Most projects start with the owner(s) explaining his or her vision to architects and engineers. The HVAC engineer then designs the mechanical system and documents how it is to operate on drawings and specifications. A mechanical contractor is then given the responsibility of installing the equipment and adjusting the systems to operate per plan and specifications.
A question that frequently arises is “does the mechanical contractor need an electrician to power up the equipment”? Codes require a disconnect in the area of the HVAC equipment. A licensed electrician runs power from a panel to this disconnect. The HVAC technician, as part of his installation, will connect the equipment that he has installed to the adjacent disconnect.
As the acronym HVAC implies, the equipment may be for heating, ventilating, air conditioning, refrigerating or any combination of these. Each basic type of mechanical equipment is assembled with its individual operating controls, many of which have similar components. These common components may be grouped in general categories beyond the safety switches and fuses.
Each of the following is also subject to further differentiation based on its sensitivity and method of operation.
- Motor cycling controls: This would include thermostats, pressure motor controls, liquid or gas flow, liquid level, on/off and speed switches
- Relays: These would be based on amperage, voltage or thermal
In addition to the above, heating equipment has its own specialized controls depending on the type of fuel and whether the medium being heated is air or a liquid. The above components may be grouped as follows:
- Temperature: for heating would include steam, water and limit controls, for cooling would include refrigerant throttling controls
- Humidifying controls
- Dehumidifying controls
- Air flow controls
- Filter controls
- Safety controls
These controls may be divided into three basic groups; Operating Controls, Primary Controls and Limit Controls. All controls can be either pneumatic or electronic and trained, experienced HVAC technicians are competent to install and troubleshoot/service all of them.
Operating controls include thermostats, pressure and humidity sensors etc. which signal the start or stop of the equipment.
Primary controls are those that insure a safe start and operation of the equipment.
Limit controls are for safety and permit the equipment to operate only if the conditions are safe to continue.
The HVAC technician will install all the controls for each “stand alone” unit but when several are linked as a system, which also usually has a common control station, a controls sub subcontractor is used. These contractors are specialists and very necessary to the properly integrated functioning of all component parts of the completed system.
While economical operation has always a desirable goal, the energy shake-up of the 1970s and the publication of ASHRAE Standard 90 (Energy Conservation in New Building Design) truly initiated the push toward much more efficient mechanical systems. A major step forward was made with the introduction of DDC (Direct Digital Control) systems and more emphasis on variable frequency and variable volume drives with modulating controls. Pneumatic systems are giving way to electronic computer controlled environment management. Rather than the HVAC controls regulating a unit or group of units, they now are linked through a computer interface with the other building systems (lighting, fire safety, security/access control, telemetry and communication) into what is now known as Building Automation Systems or the current term “Intelligent Building.”
The direction now is for new buildings to be “green” and designed for LEED (Leadership in Energy and Environmental Design) certification. Carbon policy, energy prices, utilities ant the government in addition to a positive public image are acting as the incentives. The major barrier to investments in energy savings at this time is limited capital availability and the desire of many executives to see an ROI payback period of less than three years. Perceived fiscal responsibility to stockholders is balanced against environmental responsibility.