Heating, Ventilation, and Air Conditioning

Question 1

Heating Load

Answer 1

A buildings heating load is the maximum heat loss during the heating season. It consists of heat to make up for transmission and infiltration losses.

Question 2

Maximum Heating Load

Answer 2

The maximum heating load occurs when the outside temperature is the lowest. It corresponds to the minimum furnace size, even though the lowest temperature occurs only a few times each year.

Question 3

Average Heating Load

Answer 3

The average heating load is derived from the maximum heating load and is used to determine the annual fuel requirements.

Question 4

Transmission Loss

Answer 4

Transmission loss is the heat lost through the walls, roof and floor.

Question 5

Infiltration Loss

Answer 5

Infiltration loss is the heat required to warm ventilation and infiltration air.

Question 6

Infiltration Air

Answer 6

Infiltration air is the leakage of outside air into the air conditioned space by means of cracks, windows, doors and other openings.

Question 7

Design Conditions

Answer 7

Inside design temperatures for residences and office spaces range between 21.1 to 22.2 degrees Celsius.

Question 8

Total Resistance

Answer 8

Is the total resistance to heat flow through the entire thickness of the material. It is the product of the resistivity and the material thickness. Conductance is the reciprocal of the total resistance.

Question 9

Unit Resistance

Answer 9

The resistance per unit thickness of the material. Conductivity is the reciprocal of the unit resistance.

Question 10

Overall Coefficient of Heat Transfer

Answer 10

Can be calculated for each transmission path from the conductivities and resistances of the individual components in that path, or it can be obtained from tabulations of typical wall/ceiling construction.

Question 11

Ventilation Air

Answer 11

The new air that is deliberately drawn in from the outside and mixed with return air, to meet the necessary needs of the occupants in the conditioned space. It is provided to the occupied space primarily to remove heat and moisture generated in the space.

Question 12

Humidification

Answer 12

Ventilation provides humidification to the occupied space. It can be provided by evaporating water in the occupied space or by injecting water or steam into the duct flow.

Question 13

Metabolic Heat

Answer 13

Contains both sensible and latent heat

Question 14

Sensible Heat

Answer 14

the pure thermal energy that increases the air’s dry bulb temperature.

Question 15

Latent Heat

Answer 15

Moisture that increases the air’s humidity ratio.

Question 16

Ventilation for Heat Removal

Answer 16

Ventilation requirements can be calculated from sensible heat and/or latent heat (moisture generation) rates. The sensible heat loads will usually be more significant than the latent load, and ventilation will be determined solely on that basis. When large moisture sources are present, however, the latent loads may control.

Question 17

All Air Conditioning System

Answer 17

These systems maintain the temperature by distributing only air, and most systems rely on internal loads for heating, sending only cold air to the space.

Question 18

Air and Water Conditioning System

Answer 18

Air and water are both distributed to the conditioned space.

Question 19

All Water Conditioning System

Answer 19

The cooling and heating effects are provided solely by cooled and/or heated water pumped to the conditioned space.

Question 20

Unitary Conditioning System

Answer 20

The fan, condenser, and cooling and heating coils are combined in a standalone unit for window and through the wall installation.

Question 21

Cooling Load

Answer 21

The aspects of determining the cooling load is similar in some respects to the procedure of finding the heating load. The aspects of determining inside and outside air conditions, heat transfer from adjacent spaces, ventilation air requirements and internal heat gains are the same as for heating load calculations. However the cooling load is complicated considerably by many factors and its important to distinguish these factors into three terms, instantaneous heat absorption, instantaneous heat gain, and instantaneous cooling load.

Question 22

Instantaneous Heat Absorption

Answer 22

Is the solar energy that is absorbed at a particular moment.

Question 23

Instantaneous Heat Gain

Answer 23

Is the energy that enters the conditioned space at that moment. Due to solar lag, the heat gain is a complex combination of heat absorptions from previous hours

Question 24

Instantaneous Cooling Load

Answer 24

Is essentially the convection portion of the instantaneous heat gain.

Question 25

Source of Cooling

Answer 25

Once the cooling load is determined, the source and size of the cooling unit must be considered. Cooling normally comes from liquefied refrigerant passing through a cooling coil. Some or all of the airflow passes across the cooling coil. Alternatively, cold water may be used in the coil to cool the airflow. In such cases the water is cooled in a chiller running its own refrigeration cycle. The use of outside air in economizers is also another method and that air may still be conditioned by passing through coils, but less change will be required.

Question 26

Economizer

Answer 26

An economizer is an electromechanical system that changes a portion of the cooling process in order to decrease cost, usually by taking advantage of cold ambient air.

Question 27

Water Side Economizer

Answer 27

A water side economizer substitutes natural cooling from a cooling tower for the chiller’s more expensive refrigeration cycle when the ambient air temperature drops below the desired coil temperature.

Question 28

Air Side Economizer

Answer 28

An air side economizer increases the amount of outside air that is brought into a space when the ambient air characteristics (temperature, humidity, or enthalpy) are better than the return airflow.

Question 29

Instantaneous Cooling Load (Walls & Roof)

Answer 29

For exterior surfaces, the cooling load is calculated using values of CLTD. These values are dependent on the time of year, location and orientation, type, configuration, orientation of the surface, as well as other factors and are calculated using base conditions of these factors. Using the CLTD/SCL/CFL method, the instantaneous cooling load for conduction through opaque walls and roofs uses a corrected CLTD value. This occurs because base conditions generally don’t coincide precisely with actual conditions during the study period, so CLTD is corrected according to the indoor design temperature and mean outdoor temperature.

Question 30

Instantaneous Cooling Load (Windows)

Answer 30

Using the CLTD/SCL/CLF method, the cooling load due to solar energy received through windows is calculated in two parts. The first is an immediate conductive part; the second is a radiant part. Appropriate tables are needed to evaluate the shading coefficient (SC) and the solar cooling load factor (SCL) for the radiant portions.

Question 31

Cooling Load (Internal Heat Sources)

Answer 31

Latent Loads (including metabolic latent loads) are considered instantaneous cooling loads. Only a portion, given by the CLF, of the sensible heat sources show up as instantaneous cooling load. The CLF is a function of time, and depends on zone type, occupancy period, interior and exterior shading, and other factors. Although tables are necessary to evaluate the CLF, there are cases where the CLF is assumed to be 1.0. These include when the cooling system is shut down during the night, and when there is high occupant density, and when lights and other sources are operated for 24 hours a day.

Question 32

Latent Loads

Answer 32

Increase cooling loads

Question 33

Sensible Heat Ratio or Sensible Heat Factor

Answer 33

Is the sensible heat load divided by the total load (Sum of latent and sensible loads). Most air conditioning equipment is designed to operate at a sensible heat ratio in the range of 0.70 to 0.75. A sensible heat ratio of 0.77 matches the performance of typical residential vapor compression cooling systems.

Question 34

Latent Factor

Answer 34

The latent factor is the reciprocal of the sensible heat ratio. A latent factor of 1.3 matches the performance of typical residential vapor compression cooling systems.