HVAC CONTROLS (See section 18.9) Thermostat (additional information) A thermostat is the example of a sensing device. Sensing devices of an HVAC control system sense air or water temperature, humidity, and steam pressure or air pressure, and convert the sensation into a transmittable signal. A thermostat signals the heating/cooling equipment when to start and stop and how much to modulate. The temperature difference between when the equipment is shut off and when it is called to restart is referred to as the "dead band."

HVAC CONTROLS When the room temperature falls below a specified set point for heating minus the thermostat dead band, the thermostat will call for heating. If the room temperature rises above a user-specified set point for cooling plus the dead band value, the thermostat will call for cooling.

HVAC CONTROLS Thermostats must be located where they can sense the average temperature of the space to be conditioned. They should not be located where there is a possibility of exposure to conditions that are not representative of the whole space. such as in direct sunlight, in a cold draft, on an outside wall, or near a heating or cooling surface.

HVAC CONTROLS If the thermostat is desired to be concealed for certain reasons, a variation of the standard wall thermostat can be located in the return air duct. Thermostats are also available with remote-sensing wires which may be wrapped inconspicuously around an art object.

HVAC CONTROLS Dampers (additional information) Self-contained microprocessor-based thermostats can be programmed for a wide range of daily setback cycles that can change for each day of the week. These devices are simple to operate and can control a number of different zones with separate sensors. Dampers (additional information) Circulating and controlling of air extensively involve dampers.

HVAC CONTROLS Single blade: They are mostly manually operated and installed with a locking device, called a locking quadrant, by means of which they may be fixed in the desired position. May also be automatic and may either be center or edge pivoted. Barometric: Usually made with fine-tune adjustment features and equipped with adjustable counterweights. Often used in the smoke pipe of a furnace or boiler to stabilize draft conditions and permit close adjustment of fuel burners. Multiblade: Used for control of large volumes of air moving at high or low velocities in large duct systems. They are of two types: parallel blade and opposed blade. In parallel blade type, all blades turn in same direction. In opposed blade type, alternate blade turns in opposite direction.

NOISE AND VIBRATION (Additional information) Sound becomes noise when it is objectionable to the occupants of a building. Without proper precautions, sounds generated by HVAC systems may become irritating noise. Sound is a by-product of energy supplied to the moving components of HVAC equipment. During the transformation of electrical or combustion energy into useful work, energy losses appear as heat, vibration, and sound. The items contributing the most sound are pumps, fans, compressors, and room air diffusers.

NOISE AND VIBRATION The HVAC industry has established noise criteria (NC) values for evaluating the acceptability of sound levels. NC values for different types of buildings range from 30 to 40 decibels. A decibel is a unit of comparative sound measurement (a whispered conversation at a distance of 6 ft. from the ear, for example, has a sound pressure level of 30 decibels). The noise level and vibration transmission can be partially controlled by mechanical isolation, shields, baffles, and acoustical liners. Massive concrete pads and vibration isolators are placed between the equipment and the building to avoid transmission of vibration and noise through the structure. Equipment rooms can be acoustically isolated if necessary.

NOISE AND VIBRATION Rectangular ducts, particularly those with high aspect ratio, can transmit excessive noise if not properly supported. Round ducts are stronger and have better aerodynamic characteristics, and therefore experience fewer noise problems. Registers and diffusers should be selected to minimize noise output. Equipment spaces should be separated as far as possible from spaces with demands for low background sound levels. Use of flexible fabric connections is recommended at points where the ductwork connects an equipment subject to vibration.

MISCELLENEOUS (Additional information) Aspect ratio It is the width to height ratio of an air supply device (e.g. a supply duct). A rectangular duct with an aspect ratio closer to one has lower frictional resistance. Aspiration ratio As supply air leaves a supply register, it has a certain volume of flow rate expressed in cfm; this is called primary air. As soon as this air leaves the outlet, it begins to attract air already existing in the room; this room air, called the secondary air, joins the primary air and is carried along with it. The moving air stream has now a much greater volume by the time it reaches the end of its throw. This total volume, divided by the primary air volume is called the aspiration ratio. A high aspiration ratio is good, because it means that a greater quantity of air is kept in motion, with less chance of stagnation in parts of the room and with less chance of temperature stratification within the room.