A Guide On How To Choose The Appropriate Air Conditioning System?

Each of the four general types of air-conditioning systems has numerous variations, so choosing a system is not a simple task. With experience, it becomes easier. However, a new client, a new type of building or a very different climate can be a challenge.

We are now going to briefly outline the range of factors that affect system choice and finish by introducing a process that designers can use to help choose a system.

The factors, or parameters that influence system choice can conveniently be divided into the following groups:

Building design
Location issues
Utilities: availability and cost
Indoor requirements and loads
  Client issues

Building Design
The design of the building has a major influence on system choice. For example, if there is very little space for running ducts around the building, an all-air system may not fit in the available space.

Location Issues
The building location determines the weather conditions that will affect the building and its occupants. For the specific location we will need to consider factors like:

site conditions
peak summer cooling conditions
summer humidity
peak winter heating conditions
wind speeds
sunshine hours
typical snow accumulation depths

The building location and, at times, the client, will determine what national, local, and facility specific codes must be followed. Typically, the designer must follow the local codes. These include:

a. Building code that includes a section on HVAC design including ventilation.
b. Fire code that specifies how the system must be designed to minimize the start and spread of fire and smoke.
c. Energy code that mandates minimum energy efficiencies for the building and components. We will be considering the ASHRAE Standard 90.1 2004
d. Energy Standard for Buildings Except Low-Rise Residential Buildings7 and other energy conservation issues
In addition, some types of buildings, such as medical facilities, are designed to consensus codes which may not be required by local authorities but which may be mandated by the client. An example is The American Institute of Architects Guidelines for Design and Construction of Hospital and Health Care Facilities, which has guidelines that are extremely onerous in some climates.

Utilities: Availability and Cost
The choice of system can be heavily influenced by available utilities and their costs to supply and use. So, if chilled water is available from the adjacent building, it would probably be cost advantageous to use it, rather than install new unitary refrigerant-based units in the new building.

Then again, the cost of electricity may be very high at peak periods, encouraging the design of an electrically efficient system with low peak-demand for electricity. We will be introducing some of the ways to limit the cost of peaktime electricity in our final chapter.

The issues around electrical pricing and usage have become very well publicized in North America over recent years. The ASHRAE course Fundamentals of Electrical Systems and Building Electrical Energy Use introduces this topic.

Indoor Requirements and Loads
The location effects and indoor requirements provide all the necessary information for load calculation for the systems. 

The thermal and moisture loads – Occupants’ requirements and heat output from lighting and equipment affect the demands on the air-conditioning system.

Outside ventilation air – The occupants and other polluting sources, such as cooking, will determine the requirements.

Zoning – The indoor arrangement of spaces and uses will determine if, and how, the system is to be zoned.

Other indoor restrictions may be very project, or even zone specific. For example, a sound recording studio requires an extremely quiet system and negligible vibration. The methods of calculating the heating and cooling loads are fully explained, with examples, in the ASHRAE course Fundamentals of Heating and Cooling Loads.

Client Issues
Buildings cost money to construct and to use. Therefore, the designer has to consider the clients’ requirements both for construction and for in-use costs. For example, the available construction finances may dictate a very simple system.

Alternatively, the client may wish to finance a very sophisticated, and more expensive system to achieve superior performance, or to reduce in-use costs. In addition to cost structures, the availability of maintenance staff must be considered.

A building at a very remote site should have simple, reliable systems, unless very competent and well supported maintenance staff will be available. Clients’ approvals may be gained, or lost, based on their own previous experience with other projects or systems. Therefore, it is important for the designer to find out, in advance, if the client has existing preconceptions about potential systems.

System Choice
While all the above factors are considered when choosing a system, the first step in making a choice is to calculate the system loads and establish the number and size of the zones. Understanding of the loads may eliminate some systems from consideration. 

For example:  In warm climates where heating is not required only systems providing cooling need be considered. If there are significant variations in operating hours between zones, a system which cannot be shut down on a zone-by-zone basis may not be worth considering.

Typically, after some systems have been eliminated for specific reasons, one needs to do a point-by-point comparison to make a final choice. This is where the system-choice matrix is a very useful tool.

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