Heating; General - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

Control
Loop Classification
Ventilation Basic
Better
Cooling Chiller control
Cooling tower
control
Water coil control
Direct expansion
(DX) system control
Fan Basic
Better

Heating

Coil control
Boiler control
HEATING

GENERAL

Building heat loss occurs mainly through transmission,
infiltration/exfiltration, and ventilation (Fig. 3).
TRANSMISSION
ROOF
VENTILATION DUCT
EXFILTRATION
DOOR
Fig. 3. Heat Loss from a Building.
The heating capacity required for a building depends on the
design temperature, the quantity of outdoor air used, and the
physical activity of the occupants. Prevailing winds affect the
rate of heat loss and the degree of infiltration. The heating
system must be sized to heat the building at the coldest outdoor
temperature the building is likely to experience (outdoor design
temperature).
ENGINEERING MANUAL OF AUTOMATIC CONTROL
Table 1. Functions of Central HVAC Control Loops.
Coordinates operation of the outdoor, return, and exhaust air dampers to maintain
the proper amount of ventilation air. Low-temperature protection is often required.
Measures and controls the volume of outdoor air to provide the proper mix of
outdoor and return air under varying indoor conditions (essential in variable air
volume systems). Low-temperature protection may be required.
Maintains chiller discharge water at preset temperature or resets temperature
according to demand.
Controls cooling tower fans to provide the coolest water practical under existing
wet bulb temperature conditions.
Adjusts chilled water flow to maintain temperature.
Cycles compressor or DX coil solenoid valves to maintain temperature. If
compressor is unloading type, cylinders are unloaded as required to maintain
temperature.
Turns on supply and return fans during occupied periods and cycles them as
required during unoccupied periods.
Adjusts fan volumes to maintain proper duct and space pressures. Reduces system
operating cost and improves performance (essential for variable air volume systems).
Adjusts water or steam flow or electric heat to maintain temperature.
Operates burner to maintain proper discharge steam pressure or water temperature.
For maximum efficiency in a hot water system, water temperature should be reset as
a function of demand or outdoor temperature.
-7 C
PREVAILING
WINDS
20 C
WINDOW
INFILTRATION
C3971
Description
Transmission is the process by which energy enters or leaves
a space through exterior surfaces. The rate of energy
transmission is calculated by subtracting the outdoor
temperature from the indoor temperature and multiplying the
result by the heat transfer coefficient of the surface materials.
The rate of transmission varies with the thickness and
construction of the exterior surfaces but is calculated the same
way for all exterior surfaces:
Energy Transmission per
Unit Area and Unit Time = (T
Where:
T = indoor temperature
IN
T = outdoor temperature
OUT
HTC = heat transfer coefficient
=
HTC
Unit Time x Unit Area x Unit Temperature
9
CONTROL FUNDAMENTALS
- T ) x HTC
IN OUT
joule