Underfloor air conditioning

Underfloor air conditioning is generally available in two main forms, ducted and ductless. Ducted works on a floor-supply/ceiling return system. More common is the ductless floor-supply/ceiling return system that is often found in computer rooms.

But the method under consideration here is ductless floor supply and floor return. This system offers several advantages, including: high flexibility, speed of installation, ease of design and commissioning, lower overall cost due to height and time savings, an easy solution for height-restricted projects. It also offers good energy efficiency results and studies show high end-user satisfaction.

Floor supply/return systems also offer easy adaptation to suit an increasing cooling load or changing layout.

In these days when building contamination is of increasing concern, zonal systems offer compartmentalised zones free from cross-contamination (though care must be taken with positive/negative pressurisation).

There are, of course, a few caveats when considering this type of underfloor air conditioning.

Firstly, clients need to be made aware of the benefits at an early stage to gain acceptance. The construction team also needs to understand the techniques employed to achieve the best overall outcome.

For example, it sometimes happens that such systems are installed too early in the construction phase, resulting in inefficiency and needless expense. Also, the system requires a raised floor and some care with sealing up holes in the plenum boundary walls.

Making use of the space

The floor supply, floor return, or zonal underfloor air conditioning system makes use of the raised floor void directly as a plenum for the distribution of air. Supply and return channels are created by means of baffles.

Zone units serving areas up to 300 m2 are suitably located throughout the office space. These zone units deliver conditioned air into the floor void supply plena and draw spent air back through the return plena (see figure 1).

Chilled (6/11°C) and heating (71/82°C) is recommended. Fresh air (1.5l/s/m2) may be delivered directly to the zone unit from outside or via a central ventilation system. Extract air is usually taken away at high level.

Individually controlled fan terminals are let into the floor over supply plena, introducing air into the space above in accordance with the dictates of their own on-board controls system.

Return air grilles are positioned in the floor over return plena. The building is generally divided up into several zones per floor and these zones are defined by means of fire barriers to avoid mixing of different supply air streams.

The whole arrangement is controlled by means of the electronic management system and may be integrated with a bems subject to compatibility and requirements.

We know from numerous worldwide studies that floor supply systems can offer higher air quality standards than ceiling systems. Underfloor air conditioning goes further, offering full function control of all elements, while also offering potential financial benefits in installation, operation and energy consumption compared with ceiling-based systems.

Every developer wants to achieve ‘pension fund’ regulation heights in their buildings. Every architect wants to save height in their construction. Consultants know that for a ceiling-based air conditioning system to function, be serviceable and be flexible, they require 400 mm to 600 mm of clear space in the ceiling void. Electrical consultants must have a minimum of 150 mm in the raised access floor for cable distribution, and cross-overs.

Thus in a modern office the services zones make up approximately 600 mm to 900 mm of the total height of each floor. With ductless underfloor air conditioning that zone can often be reduced by between 300 mm and 500 mm, depending on specification. This offers a height saving in new build construction of 10% to 15% and increased headroom availability in old buildings of 300 mm or more.

Building development potential

On a tall building such as Canary Wharf underfloor air conditioning could have saved the equivalent of a 5 to 10 storey office block or offered more floors within the height.

In Hong Kong, the10th tallest building in the world saved 35 m in construction height, saving over £2.5 million in curtain walling, 3% in energy demand, reduced wind loadings, and reduced foundation costs.

Jersey Energy have applied the concept at a recently completed refurbishment of a 10 000 m2 powerhouse on the Channel Islands. The ceiling is a lattice of riveted steel ribs and the architects Manser Associates have achieved a very pleasing affect with up-lighting and underfloor air. Other end users of underfloor air conditioning include American Express, Land Rover Group, BBC, BAA and KLP.

Often the demand for raised floors limits the available height of ceiling void and compromised solutions are the result. By increasing the raised floor from 150 mm to 300 mm, the need for ceiling air conditioning can be eliminated. This can be achieved while offering perhaps a 200 mm to 400 mm increase in headroom in the space.

Air quality is seen to be important for marketing the building or is required by the owner or occupier. For example, the Swedish Clinic for Occupational Medicine has carried out studies on behalf of the Swedish National Pension Fund and concluded that underfloor air conditioning systems dramatically reduce symptoms of Sick Building Syndrome and offer considerable improvement in ventilation efficiency (see figure 2 on page 42).

Cross-contamination concerns

Recent events in the US have highlighted the need for engineers to design compartmentalised systems to avoid cross-contamination within buildings.

The threat of terrorism means that many postrooms are now being adapted to stop the possibility of contamination passing around the building.

Underfloor systems are inherently compartmentalised and offer highly-effective solutions in multi-tenant areas, as well as in smoking areas and other environmentally challenging applications.

Underfloor air conditioning can also offer a dramatic reduction in maintenance, risk of leakage and disruption during reconfiguration.

There are also benefits for clients seeking an energy-efficient system. Normally engineers, with this brief, look to variable volume systems (vav) to achieve this demand. The facts are that in many cases underfloor systems offer dramatic savings in the region of 25% when compared with vav (source: Building Simulation).

Supply and demand

Zonal underfloor systems only cool down the supply air to the demands of the zone served. Thus re-heat is seldom required and diversity in operation is offered, permitting individual zones to operate anytime instead of having to run complete central plants to satisfy one small space.

Unlike fan coil and vav systems, ductless underfloor systems offer the opportunity to make use of the thermal mass of the structure as a night-time coolth store.

Roger Preston & Partners carried out a study on one such system for BAA Linton’s project at Gatwick, finding energy savings of approximately 19% by using minimum fresh air at night, and a reduction in peak energy by 8%. CO₂ emissions were thus also reduced.

Cost savings

For clients looking to cut costs, normally the only way to save money is to reduce the specification and downsize plant, or reduce flexibility by increasing size of terminal outlets and reducing numbers.

Such activity can be costly in redesign terms. Often the impact of such savings places strain on available space and can increase the cost of the structure to house the services.

Understanding underfloor

Underfloor air conditioning can offer dramatic savings in overall cost but it requires a co-ordinated team approach to achieve the optimum result and a greater understanding of its impact on the building process.

Therefore, underfloor air conditioning must be introduced into the overall design philosophy at an early stage to maximise the benefits.

The Swedish National Pension Fund has reported savings in overall cost in the region of 5% to 7%. BAA Linton found savings in both cost and time, bringing about earlier rental income and reduced site establishment costs.

In real terms average office building costs in the UK are in the region of £1000/m² to £1200/m2. A 5% saving in overall cost could therefore range from £50/m² to £85/m².

The average cost of any air conditioning system is generally in the region of £120/m² to £200/m² and thus the overall saving in construction equates to something in the region of 50% of the system.