Building Design and Construction Handbook - Merritt - Ventech!

13.18 SECTION THIRTEEN
13.3.3 Outline of Design Procedure
Design of an HVAC system is not a simple task. The procedure varies considerably
from one application or project to another, and important considerations for one
project may have little impact on another. But for all projects, to some extent, the
following major steps have to be taken:
1. Determine all applicable design conditions, such as inside and outside temperature
and humidity conditions for winter and summer conditions, including
prevailing winds and speeds.
2. Determine all particular and peculiar interior space conditions that will be
maintained.
3. Estimate, for every space, heating or cooling loads from adjacent unheated or
uncooled spaces.
4. Carefully check architectural drawings for all building materials used for walls,
roofs, floors, ceilings, doors, etc., and determine the necessary thermal coefficients
for each.
5. Establish values for air infiltration and exfiltration quantities, for use in determining
heat losses and heat gains.
6. Determine ventilation quantities and corresponding loads for heat losses and
heat gains.
7. Determine heat or cooling loads due to internal machinery, equipment, lights,
motors, etc.
8. Include allowance for effects of solar load.
9. Total the heat losses requiring heating of spaces and heat gains requiring cooling
of spaces, to determine equipment capacities.
10. Determine system type and control method to be applied.
13.3.4 Temperatures Determined by Heat Balances
In cold weather, comfortable indoor temperatures may have to be maintained by a
heating device. It should provide heat to the space at the same rate as the space is
losing heat. Similarly, when cooling is required, heat should be removed from the
space at the same rate that it is gaining heat. In each case, there must be a heatbalance
between heat in and heat out when heating and the reverse in cooling.
Comfortable inside conditions can only be maintained if this heat balance can be
controlled or maintained.
The rate at which heat is gained or lost is a function of the difference between
the inside air temperature to be maintained and the outside air temperature. Such
temperatures must be established for design purposes in order to properly size and
select HVAC equipment that will maintain the desired design conditions. Many
other conditions that also affect the flow of heat in and out of buildings, however,
should also be considered in selection of equipment.
13.3.5 Methods of Heat Transfer
Heat always flows from a hot to a cold object, in strict compliance with the second
law of thermodynamics (Art. 13.2). This direction of heat flow occurs by conduction,
convection, or radiation and in any combination of these forms.