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By Salas O’Brien Engineers, Inc., San Jose, CA

With the Statewide K-3 Class Size Reduction Program, the number of portable classrooms in California has increased dramatically. Many of these portables were brought in quickly to satisfy the classroom size requirements. Some have been in place for a number of years and may not have been originally designed for full time classroom use.

Often because portable buildings are considered temporary, they do not get the same HVAC system design consideration that a permanent school building would.

Some of the criteria that school districts should consider are summarized as follows:

1. Noise: The typical portable classroom uses a wall-mounted heat pump. The return air contains no ductwork to separate the unit from the classroom (In some instances, a student's head may be right in front of this return air opening). As such, noise generated by the unit is transmitted to the classroom. In many instances, teachers turn off the heat pump rather than to talk over it. When this happens, the classroom gets no ventilation.

2. Exhaust Air: Portables often are built with no exhaust air or even relief air provisions. It becomes difficult for the air conditioning unit to bring in fresh air when no means is provided for exhaust air. In this scenario, because of the energy code requirements to keep the building of tight construction, the room is pressurized and does not provide an outlet path for the fresh air.

3. Undersized equipment: Portables may be intended to be used as offices then later changed to classroom use. This may result in maintenance technicians minimizing outside air ventilation to enable the unit to maintain temperature setpoint on hot or cold "design days". The system needs to be re-evaluated when a change in use occurs. In some cases, portables are converted to computer rooms with no changes to the HVAC system.

4. Economizers, which can provide for more than minimum outside air when conditions are beneficial (cooler outside than inside when unit requires cooling), are poorly designed for the typical heat pump. Heat pumps typically use the condenser section as the exhaust section in economizer mode. This exposes the condenser fan to the classroom and creates more of a noise concern than normal operation. The typical economizer control also does not allow for "integrated economizers" (integrated economizers allow both economizer and cooling simultaneously). As such economizers are rarely utilized in the portable application.

5. Excessive duct leakage: After a period of years, settling and often movement of the portable building, excessive duct leakage from loose connections occurs (duct tape is often used in joints which dries out over a period of years). This leakage precludes ventilation getting into the classroom.

1. Exhaust fans were added to all rooms. The exhaust fans were controlled by a dual technology occupancy sensor, so that anytime the room was occupied there would be proper ventilation. As an added feature, the lighting included with the occupancy sensor to achieve energy savings.

2. Outside air ventilation was increased from approximately 100 cfm (some even lower) to 525 cfm.

3. In a few portables the HVAC unit was replaced, as it was older and the noise generated was substantially more than in the other portables.

4. Several other units were replaced as they were undersized (they were changing from 3 ton to 4 ton units). New ductwork and supply grilles were installed on these units.

5. Two units to had the return modified to include a sound plenum.

6. Supply ducts on all existing systems were repaired which increased total supply air by approximately 20% on the worst units.

As is the case in any classroom, portables should be designed to meet ASHRAE Std. 62-1989 ventilation standards. The system must be commissioned to ensure that the ventilation standards can be met continuously. The costs to this project were not prohibitive. It is in the best interests of the students, teachers and staff, if all districts consider these upgrades where problems of this nature are identified.