Acoustical Design Guidelines for HVAC Systems in Schools

Acoustical Design Guidelines for HVAC Systems in Schools Seminar 9, ASHRAE Summer Meeting Albuquerque, New Mexico Sponsor: TC 2.6 Sound and Vibration Robert M. Lilkendey Associate Principal Consultant Siebein Associates, Inc. Gainesville, Florida

CLASSIFICATION OF NOISE SOURCES IN HVAC SYSTEMS 2 1 5 3 4 4 4 4 1. Airborne Noise 2. Self-generated Noise 3. Duct-borne Noise 4. Structure-borne Noise 5. Break-out Noise

Guideline #1: Coordinate mechanical equipment selection with space planning.

Consider: Locate Fan Coil Units and Small Heat Pumps in fully enclosed closets of relatively high mass, doors to corridor or outside Locate Rooftop Units over non-critical spaces with concrete on roof and adequate distance to spaces served Locate Central Station AHUs in Mechanical Rooms separated from classrooms by buffer spaces, VAV Units in corridors

Return Air Supply Air Classroom Classroom Restroom Restroom Janitor Closet Mechanical Room Plan view: Mechanical room buffered from quiet space

Existing Small RTU Over High School Classroom 50 dba STANDARD ROOF CURB NO CONCRETE INSIDE CURB

Recommended Acoustical Modifications to Meet <40 dba

Accepted Acoustical Modifications to Meet <45 dba

80 70 Sound Pressure Level (db) re: 20 micropascals 60 50 40 50 dba/nc 50 pre-renovation NC 65 NC 60 NC 55 NC 50 NC 45 30 44 dba/nc 40 with gypsum board only NC 40 NC 35 NC 30 20 NC 25 NC 20 NC 15 10 31.5 63 125 250 500 1000 2000 4000 8000 16000 Octave Band Center Frequency (Hz)

Guideline #2: Construct Mechanical Equipment Room enclosure of sufficient mass to isolate airborne noise.

Mechanical Room Provide adequate mass in walls and floors Avoid locating near critical occupancies Watch for doors and other sound leaks Chapter 47 of the 2007 ASHRAE Handbook HVAC Applications, contains recommended assemblies

Provide project specific construction assemblies at critical adjacencies based on: Sound levels of Equipment in Mechanical Room Transmission Loss of wall types Sound absorbing characteristics of the receiving room Background NC level requirement in receiving room

Guideline #3: Reduce source sound levels to the extent possible.

Consider: Avoiding Forward Curved Centrifugal Fans in high static pressure systems Using Plenum Fans in lieu of Centrifugal Specify Sound Levels of Equipment AHRI Standard 260, Sound Rating of Ducted Air Moving and Conditioning Equipment

Comparison of Airfoil and Plenum Fan Acoustic Data on Recent School Project: Octave Band Center Frequency, in Hz Fan Type 63 125 250 500 1000 2000 4000 Airfoil 97 100 99 98 93 86 79 Plenum 91 81 89 91 90 81 74 Difference 6 19 10 7 3 5 5 AHU: 11,350 CFM, 6.18 TSP Discharge Sound Levels Based on AHRI Standard 260 Test Data

Guideline #4: Install duct borne noise control devices as required.

Summary of General Duct Length, Silencer Length, and Flexible Duct Length Required to Achieve Various dba Levels Design NC/dBA Level of Room Served Velocity in Duct near Terminal Supply/ Return Duct Silencer Length Duct Length to First Inlet or Outlet Acoustic Lined Flex Duct NC 25 to 30/ 30 to 35 dba NC 30 to 35/ 35 to 40 dba NC 35 to 40/ 40 to 45 dba 350 / 425 fpm 7-10 ft 90 ft 6-8 ft 400 / 500 fpm 3-7 ft 50-70 ft 6-8 ft 500 / 600 fpm 3-5 ft 50 ft 6-8 ft Chapter 47 of ASHRAE Handbook - HVAC Applications, describes system analysis process

CASE STUDY: AHU in Mechanical Penthouse with VAV Terminal Units in Corridors NC 25-30 35 dba Classroom AHU Classroom Silencers in Supply and Return Ducts Floating Concrete Floor Slab in Penthouse Mechanical Room Spring Hung Multi-Layered Gypsum Board Ceiling Classroom Classroom Classroom Classroom Main Ducts to VAVs in Corridor Beyond Classrooms Masonry Chase Walls 6' of Acoustic Flex Duct Supply from Air Handling Unit Return to Air Handling Unit

CASE STUDY: AHU in Mechanical Penthouse with VAV Terminal Units in Corridors VAV Terminal Units in Corridor Radiused Elbows Self-Balancing Supply Air System Masonry Chase Walls to Reduce Breakout From High Velocity Main Ducts Supply from Air Handling Unit Return to Air Handling Unit

Guideline #5: Follow guidelines in the literature related to air velocity, air flow, and air balancing to reduce flow generated noise.

Follow recommended air velocity guidelines Design NC/dBA of Room Served NC 25 to 30/ 30 to 35 dba NC 30 to 35/ 35 to 40 dba NC 35 to 40/ 40 to 45 dba Supply vs. Return Through the Air Terminal Device (free area) Device to 10', Including the Neck 11' to 20' 21' to 30' Supply 350 425 550 700 Return 425 500 650 800 Supply 425 500 700 850 Return 500 600 800 1000 Supply 500 600 800 1000 Return 600 700 900 1150

Follow recommended air velocity guidelines Design NC/dBA of Room Served NC 25 to 30/ 30 to 35 dba NC 30 to 35/ 35 to 40 dba NC 35 to 40/ 40 to 45 dba Supply vs. Return Through the Air Terminal Device (free area) Device to 10', Including the Neck 11' to 20' 21' to 30' Supply 350 425 550 700 Return 425 500 650 800 Supply 425 500 700 850 Return 500 600 800 1000 Supply 500 600 800 1000 Return 600 700 900 1150

Use radiused elbows with several duct diameters of straight duct before and after elbows near AHU inlets and outlets Radius elbow typ. Resilient hangers Return Flexible collar typ. Supply Air Handling Unit (AHU) 3 duct diameters min. Duct silencer typ. Seal penetrations air tight Housekeeping pad External Springs

Design and install flex ducts without kinks, especially near outlets (From ASHRAE 2007 Handbook HVAC Applications, Pg. 47.10) +0 db +0 db +12 db +0 db +12-15 db

Use volume dampers or design self-balancing systems, not OBDs at air terminal devices VAV Terminal Units in Corridor Radiused Elbows Self-Balancing Supply Air System Masonry Chase Walls to Reduce Breakout From High Velocity Main Ducts Supply from Air Handling Unit Return to Air Handling Unit

Guideline #6: Avoid common duct routing pitfalls.

Pitfall #1: Un-ducted/Plenum Returns Although it may be possible to meet the HVAC system sound level requirements, plenum returns make it difficult to meet the STC requirements for walls.

Pitfall #2: Routing Noisy Ducts above Sound Sensitive Spaces NC 40 44 to 46 dba - Break-out noise from main supply and return - Airborne sound through gypsum board MER wall - Duct-borne noise from short return duct

Pitfall #3: Routing Common Ducts above Adjacent Sound Sensitive Spaces (Crosstalk) Design Problem (Schaffer, Mark E. - A Practical Guide to Noise and Vibration Control for HVAC Systems, 2005) Any use of materials in this presentation, including reproduction, modification, distribution, or republication, (Schaffer, without Mark E. the - A prior Practical written Guide to Noise and Vibration Control for HVAC Systems, 2005)

Pitfall #3: Routing Common Ducts above Adjacent Sound Sensitive Spaces (Crosstalk) Design Solution VAV Terminal Units in Corridor Radiused Elbows Self-Balancing Supply Air System Masonry Chase Walls to Reduce Breakout From High Velocity Main Ducts Supply from Air Handling Unit Return to Air Handling Unit

Pitfall #3: Routing Common Ducts above Adjacent Sound Sensitive Spaces (Crosstalk) Retrofit Problem - Teachers voices plainly audible in adjacent rooms through common return duct - No internal duct liner allowed - Minimal space available

Pitfall #3: Routing Common Ducts above Adjacent Sound Sensitive Spaces (Crosstalk) Retrofit Solution - Close off return opening on bottom of duct - Add elbow opening toward deck above - Install 2 thick sound absorbing acoustical panel on underside of deck - Repeat for each room

Guideline #7: Beware of hidden noise sources.

Such As: Leaky Dampers in Exposed Ducts Condenser Fans and Compressors in RTUs Exhaust Fans Reheat Coils Fume Hoods Chases with High Velocity Ducts Condensing Units outside Windows

Guideline #8: Install vibration isolation devices on HVAC equipment.

In accordance with ASHRAE 2007 HVAC Applications Handbook At minimum include: Vibration isolators for all rotating or vibrating equipment (AHU s, RTUs, pumps, chillers, etc.) Flex connectors on duct and pipes Slab breaks at ground floor MERs

Guideline #9: Monitor the Value Engineering process.

Watch out for: Elimination of duct liner, silencers, etc. Cheaper (noisier) fans. Re-location of RTUs to top of space served. Reduction in ceiling cavity space. Shorter duct runs

Guideline #10: Review acoustical submittals and monitor the implementation of acoustical details during the construction process.

This includes: Review submittals for vibration isolators, equipment sound levels, duct sizes, duct silencers, etc. On sound critical projects, conduct acoustical kickoff meeting with contractors to review specific details and areas requiring coordination Conduct site reviews

5 Common Acoustical Misconceptions/Mistakes 1. Sound attenuators not required on return air side. 2. Making the Contractor or Equipment Manufacturer responsible for HVAC system sound levels in rooms. 3. Duct silencers cause more harm than good. 4. Duct elbows take care of fan noise problems. 5. Relying on published NC ratings of VAV s and Air Terminal Devices.