11
2D) Heating and Cooling Weight Zone Selection
For any system to properly operate, care must be taken to select which zones will be driving the system
and their weight attached to the calculations.
The values below are provided as an initial rule of thumb and need to be re-evaluated on a job per job
basis depending on the specifics of the system design and layout.
Total number of
zones
System layout
Recommended initial
number of master voting
zones with weight
1 to 5
All internal or external zones
1 to 3
3 to 5
Mix and match of internal and external zones
2 to 3
6 to 20
Mix and match of internal and external zones
3 to 8
21 +
Mix and match of internal and external zones
8 +
Notes regarding the master voting zones selection:
o
Not all zones in the system need to be masters. A good rule of thumb is to provide a ratio of 1/3 to
1/2 of the total number of zones which can be master to the system.
o
On larger installations where internal zones are present in the system. I.E. zones not exposed to
an outside wall. The ratio of internal to external master zones should be in the approximate range
of 1 internal zone to 4 external zones.
o
Zones selected to be masters for demand calculations should represent either:
-
Typical zones or areas that will be exposed to some of the highest peak heating and
cooling loads.
-
Zones or areas that represent a significant portion of the equipment peak load capacity.
Example, if a system has five zones where a single zone represents ½ of the total MAX
CFM of the equipment, then for sure this zone needs to be master to the system.
-
Zones or areas that are subject to temporarily larger occupancy need to be part of demand
calculations if the zones are to be expected to respond during those spikes of occupancy.
Typical examples are: Conference room, cafeteria and other such common areas.
Attaching a zone as a master to the system which is either undersized or was commissioned with
operational flaws and errors may result in erratic system behaviour by adding total demand that cannot be
met by the system.
Notes regarding the weight parameter value of the master zones:
o
Internal zones do not need to affect heating demand calculations. They should only affect the
cooling demand calculations. Such zones will always call for cooling during occupied periods even
during winter. If they where to call for heating at a certain point in time, then the surrounding
external zones would typically already be in heating mode.
-
It is possible for an internal zone to be slightly overcooled during peak summer cooling
loads because of the dampers minimum position during occupied periods. The RTU is
providing its maximum cooling capacity and the amount of cold air provided by the
minimum position is already providing more capacity to the internal zone.
-
Alternately, it is also possible for an internal zone to be slightly overheated during peak
winter heating loads because of the dampers minimum position. During occupied periods
the RTU is providing its maximum heating capacity and the amount of hot air provided by
the dampers minimum position will provide more heat to the internal zone than necessary.
