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Rooftop equipment noise reduction

24 Jan
Rooftop equipment noise reduction
Hydronics, Air Conditioning

Installing an air-conditioning outdoor unit on your roof frees up space on the ground and gives better air flow into and out of the outdoor unit so it will work more efficiently. However, the installation needs to be done correctly to avoid noise penetrating through to your living rooms and bedrooms, which can be annoying. Noise can sometimes can come from other factors such as vibrating piping connected to outdoor unit.

It is difficult to list a definitive set of considerations for your specific situation because there are many factors involved in noise creation and noise abatement. However, the following article discusses some practical ideas and steps you can take to produce a good solution.

Design Considerations for Installation of your Outdoor Unit

When installing the outdoor unit on your roof top, your installer should consider the following points: 

  1. The sound power of the noise source
  2. The path of the noise
  3. The cost of reducing the noise
  4. Where the noise is likely to be entering your home

Noise Source

The first consideration is the noise source, which is the sound power level of your outdoor unit. 

Inside your home, you will be located some distance from the noise source and so the sound pressure will be naturally lower the further you move away from the source. Also, airborne noise will need to penetrate roofs and walls to reach your ears and this will quieten the noise from the source. The more mass in the roofs and walls, the better the noise reduction.

Noise Pathway

You need to check the noise pathway. For instance, is the noise mainly an airborne noise coming through roofs and/or walls or is it mainly transmitting through your ductwork, or is it a vibration sound transmitting through your roof? The noise may even take more than one path. A properly constructed roof platform (refer below) for your outdoor unit will reduce noise by reducing vibration and airborne noise transmitting though your roof.

Installing material with mass such as rockwool batts in or on the walls adjacent to the outdoor unit is another measure to consider, particularly if it is a living room or bedroom.

For ducted systems, noise will travel through the duct work so air handling units should have quiet fans and ideally the ability to run slowly in ‘’sleep mode’’ at night, which will be quieter. This is something you should check before purchasing the equipment.
 
Air conditioning equipment vibration eliminators

Roof Platform

Consideration of a roof platform is important both from a noise perspective and for ease and safety of servicing. The roof platform supporting your outdoor unit should be stiff enough to deflect no more than 6mm under the combination of the operating load of the unit and associated equipment.

Ideally, the platform will be fabricated from aluminium although hardwood is another option and should last the expected life of an outdoor unit, 15 to 20 years. If you have a flat surface, you may use a free-standing system without penetrations through the roof surface. 

Rubber matting or rubber feet should sit under the platform or feet of the platform and also under the feet of the outdoor unit to prevent vibration noise transmitting through the roof. A layer of concrete sheeting under the outdoor unit or platform will add mass to further prevent sound transmission of airborne noise through the roof. 

Cost of Reducing the Noise

The easiest step is to place large rubber vibration noise isolation pads under all feet in contact with the roof. If it is a new home, measures to reduce noise transmission are less costly if added during construction. 

Retrofitting will be costlier and some measures during new construction may not be cost effective options after your home has been built such as installing rockwool in the adjacent walls. However, there are cost effective work-around options that you can consider to reduce noise transmission through walls and roofs or vibration through the roof. For example, it may be cost effective to apply sound absorptive material with cement sheet backing to the outside of the wall near the outdoor unit to attenuate the sound waves and reduce their ability to transmit through the wall. Also cement sheeting under the outdoor unit will reduce transmission through the roof.

Rubber matting or rubber feet should sit under the platform or feet of the platform and also under the feet of the outdoor unit to prevent vibration noise transmitting through the roof. You should also consider vibration isolation feet if the platform is installed over a living area.

To some extent you may be able to time your outdoor unit not to operate during sensitive times such as at night, or at least minimally by turning down the set temperature. If your outdoor unit has inverter technology, it will modulate and reduce speed and hence noise as it nears the set temperature.

Benefit of Reducing the Noise
A good outcome needs a holistic approach looking at your comfort levels, the location of the noise source, the location of the noise issue/s inside, your budget and your equipment. 

The benefit relates to where the noise is heard and your sensitivity to the noise. If it is in a service room such as a laundry or bathroom it may not matter greatly. However, if the outdoor unit is transmitting noise to a living room or bedroom it may matter a lot and doing something about reducing the noise will be of great benefit.  

You do not need the same level of noise abatement through your whole home. Your bathrooms and hallways do not need to be as quiet as your living rooms and bedrooms.

If you are sensitive to noise or if it disrupts your sleep then the benefit of doing something about it is significant. 
 

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