In HVAC systems, efficient heating and cooling rely heavily on adequate airflow. Plenums are essential in this process, ensuring even air distribution throughout large spaces in a wide range of residential, commercial, and industrial buildings.
However, to prevent unwanted air leakage and maintain peak performance, these plenums must be properly sealed. This is where plenum testing comes in, identifying potential air tightness improvements for achieving greater efficiency, comfort, and energy savings.
Explore our answers to 10 frequently asked questions about plenums, their applications, and the important role of plenum testing in HVAC systems.
1. What Is a Plenum in a Building?
In buildings, a plenum is the space between the main floor structure and a raised access floor, or the structural ceiling and a dropped ceiling.
These voids are typically used to distribute conditioned air evenly throughout large rooms, ensuring all areas receive sufficient heated or cooled air.
2. How Does a Plenum Work?
Plenums work by distributing newly cooled or heated air through diffusers, registers, or grilles, which control the flow of conditioned air into the desired room.
A properly sealed plenum ensures these devices can deliver air to the right place at the right time. That is, a plenum with weaknesses in its walls or joints will allow conditioned air to enter the room in ways other than the diffusers, registers, or grilles. Consequently, the rate of airflow would be uncontrolled, leading to substantial energy consumption.
Additionally, if the plenum is not properly sealed, the conditioned air may not enter the desired room at all – leaking into cavities, stairwells, risers, heating trenches, or other unintended zones outside the room. This would not only increase energy consumption and costs but also result in lower indoor air quality and discomfort for occupants.
3. When Are Plenums Required?
Plenums can be beneficial for a wide range of buildings, especially those with a high demand for conditioned air, such as data centres and computer rooms that generate a lot of heat. Whether a plenum is required depends on individual building specifications and design needs.
Underfloor and in-ceiling plenums can improve air circulation for applications such as:
- HVAC Return Air Systems: Commonly used in data centres and computer rooms.
- Fire and Building Code Compliance: Codes NFPA 90A and ASHRAE 62.1 require plenum spaces for the use of fire-resistant materials.
- Offices & Commercial Buildings: To provide flexible workspace setups in different areas of large buildings.
- Industrial & Healthcare Facilities: To manage airflow and contamination control for health and safety and compliance.
- Electrical, Mechanical & Plumbing Systems: Where dedicated space is required to house / conceal the systems.
4. What Is Plenum in HVAC?
When it comes to HVAC (Heating, Ventilation, and Air Conditioning) systems, plenums are the primary distribution points for conditioned air.
They are essentially chambers, typically located in the ceiling or below the floor, that connect to the HVAC system and control the flow of air.
These plenum voids can also be referred to as “plenum boxes”, and nearly all HVAC units have two boxes: the supply plenum box and the return plenum box.
5. What Does a Plenum Box Do?
While a plenum box can be used to discreetly house electrical, mechanical, and plumbing systems, its primary use is to circulate heating, ventilation, and air conditioning by providing pathways for unconditioned and conditioned air.
- Supply Plenums: This plenum box distributes conditioned air from the HVAC system to different parts of the building. Typically located above the ceiling, they have several outlets that direct the air to specific areas.
- Return Plenums: This plenum box collects unconditioned / used air from different parts of the building and returns it to the HVAC system for reconditioning. Typically located below the floor, they gather air from various outlets, including grilles and registers.
6. How Do Plenums Optimise HVAC Performance?
Supply and return plenums work together to deliver efficient and even air distribution in large or multiple rooms. Without plenums, HVAC systems would have to work much harder to distribute air into the space, leading to higher energy bills and greater strain on the equipment.
Plenums for HVAC systems help to:
- Conceal HVAC equipment
- Improve airflow & quality
- Reduce system noise levels
- Protect systems from dust & debris
- Increase energy efficiency
- Provide comfortable, safe environments
To ensure optimal performance, however, these plenums must be regularly tested for air leakage. A plenum that allows air to leak elsewhere can cause high energy consumption, CO2 emissions, and energy bills, as well as low indoor air quality and discomfort for occupants.
7. What Is Plenum Testing?
Plenum testing is a type of air tightness test, which is a mandatory requirement under the Building Regulations Part L for all new dwellings and commercial buildings.
The BSRIA test standard provides the methodology for testing plenum floors and places upper limits on air leakage from both the underfloor void and the Raised Access Floor (RAF):
- Plenum (RAF tiles taped): Less than 0.7 l/s per metre square of floor area at 50Pa.
- RAF (tiles not taped): Less than 1.1 l/s per metre square of floor area at 50Pa.
8. How Is Plenum Testing Carried Out?
Most floor plenums are built using floor tiles laid on pedestals, typically positioned on a 600mm x 600mm matrix above a concrete subbase.
Because concrete has a low air leakage rate, it isn’t normally considered in air leakage calculations. Rather, the plenum walls and service penetrations are usually the main leakage paths and thus the focus of the investigation.
Plenum testing involves two tests (one with the floor tiles taped and one without tape), enabling assessors to calculate the leakage of the entire plenum and the floor tiles separately.
In the event of a plenum test failure, a professional assessor like Stroma can help you locate air leakage areas using smoke testing or thermal imaging.
9. How Often Should Plenum Testing be Carried Out?
Plenum testing is required as soon as a new build project is complete to ensure its air tightness is compliant with regulations.
From then onwards, it’s recommended to carry out plenum testing annually or as part of regular HVAC maintenance (which is typically at least once per year).
10. What Common Issues Occur When Regular Plenum Testing Is Not Maintained?
Failing to maintain regular plenum testing for HVAC systems can lead to ineffective ventilation and distribution of heating around the desired spaces. Rather than assisting with the comfort of occupants, the air will escape elsewhere, consuming more energy and resulting in higher energy costs.
Incorrectly maintained plenums can also aid the spread of fire, exposing building materials not intended to be exposed to heat or fire.
Plenum Testing Services from Stroma Built Environment
Our experienced and certified team can confirm that plenums and raised access floors meet necessary air tightness limits to achieve compliance, improve energy efficiency, and ensure comfort.
We can tailor a range of plenum testing services to suit your project at every stage of development, including:
- Desktop design reviews
- Pre-test site inspections
- Plenum & raised floor testing
- Identification of air leakage paths
- Expert consultancy & advice
Contact operations@stroma.com today to ensure your plenums are performing as intended and designed. We can also carry out design reviews of your acoustic details, advising on how a plenum can provide a dedicated area for unwanted acoustic travel.