HVAC System
New Office Building of [COE]

When designing public buildings or office buildings with large numbers of occupants, one factor that has always been considered is efficient energy use. The HVAC system — which accounts for over 70% of a building's total energy consumption — is always at the center of that discussion. Energy-saving devices have been adopted in many forms, sometimes as a transfer of technology and knowledge from Western countries. However, due to differing climates, some approaches may not be suitable for the hot and humid tropical climate near the equator. Furthermore, external pollution problems that affect buildings and their occupants — such as PM2.5 dust, the COVID-19 pandemic, and new threats that will inevitably arise in the future — must all be considered in building design. The design must be adaptable to accommodate new challenges. The concept of creating a healthy indoor environment (Well-Being) for building users is increasingly discussed, and the question "Are we in a safe place?" will grow louder from occupants who spend many hours working, conducting activities, trading, or living in air-conditioned spaces.

The HVAC design for the Council of Engineers Building attempts to be appropriate for Thailand's tropical climate. It is well known that complying with standard design guidelines requires bringing in fresh outside air — air that carries both heat and high humidity — to be conditioned as fresh air supply into the building. This consumes a great deal of energy to reduce both temperature and humidity, far more than in Western countries due to the different climate. Furthermore, creating a high-quality indoor air environment requires supplying at least 30% more fresh air than standard design specifications — both for well-being reasons and energy conservation. In this context, a high-efficiency cooling system must be selected. The Council of Engineers Building chose a water-cooled chiller plant system, which can achieve very favorable energy performance figures (kW/TON).

On the air-side, the design separates the fresh air supply using a Dedicated Outdoor Air System (DOAS). This system delivers fresh air at 30% above standard requirements and includes supplementary equipment to reduce energy consumption. The DOAS is also a critical component for PM2.5 protection and is the primary device responsible for maintaining the building as a 24/7 environment.

Because HVAC energy consumption represents 70% of total building energy, a high-efficiency HVAC system must be selected. Even though the Council of Engineers Building has a total floor area of less than 10,000 square meters — and is not an extra-large building — the decision was still made to use a chiller plant with a water-cooled chiller, which delivers superior efficiency.

The selected chillers use the latest compressor technology — oil-free compressors driven by magnetic levitation (MAGNETIC OIL FREE COMPRESSOR).

These chillers achieve a satisfactory energy consumption rate below 0.6 kW/TON under Thailand's climate conditions. The total cooling load is approximately 280 tons of refrigeration, and three 150-TON chillers are selected. The chilled water piping circuit and chilled water temperature production are divided into two separate loops. The low-temperature chilled water circuit for the DOAS and the server room is maintained at a constant 7.2°C (45°F), while the high-temperature chilled water circuit for all general office areas varies with outdoor temperature and season between 7.2°C and 10°C (45–50°F).

The chilled water temperature setpoint is controlled by an automatic control system that processes both indoor and outdoor temperature and humidity data, then sets the target chilled water temperature for the high-temperature chilled water circuit to maximize energy savings. However, this design approach requires selecting larger air conditioning units that can still adequately cool the spaces when supplied with higher-temperature chilled water. For approximately 70% of annual operating hours, the units will receive chilled water at 10°C (50°F).

Dividing the chilled water circuits with different temperatures reduces energy consumption from the chiller plant side. On the air side (AIR SIDE), the DOAS is designed to handle moisture and supply dry fresh air into the building. With the indoor humidity load from people and building activities kept relatively low, the chilled water temperature in the circuit serving the office areas can be raised while still maintaining desired temperature and humidity control.

Raising the chilled water temperature to save energy is a well-established practice in large buildings in Thailand, because doing so directly reduces the compressor workload of the chiller, saving as much as 10–35% energy depending on the degree of temperature rise. However, in a conventional single-loop chilled water system, raising the chilled water temperature improves energy efficiency at the cost of reduced dehumidification performance — causing indoor humidity to rise and sometimes leading to mold problems. To suit the hot and humid tropical climate, the chiller plant is therefore designed as a Dual-Temperature Dual-Loop system (DUAL TEMPERATURE DUAL LOOP Chiller Plant, DTDL).

On the fresh air supply side, the design includes a dedicated fresh air unit (DEDICATED OUTSIDE AIR UNIT) with a supply capacity 30% above the design standard. Supplementary equipment is included to reduce energy consumption. The DOAS unit is also the key device for PM2.5 protection and for maintaining the building as a 24/7 operation.

Creating a healthy indoor air environment is increasingly necessary today due to worsening external environmental conditions — including PM2.5 dust pollution that affects every major city and the urban heat island effect, among others.

The Council of Engineers Building is designed with a dedicated fresh air supply unit — separate from the air conditioning units — called a DOAS (Dedicated Outdoor Air System). It is installed on the 7th floor of the building and delivers conditioned fresh air directly to individual zones and rooms throughout the building, without routing it through any air conditioning unit. The DOAS removes moisture from the outside air, bringing the moisture content of the fresh air supply below the indoor air moisture level. The result is dry fresh air that can be introduced into the building without causing humidity problems.

The fresh air supply rate is designed to create a positive indoor air pressure — at a flow rate 30% above ASHRAE 62.1 standards. The DOAS is equipped with MERV 8 and MERV 14 air filters to remove PM2.5 dust before air enters the building. The PM2.5-free, positively pressurized fresh air prevents PM2.5 from infiltrating the building.

While the DOAS might appear to consume high amounts of energy, energy conservation measures are incorporated. A heat pipe is installed to pre-dry the incoming fresh air, preventing mold growth in the ducts. A heat recovery unit is also installed to recapture energy from the exhaust air — which would otherwise be lost — and use it for a first-stage pre-cooling of the incoming fresh air. In high-occupancy rooms such as examination halls and the 300-seat conference room, CO₂ sensors are installed to control the fresh air supply rate as needed, saving energy.

It is common in large commercial office buildings to use large air handling units (AHU) with supply air ductwork. However, this approach typically leads to contamination inside the ducts that is difficult to clean — because the cool air leaving the cooling coil carries humidity of 90–95% RH, which then condenses and accumulates dust and mold as it travels through the supply ducts. To create a healthy indoor air environment, the design instead installs air conditioning units as individual point-cooling devices, with virtually no supply air ductwork throughout the entire building. The air conditioning units used are of the CASSETTE TYPE 4-WAY or 1-WAY type, replacing all ductwork.

Furthermore, cooling via supply air ductwork is less efficient than cooling via chilled water piping — fan energy losses in ductwork are higher than pump energy losses in a hydronic system. All air conditioning units in the project are supplied as complete factory-assembled sets, pre-fitted with PICV valves and other control valves from the manufacturer.

Because of this design, no additional air filtration equipment is required in any area of the building. During seasons when PM2.5 dust is not a concern, the high-efficiency MERV 14 filter panels can be removed and stored to reduce building operating costs.

The Council of Engineers Building is designed to maintain an indoor environment that promotes the health and well-being of its occupants. To sustain this standard of well-being continuously, the building's air conditioning system must never be shut down — which may seem unusual compared to other office buildings, where the air conditioning is typically turned off at the end of working hours or overnight, and left off entirely on holidays. The harmful consequence of shutting down the HVAC system is that dust, moisture, and the formation of various molds begin to accumulate.

The Council of Engineers Building is designed to run its air conditioning system continuously — 24/7, or 24 hours a day, 7 days a week. However, during periods of low occupancy or on holidays, the system operates in a reduced mode: the DOAS continues to run but is adjusted to maintain only a slight positive pressure inside the building. With fewer people entering and exiting, fewer door openings occur to dissipate that pressure. The system supplies conditioned fresh air that maintains temperature and humidity within the range of 28–30°C and 50–60% RH — keeping the building breathing continuously, as though the building itself is still alive.

Due to the complex control requirements of the Council of Engineers Building's air conditioning system — where the chiller operating commands may need to change every hour — and the need for efficient operation and energy monitoring visible to building users, the chiller plant and HVAC control system is designed with a control room on the 1st floor and a web-based Chiller Plant Management program. This allows authorized users to log in and control the system remotely from anywhere via the internet, and displays live system status on an LED screen on the 1st floor for all building users to see.

In addition to controlling the chiller plant and HVAC automation system — which manages all equipment including pumps, cooling towers, and chillers — it also controls all air conditioning units throughout the building.

The Council of Engineers Building features an open semi-outdoor area on the 5th floor — a space designated for semi-outdoor activities, relaxation, an indoor garden, and a dining area for members attending examinations or seminars at lunchtime. Dining in a semi-outdoor open space helps avoid the problems that typically arise from eating in air-conditioned spaces, such as odors, excess humidity, and waste.

However, to ensure comfort and usability in this space — which has no full air conditioning — supplementary systems are designed for the semi-outdoor area.

Around the plant pots and the living green wall (which uses real plants — no artificial plastic plants are used in this project), additional oxygen is generated. A DEHUMIDIFIED GREEN WALL moisture system is installed, using condensation on the chilled water pipe surface to collect water for irrigating the plants while simultaneously reducing humidity.

To improve air circulation and create a breeze for building users, ceiling fans are installed. Since the floor-to-ceiling height of this level is not very high, BLADELESS CEILING FANS are selected.

Despite these installed systems, there was concern about the heat during midday — especially during Thailand's hot season — which might make the space uncomfortable for full use. Therefore, the design also incorporates supplemental cool air supply: fresh air from the DOAS at approximately 24°C is routed through a cooling coil operating at 100% sensible heat only, so the air remains dry while being cooled. This cool dry air is distributed via UFAD (Underfloor Air Distribution) — floor-embedded diffusers positioned at seating locations to provide localized cooling for the space.

In order to achieve maximum design efficiency and minimize inter-discipline conflicts, this project employed 3D virtual building model (BIM) technology to create a digital simulation of the building to assist in the design process. This was particularly important for the HVAC system's core component — the chiller plant room and cooling towers. The chiller plant room is located on floor 7.5, which is the sub-roof level of the building and features a sloping roof structure.

Using BIM (Building Information Modeling) technology allowed the team to identify constraints in each discipline from the very beginning of the design process, and to resolve those constraints through design modifications. For example, the structural roof framing above the chiller plant room — once the 3D models from each discipline were combined in the early design phase — revealed height conflicts. As a result, the structural framing was changed from a space-frame truss to a beam-and-column steel structure, to provide the required ceiling height for the chiller plant room at the critical locations.

Once the full building model was completed across all disciplines, it was further used for material quantity take-offs, fire and smoke simulation, evacuation simulation, as construction drawings, and as a database for building maintenance going forward.