for a solution. All of the IAQ seminars he had attended taught that “dilution is the solution to indoor air pollution.” But the solution for dilution was not so obvious.
Increasing the ventilation on existing equipment presented a whole new set of challenges:
Simply adding more ventilation to the existing HVAC equipment would exceed the unit’s BTU capacity sacrificing both comfort and kilowatts during design ambient conditions.
Upsizing all HVAC equipment to compensate for the increased ventilation load would also involve upsizing ductwork and electrical power supply making this option much too expensive. It would also result in oversized equipment for milder weather conditions causing compressor short cycling, poor dehumidification and refrigerant flood back to the compressor.
Energy recovery products (ERVs) had been developed to reduce the increased thermal load entering the ventilation system. However, they are of no value during mild weather conditions and over-ventilate when the building is not occupied to full capacity. In addition, the ERVs require periodic cleaning. The district didn’t need more equipment to maintain with an already strained staff.
Demand ventilation controls were available but the units didn’t have any dampers to control. Since the units were small tonnage, operating on 230 volt AC, single phase power, they were considered by the manufacturers to be a residential application and did not have an available economizer option.
What’s the solution for dilution?
David Peterson had to decide what to do with the ventilation needs of nearly 800 classrooms and the district could not afford to implement the wrong solution 800 times over. The engineering community was pointing him towards an ERV solution but he wasn’t completely comfortable with that based upon the maintenance required and over-ventilation when classroom occupancies are low. Peterson decided to set up a test site. Patterson Elementary had eight new modular classrooms. He replaced the existing units with a new 4-ton AC unit on each classroom to make the evaluation equal and installed a different ventilation solution for each one. The solutions included ERVs from several different manufacturers using both fixed core heat exchangers as well as rotating wheels. Other applications included a simple fixed damper fresh air intake, and a custom split DX unit manufactured in Milwaukee specifically for this project. Then there was a new, unheard of, ventilation module called a VentPak™ ventilation control system that used CO2 concentrations to modulate the fresh air intake to the unit.
Mesa Schools installed an electronic monitoring system to record temperature, humidity, CO2 in each classroom and energy consumption on each unit. After one year of daily monitoring, the results were in…The VentPak ventilation system was selected as the best overall solution.
The VentPak™ ventilation control system is a self-contained demand-control ventilation module consisting of a filtered fresh air intake damper assembly and control panel that couples directly to the return air portion of the HVAC unit or ductwork. Inside the control panel is a full modulating damper actuatordirect-coupled to a patented two-stage damper, a DDC controller and an outdoor temperature sensor. The VentPak system also includes a special wall sensor to replace the existing thermostat. The wall sensor includes CO2 and temperature sensors as well as a push button override and temperature adjustment lever. Essentially, the VentPak provides the missing damper assembly, CO2 control, economizer functionality, and DDC HVAC controls in a pre-programmed, plug-and-play package. The VentPak does more than simply control outside air flow, it also controls the sequencing of compressors and heat stages. The VentPak system was designed to be a universal retrofit device so it would couple directly to any rooftop package unit or even the wall-mounted “Bard” style units that are often found on portable classrooms. Since the VentPak system only consumes 4 watts of power, it utilizesthe unit’s existing 24-volt transformer for its power supply. There are no additional electrical supply requirements, like the ERV options, and no roof penetrations. The VentPak offered the lowest installed cost and was maintenance free.
VentPak installed on Trane rooftop a/c unit
VentPak installed on wall-mounted a/c unit.
Not only did the VentPak cost less and save more energy than all of the other options, but it could be used on virtually any unit in the district. Mesa School District could implement a standard ventilation solution and get economizer functionality and DDC controls as well. The DDC controller is capable of time scheduling, holiday scheduling, trend logging, alarming and remote access. The ERV’s didn’t provide any of these benefits.
The VentPak™ system worked well when the classroom was not fully occupied or ambient conditions were not extreme but what about the worst case scenario? The VentPak algorithm allows the outside air damper to modulate toward the closed position when the room temperature deviates more than 2 degrees from set point. This allows the unit to temporarily compromise the ventilation in favor of thermal comfort. This condition is estimated to occur less than .2% of the total annual run time. It should be noted that even during this “extreme ambient reset” cycle, the damper is never allowed to fully close in order to provide minimum ventilation and positive building pressure at all times.
What about CO2 sensor drift? The VentPak system uses a pre-occupancy purge cycle for two reasons. First, it is a good IAQ procedure to flush built up pollutants in the space prior to occupancy. Secondly, the VentPak marks the CO2 level at the end of the purge cycle and records it as the outdoor CO2 level for that day, assuming that indoor and outdoor CO2 levels will be in equilibrium after the space is purged. From there, the controller maintains the space CO2 level as a delta between indoor and outdoor levels, in compliance with ASHRAE standard 62. This means that if the sensor happens to drift out of calibration, the operation of the system remains unaffected because the same sensor is used to measure both indoor and outdoor levels. 700ppm delta is still 700ppm delta even if the sensor has drifted.
The VentPak has a patented “two-stage damper” that restricts the air intake during the first 50% of the stroke to allow more resolution when controlling at low air flows, solving a common problem associated with applying CO2 controls to economizer sections. As the damper travels past the restrictor plates during the last 50% of the stroke, the air bypasses the restrictor plates to allow full air flow through the VentPak for economizing.
The VentPak system worked so well, in fact, that the district also decided to use them on new and existing brick-and-mortar buildings as well. On these buildings, the district took advantage of the VentPak’s open LonWorks™ communications protocol and tied them into the district’s standard LonWorks™ control network. LonWorks technology, developed by Echelon Corporation, is an open, interoperable communication protocol that allows the district to add VentPaks, and other LonWorks-based devices, to existing control networks regardless of who originally installed the system. The non-proprietary nature of the LonWorks™ network gave the district flexibility to choose “Best-of-Breed” products to integrate into their control systems without the need for cumbersome gateways used to translateproprietary communication languages. Without the use on LonWorks technology, the VentPak solution would have been difficult, if not impossible, to implement in many of their buildings.
The VentPak is an electronic solution that solves operational issues. ERV’s are a mechanical solution aimed at reducing peak loads but typically leave these issues unresolved:
ERV’s work against the cooling load during economizer conditions. That is, when the classroom is warm and humid from little hyperactive bodies, instead of replacing the indoor air with cooler, dryer air, the ERV’s would recycle the warm, humid air back into the classroom. They worked fine when the system was operating at design conditions and the classroom was fully occupied but those conditions only accounted for a small fraction of the overall operating time during the school year.
Because there was no provision available for holiday scheduling, all the other units, except the VentPak system, continued to run during all of the holiday breaks year round, including the summer!
With the exception of the VentPak system, there was no way to ensure that the fan ran during occupied hours to provide ventilation. Once the teacher turned the fan off, ventilation ceased.
Demand controlled ventilation is self adjusting to compensate for filter loading, duct leakage and open doors. ERV’s are not.
ERV’s require energy to save energy. The heat transfer efficiencies for ERV’s assume a clean heat exchanger media and do not include the added fan horsepower required to overcome the addition pressure drop of the media.
IAQ vs. energy cost
One of the major problems when attempting to provide adequate ventilation is that bringing in more outside air usually means higher utility costs which also adds KW load to an already over-taxed power grid. This was not the case with the VentPak system, as Mesa School District was awarded the Arizona Governor’s Award for energy conservation as well as the EPA’s highest honor for indoor air quality as part of the “Tool for Schools” program. David Peterson estimates that the district has reduced energy consumption by 22% at the facilities where the VentPak system is being employed. This savings was significant considering the VentPak system actually saved energy while adding fresh air into classrooms while contending with the extreme Arizona climate.
There are applications for ERV’s where their performance exceeds any CO2-based solution. However, in a single-zone, small tonnage application with fluctuating occupancy, the VentPak proved to be the solution for dilution.
The VentPak™ system is manufactured by Dilution Solutions, Inc. and distributed by Mechanical Products BAS, Phoenix, Arizona 85009. For information, please contact Kevin Estepp (602) 233-0404 or www.mpbas.com .