In this Article:
- Introduction
- Why are Vibration Isolators needed
- Understanding Deflection
- Vibration Isolators for Various Equipments
- Common Mistakes
- Flexible Pipe Connections
- Fire Retardant Duct Connectors
- Contact Us
- FAQ
Every HVAC system that runs inside a building creates vibration. Chillers, cooling towers, air handling units, pumps, fans, and fan coil units are all rotating machines — and every rotating machine generates mechanical vibration at its operating frequency. Without isolation, that vibration travels through the equipment base, into the structural slab, through the columns and walls, and eventually radiates as a low-frequency hum in occupied spaces far from the plant room.
In most commercial buildings, HVAC plant rooms are located in basements, on rooftops, or on intermediate plant floors — all positions that place them in direct structural contact with occupied spaces. A chiller room directly below a hotel ballroom, a pump room above a hospital ICU, or a rooftop AHU above a residential apartment are all situations where inadequately isolated HVAC equipment causes noise complaints, tenant dissatisfaction, and in some cases, regulatory enforcement action.
Vibration isolation for HVAC systems is not a single product — it is a system that must be designed and specified for each piece of equipment and each installation condition. This guide covers every major piece of HVAC equipment, the correct isolator type for each, and the common mistakes that cause otherwise well-designed HVAC systems to fail acoustically.
RMS Corporation has supplied vibration isolation systems for HVAC installations across India and internationally for over 45 years, Our engineering team provides product selection support for HVAC and MEP projects of all scales.
Why HVAC Equipment Needs Vibration Isolation?
HVAC equipment generates vibration for two fundamental reasons. The first is rotational imbalance — even a perfectly balanced rotor generates some centrifugal force at its operating speed, and in practice all rotors have some residual imbalance that increases with wear. The second is fluid-induced forces — the pressure pulsations generated by pumps, compressors, and fans create dynamic forces in addition to the rotational ones.
The vibration generated by HVAC equipment has two transmission paths into the building structure. The first is the structural path — directly through the equipment base and mounting surface into the slab. The second is the acoustic path — through the air inside the plant room, exciting the walls and slab directly. Vibration isolators address the structural path. Acoustic enclosures, room lining, and duct silencers address the acoustic path. Both are needed for a complete solution in noise-sensitive environments, but the structural path is almost always the dominant one in HVAC plant rooms.
The effectiveness of isolation is measured by the vibration isolation efficiency, expressed as a percentage. A well-selected spring isolator system on a chiller can achieve 90-92% isolation efficiency, meaning less than 10% of the generated vibration force reaches the building structure. An incorrectly selected isolator — or no isolator at all — allows 100% transmission, and in resonance conditions can actually amplify the transmitted force above the source level.
Understanding Static Deflection and Natural Frequency
Before specifying any vibration isolator for HVAC equipment, two quantities must be calculated: the disturbing frequency of the equipment, and the required static deflection of the isolator.
The disturbing frequency in Hz is simply the equipment’s operating speed in RPM divided by 60. A chiller running at 1500 RPM has a disturbing frequency of 25 Hz. A cooling tower fan running at 400 RPM has a disturbing frequency of 6.7 Hz.
The static deflection required for effective isolation is determined by the ratio between the isolator’s natural frequency and the equipment’s disturbing frequency. For 90% isolation efficiency, the isolator’s natural frequency must be less than one-third of the disturbing frequency. The natural frequency of an isolator decreases as its static deflection increases — which is why heavy, slow equipment needs high-deflection spring isolators, and lighter, faster equipment can be effectively isolated with lower-deflection rubber mounts.
As a practical guide for HVAC applications:
- Equipment at 2900 RPM (25 Hz) — requires under 1 mm deflection for 90% efficiency
- Equipment at 1500 RPM (25 Hz) — requires at minimum 4 mm deflection for 90% efficiency
- Equipment at 900 RPM (15 Hz) — requires at minimum 12-16mm deflection for 90% efficiency
This calculation is why there is no single correct isolator for all HVAC equipment — the selection is always equipment-specific.
Vibration Isolators for Chillers
Chillers are the heaviest and typically most vibration-sensitive piece of HVAC primary plant. Centrifugal chillers operating at compressor speeds of 3000–3600 RPM generate relatively high-frequency vibration, but the large mass of the chiller means the force levels involved are significant. Screw chillers and reciprocating chillers operate at lower compressor speeds and also generate strong low-frequency components from the reciprocating or meshing action of the compressor elements.
The standard vibration isolation specification for chillers in commercial projects is spring isolators with a minimum of 25mm static deflection. The isolators must be positioned under the chiller base frame at the manufacturer’s specified support points, which are typically at the four corners and any intermediate support feet for larger units.
Recommended RMS Products for Chillers:
- Open Spring Mountings — for standard commercial chiller installations where access around the isolator is not restricted
- Enclosed Spring Mounts — for outdoor or rooftop installations where weather protection of the spring is required
- Seismic Restrained Spring Mounts — for projects in seismic zones or where the project specification requires seismic-rated isolation
- Spring Isolators with Viscous Dampers – for extreme vibration control.
For very large or tall chillers where the centre of gravity is high, an Inertia Pouring Frame (concrete inertia base) beneath the chiller is recommended to lower the combined centre of gravity and improve isolation stability. The inertia base itself then sits on spring isolators.
Flexible pipe connectors — rubber expansion bellows from Resistoflex — must be installed on all chilled water, condenser water, and refrigerant pipework connections at the chiller to prevent vibration from bypassing the isolation mounts through rigid pipe connections. This is the most common single point of failure in chiller vibration isolation installations.
Vibration Isolators for Cooling Towers
Cooling towers present a different vibration isolation challenge from chillers. The fan in a cooling tower operates at a relatively lower speed, large induced-draft towers — generating low-frequency vibration that requires high static deflection for effective isolation. At the same time, cooling towers are almost always installed outdoors on rooftops or on dedicated plinths, exposing the isolators to weathering, UV, and thermal cycling.
Spring isolators are the standard specification for cooling tower isolation, with deflection requirements typically in the range of 12-20mm depending on fan speed. Because of the outdoor installation environment, enclosed spring mounts or open spring mounts with corrosion-resistant finishes are required. For coastal installations or highly corrosive environments, stainless steel springs should be specified.
Recommended RMS Products for Cooling Towers:
- Enclosed Spring Mounts — primary recommendation for all cooling tower installations, providing spring isolation with weather-resistant housing
- Open Restrained Spring Mounts — where lateral restraint is required in addition to vertical isolation, such as in locations subject to high wind loading
- Seismic Restrained Spring Mounts — for cooling towers in seismic zones
Cooling towers also require Expansion Bellows on all water supply and return pipework and cable tray connections where cables connect to the fan motor. The isolation system is only as effective as its weakest rigid connection.
Vibration Isolators for Pumps
Pumps are the most numerous piece of vibration-generating equipment in any HVAC system — a large commercial building may have dozens of chilled water, condenser water, heating, and domestic water pumps all requiring isolation. Pump operating speeds range from 750 RPM for large slow-speed centrifugal pumps to 2900 RPM for small direct-drive units, giving a wide range of disturbing frequencies and corresponding isolation requirements.
For large base-mounted centrifugal pumps, spring isolators with 12–25mm deflection are the standard specification. For smaller inline pumps mounted directly in the pipework, rubber anti-vibration mounts under the pipe supports or expansion bellows on either side of the pump are the practical isolation measures. For pumps on elevated floors or in noise-sensitive locations, inertia bases beneath the pump and motor set are recommended to stabilise the isolation system and reduce rocking.
Recommended RMS Products for Pumps:
- Open Spring Mountings — for standard base-mounted pump installations
- Enclosed Spring Mounts — for outdoor pump installations
- Cushyfoot Mounting — heavy-duty rubber-to-metal bonded mounts for medium-weight pump sets where spring isolators are not specified
- Combi Mounts — compact rubber mounts for smaller pump sets and inline pump applications
- Inertia Pouring Frame — for pump and motor sets on elevated floors requiring inertia base stabilisation
Expansion Bellows are critical for pump isolation. The standard installation requires a rubber expansion bellow on both the suction and discharge connections of every isolated pump. The Resistoflex Rubber Bellow and Metal Bellows with Fixed Flanges are the standard products for HVAC pump connections, available in all standard pipe sizes.
Vibration Isolators for Air Handling Units
Air handling units (AHUs) are typically the largest ceiling-suspended or floor-mounted pieces of HVAC equipment in a building, and they contain multiple vibration sources — supply and return air fans, motors, compressors in packaged units, and damper actuators. Floor-mounted AHUs use spring isolators or rubber mounts depending on the fan speed and acoustic specification. Ceiling-suspended AHUs require spring or rubber hanger mounts.
For floor-mounted AHUs with direct-drive fans operating at 1450–2900 RPM, rubber anti-vibration mounts are typically sufficient. For AHUs with belt-drive fans operating at lower speeds, or for AHUs in acoustically sensitive locations, spring isolators with 12–25mm deflection are specified. For ceiling-suspended AHUs, spring hanger mounts provide the necessary deflection while supporting the unit from the structural slab above.
Recommended RMS Products for AHUs:
- Open Spring Mountings — for floor-mounted AHUs with belt-drive fans
- Cushyfoot Mounting — for floor-mounted AHUs with direct-drive fans in standard commercial locations
- Spring Hanger Mounts (from the Hangers range) — for all ceiling-suspended AHU installations
- Rubber Hanger Mounts — for ceiling-suspended AHUs in standard acoustic conditions
Duct connections from AHUs to the supply and discharge ductwork must use fire-rated flexible canvas connectors to prevent vibration from transmitting into the duct system and radiating as noise throughout the building. RMS Corporation supplies Fire Retardant Canvas Duct Connectors that meet fire rating requirements for AHU installations in commercial and institutional buildings.
Vibration Isolators for Fans and Blowers
Standalone supply, exhaust, and process fans represent a significant vibration isolation requirement in commercial buildings — particularly extract fans, staircase pressurisation fans, and kitchen exhaust fans which often run at high speeds and are located in positions that make noise transmission particularly problematic.
Centrifugal fans operating at 960–1450 RPM require spring isolators with 12–25mm deflection. Axial fans and high-speed centrifugal fans operating above 1450 RPM can typically be isolated with rubber anti-vibration mounts. For rooftop fans, enclosed or weather-resistant spring mounts are required.
Recommended RMS Products for Fans and Blowers:
- Open Spring Mountings — standard specification for centrifugal fans in plant rooms
- Enclosed Spring Mounts — for rooftop fan installations
- Double U Shear Mounts — heavy-duty rubber shear mounts for large axial and centrifugal fans with significant lateral forces
- Combi Mounts — for smaller fans and blowers in standard locations
Vibration Isolators for Fan Coil Units
Fan coil units (FCUs) are the most numerous individual piece of HVAC equipment in any commercial building — a large hotel or office building may have hundreds of ceiling-suspended FCUs. Each FCU contains a small fan motor that generates vibration at the fan operating speed, typically 900–1450 RPM. Because FCUs are ceiling-suspended directly above occupied spaces, even modest vibration transmission can be perceived as noise.
Rubber hanger mounts are the standard isolation product for ceiling-suspended FCUs. For FCUs in acoustically sensitive locations such as hotel guest rooms, hospital patient rooms, or executive offices, spring hanger mounts with 12–25mm deflection provide a higher level of isolation. Flexible pipe connections on the chilled water supply and return connections to each FCU prevent water-borne vibration from bypassing the hanger isolation.
Recommended RMS Products for FCUs:
- Rubber Hanger Mounts — standard specification for FCUs in commercial office buildings and standard hotel rooms
- Spring Hanger Mounts — for FCUs in acoustically sensitive locations — luxury hotel rooms, hospital patient rooms, executive boardrooms
Common HVAC Vibration Isolation Mistakes
Mistake 1 — Rigid pipe connections to isolated equipment The single most common installation error. If an isolated chiller or pump has a rigid pipe connection to the building pipework, vibration bypasses the isolation mounts completely through the pipe. Every isolated piece of HVAC equipment must have flexible connectors on all pipe connections.
Mistake 2 — Incorrectly loaded isolators Spring isolators only achieve their rated deflection at their rated load. An overloaded isolator bottoms out and provides zero isolation. An underloaded isolator operates too high in its travel with poor stability. Always calculate the actual operating weight at each mount point — including fluid — and select isolators rated for that load.
Mistake 3 — Direct contact between isolated equipment and building structure A single bolt, pipe hanger, or electrical conduit connecting an isolated piece of equipment to the building structure short-circuits the entire isolation system. All connections to isolated equipment must be flexible — pipes, ducts, electrical conduits, and drainage pipes.
Mistake 4 — Incorrect isolator type for the equipment speed Using rubber mounts on a slow-speed cooling tower fan running at 300 RPM will result in near-zero isolation efficiency — the rubber mount’s natural frequency is too close to the fan’s disturbing frequency. Always calculate the required deflection before specifying a product.
Mistake 5 — No inertia base for tall or unbalanced equipment Tall, narrow equipment such as vertical pumps or large base-mounted chillers with a high centre of gravity can rock on their isolation mounts, reducing isolation efficiency and causing instability. An inertia base lowers the effective centre of gravity and stabilises the system.
Flexible Pipe Connections — Completing the Isolation System
Vibration isolation mounts alone are not sufficient to prevent vibration transmission from HVAC equipment. Every rigid connection between isolated equipment and the building structure is a potential vibration flanking path. The most important rigid connections to address are the pipe connections — chilled water, condenser water, heating water, and refrigerant lines that connect isolated equipment to the building pipework network.
RMS Corporation supplies the complete Resistoflex range of rubber expansion bellows for HVAC pipe connections:
- Rubber Expansion Bellow with Floating Flanges — the most flexible option, allowing axial, lateral, and angular movement; used on all standard HVAC chilled and condenser water connections where the pipe configuration allows free movement
- Rubber Bellows with Fixed Flanges — for installations where the pipe flanges must be fixed in position; suitable for most inline applications
- Double Arch Rubber Bellow — for applications requiring higher movement capability; used on large-diameter connections and where equipment movement is significant
- Metal Bellows – for applications where higher temperature medium of flow or as specified by user
All bellows should be installed without pre-stress and with control rods where required by the system operating pressure to prevent pressure thrust forces from overextending the bellow.
Fire Retardant Canvas Duct Connectors
Between AHUs, fans, and the duct system, rigid metal duct connections are replaced with fire-rated flexible canvas connectors that isolate vibration transmission into the ductwork while meeting fire safety requirements. RMS Corporation supplies Fire Retardant Canvas Duct Connectors and Fire Retardant Fabric for AHU and fan discharge and return connections.
The canvas connector must be long enough to remain flexible under all operating conditions — a minimum of 150mm length is recommended for most HVAC applications. Connectors must be installed without tension and with adequate clearance to the duct edges to prevent contact under movement.
RMS Corporation — HVAC Vibration Isolation Selection
RMS Corporation’s engineering team provides free product selection support for HVAC vibration isolation on projects of all scales. To receive a product recommendation, share the following information:
- Equipment type and model or specification
- Operating weight (including fluid where applicable)
- Operating speed in RPM
- Number and position of mounting points
- Installation location — floor level, rooftop, basement, ceiling-suspended
Frequently Asked Questions
Spring isolators with 25 mm static deflection are the standard specification for chillers in commercial buildings in India. Open Spring Mountings from Resistoflex India Pvt. Ltd. are the most widely specified product for standard chiller installations. For rooftop or outdoor installations, Enclosed Spring Mounts with weather-resistant housing are used. Cushyfoot mountings are also used in some instances. For seismic zones, Seismic Restrained Spring Mounts are required. Flexible rubber expansion bellows must also be installed on all pipe connections to prevent vibration flanking.
Open spring mounts expose the steel spring directly — they provide the same isolation performance as enclosed mounts but are suitable only for indoor, protected locations. Enclosed spring mounts house the spring inside a steel casing with a neoprene base pad, protecting the spring from moisture, dust, and UV exposure. Enclosed mounts are the correct specification for all outdoor, rooftop, and semi-exposed HVAC equipment installations.
Yes. Cooling tower fans — particularly large induced-draft towers generate significant low-frequency vibration that requires high static deflection spring isolators to isolate effectively. Without isolation, cooling tower vibration transmits through the rooftop slab into the floors below. Enclosed spring mounts with 25 mm deflection are the standard specification for cooling tower isolation.
Rubber hanger mounts are the standard product for ceiling-suspended FCUs in standard commercial locations. For FCUs in acoustically sensitive locations — luxury hotel rooms, hospital patient rooms, executive offices — spring hanger mounts with 12–25mm deflection provide a significantly higher level of isolation. RMS Corporation supplies both rubber and spring hanger mounts through its Polybond India and Resistoflex partnerships.
The most common reason is vibration flanking through rigid pipe connections. If the pump is correctly mounted on spring isolators but the pipe connections to the building pipework are rigid metal flanges without flexible connectors, vibration bypasses the isolation mounts completely through the pipes. Every isolated pump must have expansion bellows on both the suction and discharge connections. Secondary causes include incorrectly loaded isolators that are bottomed out, and rigid electrical conduit connections between the isolated pump motor and the building structure.
RMS Corporation supplies the complete Resistoflex range of rubber expansion bellows for HVAC applications — including Rubber Expansion Bellows with Floating Flanges, Rubber Bellows with Fixed Flanges, Metal Expansion Bellows and Double Arch Rubber Bellows. The floating flange type is the most commonly specified for new HVAC installations because it accommodates the widest range of movement. Fixed flange bellows are used where pipe alignment is fixed and flanges cannot rotate. All products are available in standard pipe sizes from DN25 to DN600 and above for special applications.
Vibration isolation is not universally mandated by a single national code for all HVAC installations in India. However, project specifications for hospitals, hotels, IT parks, and residential buildings routinely include vibration isolation requirements as part of the acoustic specification. The National Building Code of India (NBC) sets noise limits for occupied spaces, and the most practical way to achieve these limits in buildings with HVAC plant rooms is through vibration isolation of all primary plant. Many international project specifications — particularly for hotel chains and hospital groups — specifically require vibration isolation products tested to international standards.
