8
2A) Demand Based Heating and Cooling System
System operation as a whole consists of selecting which zone thermostats will have heating and cooling
weighted votes used by the RTU thermostat to which they are attached. The weighted heating and cooling
demand values from the selected master zones are then used by the RTU thermostat to determine if
heating or cooling action is required for the system as a whole.
Both internal and external zones are typically serviced by the same unit. This means that the system may
be exposed to conflicting heating and cooling demands in mid-seasons. The conflicting demand conditions
are addressed with the heating and cooling lockouts based on the outside air temperature value at the
RTU.
The heating or cooling action at the zone is dependent on how the RTU thermostat treats and calculates
what will be delivered point in time to the zones. Many factors can influence the delivery or availability of
hot air or cold air to satisfy the current zone demand point in time.
The following is an example of a RTU system mode calculation based on highest, average of the three
highest demands or the average of the five highest demands.
•
RTU system mode calculations based on, average of the three highest demands or average of
the five highest demands.
•
Example 1 with 3 voting master zones only
Voting Zone 1
Voting Zone 2
Voting Zone 3
RTU Control Type
Current heat
demand
Current heat
demand
Current heat
demand
Highest
Average of 3
highest
50% 0% 0%
Heat weight set Heat weight set Heat weight set
50% 100% 100%
Resulting heat
weight to RTU
Resulting heat
weight to RTU
Resulting heat
weight to RTU
25%
0%
0%
25%
8.3%
Current cool
demand
Current cool
demand
Current cool
demand
0% 100% 100%
Cool weight set
Cool weight set
Cool weight set
100% 100% 50%
Resulting cool
weight to RTU
Resulting cool
weight to RTU
Resulting cool
weight to RTU
0%
100%
50%
100%
50%
It can be seen here that the resulting demand used by the RTU thermostat for the three master
voting zones are different and will result in different heating and cooling actions simply based on the
RTU configuration.
