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In many large commercial buildings, central-station air handlers are used to maintain occupant comfort. Often, these units include return fans, which draw air from occupied spaces for recirculation or exhaust. Commonly, the return fans are controlled with variable-frequency drives VFDswhich receive a speed signal based on building differential static pressure—the difference between interior-space static pressure and outside-air static pressure.
Building differential static pressure commonly is measured with a sensor with two ports: Tubing extends from both ports. Commonly, low-port tubing is extended to the outside of a building, while high-port tubing is extended to an interior space.
The sensor determines the difference in static pressure between the two ports, which it can report to a building-automation system. The recent retrocommissioning of two office buildings—one 25 stories andsq ft, the other 24 stories andsq ft—served by central-station variable-air-volume air handlers in California revealed substantial opportunities for optimizing pressurization-control systems in large commercial buildings.
This article will discuss two such opportunities: In both retrocommissioned buildings, the low port of each pressure sensor was open to the control panel in which the sensor was installed, not extended to the outside via tubing. In one building, the control panel click in the penthouse mechanical room, which was open to the outside via boiler outside-air-intake louvers.
He made sure I understood the process which gave me much relief. In these airtight homes, not much air enters or leaves on its own, so the ERV is needed to maintain good indoor air quality. They are sawing wood and so there is sawdust in the air but the building thinks there is no problem. The sensor determines the difference in static pressure between the two ports, which it can report to a building-automation system.
While that location was good for minimizing wind effects and protecting the sensor from rain, readings likely were not representative of true outside-air static pressure because the panel was in a room containing equipment boiler burner fan, exhaust fans, etc.
In the other building, the control panel containing the sensor was located in the return-air plenum, on the discharge side of the return fan. At this location, with the low-side port open to the panel and, thus, the plenum, the pressure sensed by Atlanta Speed Hookup Companies That Test Air Quality low-side port was higher than the pressure sensed by the high-side port.
There was intent to extend the tubing to the outside, as evidenced by a hole in the adjacent exterior wall, but the tubing never was installed Photo A. The termination of low-port tubing is important, as wind can have a significant impact on measured outside-air pressure. For example, a mph outside breeze translates to wind pressure of 0.
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This light breeze can have a significant impact on pressure control, as pressurization systems typically operate to maintain a slightly positive building pressure in the same range 0. Many outside-air-pressure-sensing devices are available for installation at low-port-tubing terminations. Designed to minimize the effects of wind speed, these devices typically consist of two plates, with the sensing element in the middle of the inside surface of one of the plates. The building-pressurization-control scheme for the two retrocommissioned buildings is similar to the one employed in many buildings with central-station air-handling systems: Return-fan speed is varied to maintain a set differential between interior-space and outside-air pressure.
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The two buildings also employ an economizer-damper-control scheme that is fairly typical: The economizer-damper controller sends a signal to control outside- return- and relief-air dampers, with the return-air dampers operating opposite of the outside- and relief-air dampers Figure 1. These control schemes can cause performance issues because the control sequences overlap somewhat: The relief-air damper operates based on the economizer-damper signal temperature- or enthalpy-based controlbut is an integral part of the building-pressurization-control scheme pressure-based control.
For example, on a past retrocommissioning project, the pressure in the main air handler's mixed-air plenum was positive during minimum-outside-air operation.
Normally, a mixed-air plenum should be at a negative pressure to draw in outside air.
Instead of outside air being drawn in, return air was being exhausted through the outside-air damper; in other Atlanta Speed Hookup Companies That Test Air Quality, the system was operating in percent recirculation mode.
This issue was attributable to oversized return-air dampers—typically, return-air dampers should be smaller than outside- and relief-air dampers in return-fan systems 2 —and the overlap of temperature- and pressure-control strategies. A better method may be to control the relief damper to maintain building differential pressure and control the return fan to maintain return-air-plenum pressure Figure 2.
The outside- and return-air dampers would continue to be controlled based on the economizer cycle. Converting a control system from the arrangement in Figure 1 to the arrangement in Figure 2 would require minimal capital outlay, as no source system modifications beyond the addition of a pressure sensor and related points and the modification of control sequences would be required.
Depending on the as-found performance of a system, such a conversion could result in reduced energy use through return-fan-speed reduction and increased indoor environmental quality through adequate outside-air intake.
Another possible improvement involves the use of a relief fan instead of a return fan. A relief fan is located downstream of the return-air plenum, drawing air from the plenum and pushing it through the relief-air damper Figure 3. The relief fan should be controlled to maintain building pressurization, not used as part of an economizer-control scheme. Typically, it will not http://24dating.me/ha/a-love-to-last-january-17.php to operate during minimum-outside-air mode.
In buildings utilizing return fans, changing to a relief-fan system may not be feasible because of space constraints and cost.
In buildings with a relief-fan arrangement already in place, an investigation of system performance is worthwhile; as with a return-fan system, check the low-port-tubing termination and control sequences. For an existing building with a direct-measurement control system already in place, this may not be feasible because of the space required for proper installation of airflow-measuring stations. Following are indicators of a possible issue with a direct-measurement building-pressurization-control system's operation:.
Many outside-air-pressure-sensing devices are available for installation at low-port-tubing terminations. Another possible improvement involves the use of a relief fan instead of a return fan. The baby is in the hospital with a breathing problems.
From the Fundamentals to the Field. Many large commercial buildings utilize direct-measurement building-pressurization-control systems.
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Focusing on commercial-building energy efficiency, he manages the technical aspects of utility retrocommissioning programs, leads in-building retrocommissioning projects, and conducts research. Did you find this article useful? This article discusses two methods of optimizing pressurization control in large buildings: More information about text formats. Text format Comments Plain text. Web page addresses and e-mail addresses turn into links automatically.
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