a. Select and layout supply-air outlets and exhaust and return-air inlets in accordance with current ASHRAE Guidelines and acoustical requirements.
4. Fans
a. Select fans to minimize noise and to meet noise level criteria in occupied spaces.
b. Provide direct drive fans for smaller exhaust fans of the centrifugal roof exhauster, ceiling, inline or cabinet type.
c. Special Exhaust Fans: Exhaust from kitchen hoods, fume hoods, kiln hoods, spray booths, and dust and sawdust collection systems require special attention to construction details, explosion hazards, noise and location.
1. Roof fans handling exhaust from kitchen hoods require a shaft seal and a special insulated plate to separate fan from
motor compartment.
2. Fans exhausting fume hoods require spark resistant construction, and special coatings to prevent chemical action on fan and housing. The motor shall be explosion proof and located outside of the air stream.
3. Fans exhausting paint spray booths require spark resistant construction. The motor shall be explosion proof and located outside of the air stream.
April 2007 13 3.6 HVAC d. Pre-fabricated duct collection systems shall be used for removal of saw
dust in wood shops.
e. Exhaust from kiln hoods requires special fans with force-vented motor compartment and special construction to withstand high temperature. 5. Economizers for Outside Air
a. General:
1. The California Energy Commission prescribes 100% outside-air economizers for equipment with supply-air capacities over 2,500 cfm or 6.25 tons cooling capacity. That is the usual low limit for satisfactory payback at locations where weather conditions are least conducive to 100% outdoor air economizer operation with fan- assisted relief. For units of 7 ½ tons capacity and higher, which are usually used for administrative areas, multi-purpose rooms and gymnasiums, the District requires outside-air economizers and recommends power-exhaust systems.
2. The District also prefers 100% outdoor-air economizers for classroom small rooftop unitary systems (3, 4 & 5 Ton capacities) to achieve energy savings and to more easily comply with CHPS Energy Prerequisite 1 and IEQ Credit 4.2 (both for the ability to more effectively flush out the building prior to occupancy and for the increase in fresh air during the economizer cycle). However, they are to be used with gravity relief of exhaust air, not with power exhaust systems. Small split systems are not required to be provided with 100% outdoor air economizer systems. Do not provide 100% outdoor air economizers when the outdoor air quality is low or when the outdoor ambient sound level is higher, such as for schools that are very close to a freeway. Also, economizers may be omitted when the CHPS Energy Prerequisite could be satisfied through other measures and the incorporation of gravity relief openings into the Architecture imposes substantial hardship.
3. Power exhaust is not recommended for small unitary systems for several reasons. First is capital cost and energy-cost savings:
a. The maximum cost of a typical 4-ton rooftop gas-electric HVAC unit is about $4,000 (plus installation). The cost of a power exhaust accessory for these units is about $4,000. This is a substantial increase in construction cost per classroom, and the energy savings will be substantially less because of the energy consumed by the exhaust fan.
b. The cost of a gravity-relief economizer is about $700, so the cost justifies the benefit.
4. In addition, the noise generated by the power exhaust fan adds to the classroom sound level.
5. Further, the differential pressure sensors in a power exhaust system require frequent calibration, increasing maintenance costs.
April 2007 14 3.6 HVAC
b. Design Criteria for Economizers and Gravity Relief Systems:
1. For small rooftop unitary systems, provide units with downward duct discharge, and with manufacturer-installed and warranted economizer equipment.
2. The total pressure drop through the relief system shall not exceed 0.075” water gauge.
3. Relief Louver size: The pressure loss through the louvers should not exceed 0.02” water gauge static pressure. Catalog data indicates that the majority of the commercially available louvers will have about 0.02” water gauge static pressure drop at about 250 feet per minute free area velocity.
4. Ceiling Grille size: The pressure loss through the grilles should not exceed 0.02” water gauge static pressure. Catalog data indicates that the majority of the commercially available registers will have about 0.02” water gauge static pressure drop at about 300 feet per minute free area velocity.
5. Duct size: The relief duct should be sized for 0.01” water gauge static pressure loss per 100’ of ductwork maximum for 100% of the unit capacity. The relief duct pressure loss should not exceed 0.01” water gauge static pressure.
6. Backdraft dampers. Provide the counter-balanced type that opens at about 0.01” water gauge static pressure. Check if they are opening properly and not stuck in the closed position. The total pressure loss through the damper should not exceed 0.02” static pressure.
7. The pressure loss through each component of the system is required to be adjusted so that the total does not exceed 0.075” water gauge static pressure.
c. Commissioning Measures
1. Air Balance: Systems must be balanced for both the regular and economizer modes.
a. Most unitary systems are specified with a safety factor in the static pressure. The Contractor must be required by the specifications to replace the drive sheaves and slow the fan down to achieve the required air balance and prevent energy waste and noise. If this is not done, the air flow is left higher creating higher static pressure and noise levels, as well as excessive pressure on doors and door closers.
b. If the system is not also balanced for the economizer mode, when the outdoor air dampers open fully much more air is delivered than the design capacity.
2. Corridor pressure: If the corridor HVAC unit is off, or the corridor unit is not in the 100% economizer mode when the classroom is operating on the economizer cycle, the corridor
April 2007 15 3.6 HVAC pressure will be lower than normal operating conditions, further contributing to the door-closing difficulty. Design system to prevent this occurrence.
3. Door closer pressure: During testing and balancing, door closer pressures must be set properly and not too low just to more easily achieve access compliance.
4. Remedial Measures for Incorrect Design:
a. If gravity relief is not sufficient during the 100% outdoor air economizer operation after the above commissioning is done, the maximum operation of the return and relief dampers in the economizer system should be adjusted to reduce the amount of outdoor air and return some air to the unit. An 80% outdoor air system is still more energy efficient and conducive to fresher indoor air than a minimum outdoor air system with 30% outdoor air. If this is not possible, the economizer operation is required to be deactivated.