Air Conditioning in Tall Buildings: A Comparison between Central Station VAV and Zonal Floor based Systems – AET Flexible Space

Air Conditioning in Tall Buildings: A Comparison between Central Station VAV and Zonal Floor based Systems

tall buildingsback to articles

This paper sets out to demonstrate the financial benefits of using floor based systems and the improvements in user efficiency as a result, compared with the use of VAV and Fan coil Systems.  The Centre in Hong Kong is the worlds 10th tallest building standing over 1000 feet high and offering over 1000000sq ft of office space. The building creates a new landmark standing in a 100 000 sq ft site in Queens Road Central Hong Kong.

Under construction in Malaysia the Telecom Tower creates another impressive sight offering 600 000 sq ft of space.

These buildings stand out because they are the first “super buildings” to adopt the concept of The Flexible Space System, and they offer a chance to study the issues that form the basis of this paper.

Current technology in the construction industry is under scrutiny by leading developers and construction companies simply because the costs of construction are high and time taken to build these buildings is considered to be too long.

In London many professional teams are being asked to carry out value engineering studies in an effort to reduce cost and time.

However many of these studies are confined to initial cost savings without necessarily investigating ways to save costs in ownership. Why? Obviously, because developers want to maximise their return.

In my opinion the flaw in this approach is that the teams look to make savings in each discipline instead of through a co-ordination of the disciplines and they fail to fully embrace the changes in working practice and the demands of modern office technology that demand a different type of building and the demands of users who are now far more aware of the need for efficient facilities in order to operate effectively in world markets.

It is my opinion that occupiers are now far more aware of the disadvantages imposed on them by poorly designed buildings and further that they are prepared to pay a premium for buildings which can show positive operational advantage.

Let us not forget that even in Hong Kong buildings are now expected to last many more years than they were in the early 1980’s and it is not unrealistic to expect buildings constructed today to have a 40 to 60 year life span.

AT&T recently published the figures that show the proportion of costs over a forty- year life span to be in the order of:-

11% Initial Construction Cost

14% Financing Costs

25% Reconfiguration Costs

50% Operational Costs

Whilst much effort is going into studies to reduce the initial construction cost – even if they save 5% of that element it still only accounts for 0.5% of the total cost.

The impact of such cost saving decisions often is to dramatically affect the reconfiguration and operational costs adversely. Currently this cost saving approach is still general practise because the extra costs of such decisions are passed on to the as yet unknown tenant. This must change.

If then design teams could be permitted to embrace the additional dimension of reconfiguration cost saving and if developers could market this benefit well to the potential users those Developers may find that they can rent the building at a premium and thus improve their return by sharing part of the users operational cost saving over the life of the building and in my opinion benefit greatly in commercial terms.

My area of interest is in the savings that can be made in the cost of reconfiguration and interestingly techniques we employ to achieve dramatic savings in this area also offer a major saving in the initial cost. This is not a wild claim. It is supported by the Swedish National Pension Fund, British Airports Authority and many quantity Surveyors, Consultants and Developers in many countries.

The concept is basically very simple.

More and more buildings world-wide are adopting the concept of raised access floors for the intelligent distribution of power and data cabling. The Wireless office in my opinion is still a dream imagined by data engineers and amusingly accommodated by power engineers.

However the general acceptance of the advantages that a raised floor offers for power and data has generally been overlooked by air conditioning and ventilation engineers who continue to argue for clear space in the false ceiling for distribution ductwork in the case of VAV and location of fan coil units. Leading consulting engineers and pioneering Developers are already using this raised floor to great advantage.

The Solution:

The Flexible Space System

The Flexible Space System is basically an under floor air conditioning system making use of the floor void directly as a plenum for the distribution of air. Supply and return channels are created by means of baffles and Conditioned Air Modules (CAM) are suitably located throughout the office space to generate conditioned air locally to serve the needs of the particular zonal space. The CAM units deliver conditioned air into the floor void supply plena and draw spent air back through the return plena.

Chilled and Heating water are delivered to the CAM by means of small diameter pipework and condensate water is drained away to nearby risers.

Fresh air may be delivered directly to the CAM from outside or via a central ventilation system. Extract air is usually taken away at high level in the space near risers.

Let into the floor over supply plena are individually controlled Fan terminals of either recessed or floor standing configuration. These terminals introduce air into the space above in accordance with the dictates of their own on board controls system. Each unit has fan speed and temperature set point adjustment and through a fast data link may be controlled from a central point. In a recent survey in London 78% of office workers polled requested personal control of their environment as a priority.

Return air grilles are positioned in the floor over return plena to permit the return of spent air to the CAM unit for re-conditioning.

The building is generally divided up into several zones per floor and these zones are defined by means of fire barriers placed in the floor void.

The whole system is controlled by means of the electronic management system and Flexmatic advanced controller. All CAMs and Fan terminals can be connected into the system permitting centralised monitoring and control of various functions to take place.

The system may be integrated with a Building Management System subject to compatibility and requirements.

The two buildings mentioned at the beginning of this paper are pioneers in the field of under-floor air conditioning in Asia and follow several years of dramatic success in Europe where such concepts have been operating with surprising results in cost, effectiveness and user satisfaction.

Benefits at Initial Construction Stage

Height Saving

Conventionally engineers will ask for perhaps 500mm or 600mm clear under structural beams for distribution ductwork on buildings with perhaps 2000m2 floor plate or greater.

The aspect ratio of ductwork and the problems of drumming and support make a 3/2 ratio generally the norm.

Now that raised floors are generally accepted for cable distribution we find floor to floor heights to be generally in the region of

Structure                       450mm

Raised floor                  150mm

Room space                   2600mm

Ceiling void                  600-800mm

Making a total of 3800mm to perhaps 4000mm

Now if we consider doing away with the false ceiling completely we can reduce the height by 600mm. If we then consider using the raised floor for ventilation as well as power distribution we must increase the floor void to provide a cable zone and a ventilation zone and we have found that generally a total 300mm void offers all the space that a building ever requires. Thus we find a net saving in height of approximately 450 mm.

This equates to somewhere between 10 and 15 % of the height of a building and we must study the inherent savings that are achieved

Building Programme

British Airport Authority is on record that they have reduced construction time by 25% using this concept. Such saving in time offers huge savings in financing costs and permits the developer to earn rental income sooner.

Initial Building Cost Saving

The Swedish National pension Find are on record as having saved 5% to 7% of the construction cost and this has been confirmed by BAA. The two major areas of saving come from (1)the savings in the Curtain Walling materials and all other vertical structural elements due to the height saving and (2) the savings in interest charges on modular equipment which can be purchased far later in the programme.

Builders generally agree that any solution that permits them to “Get out of the ceiling quicker” reduces management and time and often reduces damage to final finishes which can also be very costly.

Architects find reduced “snagging problems” and Engineers report reduced commissioning programmes.

Together all these advantages offer savings and increased profitability across the design and development teams.

Financing Costs

Firstly the reduction in building programme offers huge savings in the total interest charge on the development. A 20 week reduction in programme could offer perhaps $5 000 000.00 US saving in interest charges on a 100 000 sq m development

Secondly and still of considerable note the nature of a floor based system permits delivery of key mechanical and electrical elements far later in the programme and in some cases permits the equipment to be supplied at the time of the tenant fit out and thus cash flow and cash demand can be improved offering savings in interest charges on the development.

Tax Advantages

Because the raised floor is used as a plenum for air distribution, in many countries the floor is regarded as equipment rather than fixture and thus attracts capital allowances.  Further more if the floor is deemed to be equipment then anything sitting on the floor is also deemed to be equipment and so partitions, modular power, desks, chairs and carpets are all deemed to be equipment and all attract increased capital allowances.

Energy Saving and CO2 emission

The system makes use of the floor void directly for the distribution of air and thus that air is in contact with the concrete slab. Using off peak power tariffs the building manager can run the plant at night time and reduce the cost of power, while he cools the thermal mass of the building by a few degrees.

This permits the generated cooling load the following day to be absorbed into the slab and thus reduces the demand on the chillers for several hours each day.

Roger Preston and Partners have carried out computer simulation of this night time cooling performance and find even with minimum ventilation that considerable energy saving is possible.

Building Simulation Ltd were commissioned by a leading UK developer to study the difference between VAV and the Flexible Space system and compared with VAV showed a marked 26% reduction in energy and CO2 emissions.

Reconfiguration Costs

Tenants immediately find reduced time in the initial fit out of their space permitting them to start their business operations earlier.

In a report to a Facilities Management conference in Strasbourg the General Manager of Panasonic in Paris reported the results he found in comparing his original building served by ceiling based fan coil systems with his new head quarters building served throughout with the flexible space system as follows.

Direct invoiced costs of change

Building With Fan Coil System $190-$210/m2

Building with Flexible Space system $19-$35/m2

Net saving in cost of change in the order of $150/m2

Time for reconfiguration

Building With Fan Coil System 10 weeks

Building with Flexible Space system 1 week

A reduction of 9 times in the time taken.

The savings simply in the cost of reconfiguration equate to the cost of the rent in many offices

The reduction in time taken is immeasurable in terms of the increase in staff productivity but is recognised to be very significant by leading organisations.

The Future:

As more and more projects adopt this style of construction we can expect to see a noticeable reduction in the cost of construction, nation by nation and improved indoor and outdoor air quality through reduced pollution in operation, reconfiguration and eventual demolition.

Author: Glan Blake Thomas C Eng, MCIBSE