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The main purpose of the HVAC system is to achieve clean indoor air quality and human comfort (thermal comfort), there are many HVAC systems a designer or owner has the option to select based on the factors such as the type of the building, architecture, location, shape, surrounding climate, occupancy, envelop, level and frequency of activities, and the system operation schedule.
In addition to the above base factors that an HVAC system is expected to be selected upon, the energy consumption, system efficiency, initial and operational cost, and finally, feasibility (short and long term rebound positive effect) are of the owners and designers critical concerns.
This paper will discuss the elements of a typical feasible high performance low cost, fine tuned HVAC DDC integrated system to achieve the best for users, owners, and environment.
HVAC and its associated auxiliaries system are major energy consumers in a building, the rapid development of the advanced technology nowadays boosts the HVAC system feasibility as more complex control systems are developed for this industry and additional fine-tune, prompt response, standardized communication, ease of control and monitor, and remote accessibility.
The BMS/DDC (Building Management System/ Direct Digital Control) integrated system is the core of a good feasible high-efficient HVAC system.
The BMS is the most recent High-Tech energy management system that manage a building performance to the maximum desirable pre-determined set of parameters which able to control, monitor, adjust, save and record mostly all of the building facilities and utilities when integrated with all of the compatible building's Sub-LANs, a DDC is one of those LANs and can communicate with other control LANs under the supervision of the BMS.
BMS is able to supervise, control, adjust and record the illumination, electric power control, HVAC, security and observation, magnetic card and access, fire alarm, lifts, and other engineering systems.
Integrated with the BMS, the DDC performs the HVAC control management and communicates with the other building controllers via the BMS to achieve integration based on a specified, programmed event sequence.
The DDC is the heart of an efficient HVAC system, it finely tunes the digital/analog input/output communication between sensors, probes, stand-alone controllers, LANs controllers, and finally the controlled element which could be an actuator that adjust the process variable (flow, temperature, level, or pressure), and allows for a feedback signal to further adjust the desired process set-point. This whole process is reported in a real-time manner to the BMS system for further coordination with the other building's controllable systems to achieve integration based on the pre-programmed parameters.
In order to achieve the highest human comfort, energy saving, and a long term rebound effect strategy, The BMS/DDC system should be interlocked and integrated with a high-efficient and feasible HVAC system, this combination can awards energy saving, system and environment sustainability, human comfort, and business feasibility.
An Optimal Air System is a good example of a low-coast, high-performance, energy-efficient and a good investment for long-term rebound pay-back effect.
Optimal Air System concept is based on the low temperature supply system that needs, less energy consumption by the most energy consumer auxiliary that is the fan, this affects the sizing of the ducts (less duck size), air handling units and fan motors, all of which will be smaller and results in a system that requires less space and uses less power.
As this paper focuses on the HVAC/DDC integrated system application for human comfort, energy saving, and feasibility (long-term rebound effect), I will discuss and focus on the DDC and Optimal Air System integration for the above purposes and define characteristics, elements, and functions of both systems.
DDC has became the latest and the most recently used system for HVAC controls after the pneumatic and electromechanical control systems, digital pre-programmable controllers can handle extensive digital/analog data process from inputs (sensors, tranceducers and transmitters) that tyapically mesure temperature, flow, humidity, pressure or level, and outputs to final controlled devices to adjust a process variable based on a preset parameters, also recives a feedback signals from inputs again to further adjust signal command errors for best results based again on the setpoints. Digital inputs are Dry contacts from a control device, analog inputs are voltage and current signals that mesure variables such as humidity, pressure, level or flow form sensing devices and converted to percentage. Digital outputs are of 1 or 0 binary that either stops or starts equipments via a relay, analog outputs are voltage or current signals that control a process variable control devices such as valves, motors or dampers. The DDC program code may be customized for intended use such as: Time schedule, sequence of operation, trend logs, alarms.
2.1 Elements of a DDC
As described above, the three functional elements needed to perform the functions of a DDC system are:
a) A measurement element (Sensor, prob, Transmitter, Transducer)
b) An error detection element (Digital/Analog/pneumatic Controller, PCU)
c) A final control element (Motor/Piston Actuator, VFD, VSD, Relay)
2.2 DDC controled mediums
The DDC controls two variables:
I. A controlled variable is the process variable that is maintained at a specified value or within a specified range.
II. A manipulated variable is the process that is acted on by the control system to maintain the controlled variable at the specified value or within the specified range.
2.3 Functions of DDC system
In any DDC, the four basic functions that occur are:
3. DDC LAN-WAN Configuration
DDC is where mechanical and electrical systems and equipment are joined with microprocessors that communicate with each other and to a central computer BMS. This computer and controllers in the building Management system can be networked to the internet or serve as a stand alone system for the local peer-to-peer controller network only Fig 1. Additionally, the controllers themselves do not need a computer to operate efficiently as many of these controllers are designed to operate as stand-alone controllers and control the specific equipment they are assigned to control.
Fig 1. Typical peer-to-peer controller network 
With a few exceptions, each DDC or building automation controller holds their own programs and has the ability to communicate to other DDC building automation controllers. It is important for the DDC or building automation controllers to communicate to each other. If the network fails for whatever reason then the system may still function (because the DDC controllers in a BMS system are stand-alone) but it will not function as efficiently as designed.
The DDC/BMS system can be configured as independent (localized) closed-system, or DDC open-system based on accessibility options required by a group of buildings managed by a single company or property management firm (centralized), or a single property to be monitored and controlled by its own (localized) Fig 2.
Fig 2. DDC/BMS LAN/WAN configuration 
3.1 BACnet compatibility
BACnet is the term commonly used to refer to the ANSI/ASHRAE Standard 135- 1995, adopted and supported by the American National Standards Institute (ANSI) and the American Society of Heating Refrigeration and Air-Conditioning Engineers (ASHRAE). BACnet stands for Building Automation and Control network. BACnet is a true, non-proprietary open protocol communication standard conceived by a consortium of building management, system users and manufacturers .
A closed protocol is a proprietary protocol used by a specific equipment manufacturer. An open protocol system uses a protocol available to anyone, but not published by a standards organization. A standard protocol system uses a protocol available to anyone. It is created by a standards organization.
An open system is defined as a system that allows components from different manufacturers to co-exist on the same network. These components
would not need a gateway to communicate with one another and would not require a manufacturer specific workstation to visualize data. This would allow more than one vendor's product to meet a specific application requirement.
The DDC/BMS' BACnet based LANs and Sub-LANs can be accessed, controlled and monitored from remote locations via the Internet trough a centralized data management system which is capable of collecting data from multiple sites. This is accomplished by connecting with a gateway for collecting data from the lighting and air-conditioning control systems installed in each building or factory, and the center server for providing data collection, database and web server functions along with security measures applied to all transmitted data.
Based on the capability of real-time monitoring and analysis of actual energy consumption such as electricity and gas from a remote location by using a web browser, this system is able to achieve the maximum level of energy saving in buildings and factories which in turn, reduce the emissions and the environmental impacts by taking advantage of its cost effectiveness and by limiting the required energy for a specific application or function.
Fig 3, Integrated BACnet based WEB Browser BMS Control System Layout 
4. DDC/BMS integrated features, application and functions
4.1 Energy saving
DDC/BMS allows the owner to set up schedules of operation for the equipment and lighting systems so that energy savings can be realized when the building or spaces in the building are unoccupied.
Have algorithms as reset schedules for heating plants, static pressure control, and other systems where energy savings can be realized through these predictive programs.
4.2 Human comfort (thermal Comfort)
DDC/BMS system allows the equipment optimal start with pre-scheduled program. Optimal start is allowing the equipment to be brought on in an ordered and sequential manner automatically on a schedule before the building is reoccupied so that space set points can be realized before occupation. Event sequence programming features allow the system to compare space temperature, outside air conditions, and equipment capabilities so that the equipment can be turned on at an appropriate time to ensure space set points are achieved before occupation.
Have trim and respond capabilities. Based on zone demand the set point for various heating and cooling sources will change according to demand from the zones. For instance, in a Variable Air Volume system, all the VAV boxes are served from a central air handling unit. If all the zones are at set point then the supply air temperature set point of the air handler is automatically changed to prevent mechanical cooling from occurring when it is unnecessary. When the zones grow warmer the supply air temperature set point is automatically lowered to allow mechanical cooling to satisfy demand.
In conjunction with the appropriate mechanical system set-up, offer economizing based on enthalpy calculations and/or CO2 set point control.
4.3 Long-term rebound effects
Offer load shedding when power companies are at peak demand and need business and industry to cut-back on power usage to prevent black outs. Building Management systems for instance, allow the owner to cycle various things off like water heaters or drinking fountains where use of these things-
-will not be noticed even though they are off.
Management companies who acquire a good DDC/BMS can set up the system to bill tenants for energy usage (fewer employees required).
Ability to send alarms via email, pager, or telephone to alert building managers and/or technicians of the developing problems, and system failures.
4.5 Other applications and compatibilities
Have the ability to monitor energy usage including the ability to meter electric, gas, water, steam, hot water, chilled water, and fuel oil services.
Have the communications abilities to be integrated with other buildings via WAN setup using the standardized TCP/IP family of protocols. It is BACnet base web browser compatible and other open source communication protocol which allows the system to be accessed via the web browser from remote locations. (Refer to 4.2)
5. High-performance Low-energy HVAC design
Recall the Introduction, In addition to BMS/DDC System application for energy saving and high HVAC system performance, a green HVAC system design will achieve all aspects of comfort, energy saving, low initial and operational capital costs, and adds more efficient performance in conjunction with the DDC system, an example of such green HVAC system would be an Optimal Air System .
Optimal Air System concept, idea and example are taken from McQuay Air Conditionning/2002 McQuay International/Application Guide AG 31-005 as an example to illustrate its benefits for energy saving, human comfort, lower initial cost and long term rebound effects.
Optimal Air systems uses less energy than conventional systems on an annual basis, for example, In a conventional system, supply air temperatures run between 54Â°F -57Â°F from the air handling unit. With duct heat gain, the supply air ranges from approximately 56Â°F-59Â°F out of the air diffuser.
In Optimal Air System, supply air temperature run between 45-52Â°F from the air handling unit to optimize energy consumption, reduce first capital cost and improve humidity control. Optimal Air has for years been extensively used in grocery stores and is gaining increasing popularity in comfort cooling applications such as offices and schools.
There are several benefits of Optimal Air that make it an attractive system for use in a wide variety of applications.
It Saves Space and Reduces Energy and Construction Costs, increases the amount of sensible heat that each CFM delivered to a zone can absorb. While 50Â°F air may not seem much colder than 55Â°F air, the delta T rises from 20Â°F to 25Â°F. That is an increase of 25%.
This affects the sizing of the ducts, air handling units and fan motors, all of which will be smaller and results in a system that requires less space and uses less power. In many applications, fans can use more power annually than refrigeration (chillers, condensing units, pumps, and compressors).
An example of annual 10-story building energy usage of 200,000 square-feet of HVAC components, the fan energy use is high because the fans operate every hour the building is occupied providing minimum air movement, ventilation air, heating, etc. In this case, an Optimal Air system would have a very real impact on overall energy costs.
Fig 4, Annual HVAC Energy Usage 
5.2 Less Humidity, more comfort
Optimal Air systems take more moisture out of the return and ventilation air mixture as it passes over the cooling coil. The lower moisture content in the supply air reduces the "Psychrometric balance point" humidity level in the conditioned space. This allows the space temperature to be set higher while achieving the same comfort level for occupants and further reduces the supply air quantity and fan power requirement.
5.3 Quieter and Improve Indoor Air Quality (IAQ)
The lower air volume required for Optimal Air systems makes them quieter than conventional systems. Fan sound generation is a function of fan type, static pressure and air volume. By reducing air volume (and often the total fan static pressure) Optimal Air systems generate lower fan sound which can result in more desirable space conditions. This reduced sound generation can also be used to reduce the cost of any required noise attenuation in critical applications.
The lower required air volume can also be used to reduce filter face velocities, allowing more efficient filters to be used without high energy cost penalties. The lower air temperature and resultant humidity levels also reduce the chance of mold growth in the air handling units, ducts or the occupied space.
The example of the building above requires a supply air of 26,667 CFM. The HVAC system is floor by floor VAV air handling units with a two chiller primary secondary system, Optimal air works equally well with applied rooftop units or indoor vertical self-contained units.
Table 1, HVAC system performance with optimal air system 
Table 1 shows the HVAC system performance as the supply air temperature, to the duct, is lowered. It is important to differentiate between supply air temperature off the cooling coil and supply air temperature into the duct.
To accommodate the lower supply air temperature, the chilled water supply temperature (CWST) was gradually lowered, the air handling unit coils deepened to allow for closer approaches, and chiller performance was adjusted to deal will the increased lift. Because of their basic operating differences, DX rooftop and self-contained systems may have a different Optimal Air temperature than a chilled water system. When considering multiple system options, it is important to use Energy Analyzer for each in order to identify the best option.
5.4 Optimal Air Balance Point
Reduced fan energy must be "traded off" against increased refrigeration energy. This trade off varies with the type of building, the type temperature control system, the type air conditioning system and geographic locale. Therefore, the "optimal" supply air temperature is different for every job. When only energy costs are a factor and no thermal storage is involved, this optimal supply air temperature generally falls in the 47Â°F -52Â°F range. It can be determined by comparing total system energy consumption with varying supply air temperatures using an energy analysis program.
5.5 Space Design Temperature and Related Comfort
Temperature, humidity, air velocity and mean radiant temperature directly influence occupant comfort. Conventional designs are usually based on maintaining 75Â°F and 50% RH (Relative Humidity) in the occupied space. Figure 5 shows the ASHRAE comfort zone where 80% of the people engaged in light office work are satisfied. As the relative humidity is lowered, the space air temperature can be raised and still provide occupant comfort.
The leaving air condition from the air handling unit is the primarily control of the relative humidity in the occupied space.
The internal moisture gains from people, kitchens, etc, as well as infiltration also play a part.
Fig 5, Equivalent comfort chart 
In most climates, the lower the supply air temperature, the lower the humidity ratio and the drier the space. Figure 5 shows sensible heat ratio lines for conventional, Optimal and low supply air temperatures. As the space relative humidity is lowered, the space temperature set-point rises from 74Â°F to 78Â°F.
5.6 ASHRAE Compliance
The 1999 and 2001 version of ASHRAE Standard 90.1, Energy Standard for Buildings except Low Rise Residential Buildings , has mandatory requirements for refrigeration equipment and prescriptive requirements for fan work. The Standard recognizes that Optimal Air systems improve fan work significantly and provides credits to account for improved fan performance. In addition, refrigeration system performance is rated at conventional conditions or special tables are provided to account for non-standard operating conditions (as is the case with centrifugal chillers). In either case, ASHRAE Standard 90.1 does not penalize Optimal Air systems.
5.7 Design Considerations
Design of refrigeration and air handling equipment for an Optimal Air system is similar to the design of a conventional air temperature system. Attention must be paid, however, to air distribution, controls and duct design. Conventional diffusers, when properly applied, will work with Optimal Air.
Controls also require only minor changes from conventional systems. In particular, programming of economizer controls and supply air temperature reset. Finally, the ducting system must be sized for the reduced air volume to take full advantage of the potential capital savings. Duct insulation and sweating should also be reviewed to provide a trouble free system.
Not every building type is a good candidate for Optimal Air. When air volumes are dictated by air turnover rates, such as some health care applications, Optimal Air offers no advantage. In fact, there would be increased reheat costs. Office buildings are a strong candidate for Optimal Air. They have high sensible heat ratios and typically less than 20% ventilation loads. Schools can also be a possibility. Generally speaking, as the percentage ventilation load increases, Optimal Air becomes less attractive.
Location and climate also impact whether or not Optimal Air is a good candidate. Locations where weather provides significant economizer hours between 45 and 55Â°F will limit the savings. Ultimately, each project must be checked by performing the applicable specific calculations. The following should be considered:
Load and Balance Point calculations, Space Temperature Set-point evaluation, Design Load Calculation, Primary and Secondary System Selection, Parallel, mixing or series VAV-Fan powered boxes, Perimeter Heating, Air Distribution, Diffusers (based on air flow and the throw distance calculation), Duct design (considering duct heat gain, sweating and insulation).
5.8 System Life-Cycle Analysis
Evaluating different engineering solutions is always part of a good proposal. Optimal Air systems are no different.
In the case of Optimal Air, there may be no need to do any calculations because Optimal Air systems cost less to build (lower capital cost) and have the same operating cost as conventional systems (assuming the balance point was used for the design).
Duct sizing will decrease almost linearly with reduction in air volume. The installed cost will not change linearly because of the labor portion. A 20% reduction in air volume can result in 80% savings of the 20% reduction or 16% overall savings in sheet metal cost.
On the plus side, there are less pounds of steel and fewer man-hours to install it. On the minus side there is more insulation. Terminal boxes and diffusers will be a wash since there are fewer of them but the equipment cost will be higher than conventional equipment.
HVAC equipment will cost about the same. This is conservative because the air handling equipment will cost less and refrigeration equipment will be slightly more. There is typically more capital invested in air handling than refrigeration.
Building envelope should be the same for new construction. In the case of retrofit applications, it will depend on the quality of the existing building.
The cost of space may also need to be evaluated. Not accounting for space savings is conservative. There will be space savings but they may be difficult to realize. If enough plenum height savings can be realized to add another floor within the same building envelope, then that rentable space should be accounted for.
Simple payback calculations do not take into account the cost of money, taxes and depreciation, inflation, maintenance or increases in the cost of energy. A more complete analysis should include Internal Rate of Return (IRR) and net present value (NPV). In the HVAC industry, many projects fail simple back (they are in the 5-year range) while passing IRR (they offer a 25% rate of return).
Software analysis tools can be used to perform both energy and life-cycle analysis that include simple payback, IRR and NPV.
Building owners and designers faced with increased concerns for energy saving and environmental stewardship search for cost effective system options for their projects.
The DDC, integrated with a high-performance low-energy HVAC system as the Optimal Air system can deliver both low first costs and reduced energy costs in a new construction and retrofit applications. This integrated system will not only meet the efficiency and sustainability of its performance at the desired set-parameters, but when designed with advanced selection tools, installed with the most advanced DDC/BMS system, and supported by trained operators, will achieve both energy saving and long term rebound effect (pay-back), maximum human thermal comfort, in addition, it allows building owners to compare predicted energy use to actual performance, this leads to a flexible budgeting, further future system adjustment and energy consumption cut-back. The whole integrated DDC/BMS HVAC system function will also contribute in the environmental impacts reduction.
In today's challenging energy efficiency, building owners need proven system that delivers the necessary performance to meet their integrated environmental sustainability and business goals .