The advantage of this offset technique is that the control can safely enforce a vent mode without worrying about crossing set points. Even more importantly, under CCN linkage, the occupied heating set point may drift up and down and this method ensures a guaranteed separation in degrees Fahrenheit between the calling out of a heating or cooling mode at all times.
NOTE: There is a
To enter into a VAV Occupied Cool mode, the controlling temperature must rise above [OHSP minus L.H.ON plus L.H.OF plus V.C.ON].
To exit out of a VAV Occupied Cool Mode, the controlling temperature must fall below [OHSP minus L.H.ON plus L.H.OF plus V.C.ON minus V.C.OF].
NOTE: With vent mode, it is possible to exit out of a cooling mode during the occupied period if the
Supply Air Set Point Control and the Staging of Compressors
—Once the control has determined that a cooling mode is in effect, the cooling control point (Run Status→VIEW →CL.C.P) is calculated and is based upon the supply air set point (Setpoints→SASP) plus any supply air reset being applied (Inputs→RSET→SA.S.R).
Refer to the SumZ Cooling Algorithm section on page 46 for a discussion of how the A Series ComfortLink™ controls manage the staging of compressors to maintain
VAV Cool Mode Selection during the Unoccupied Period (C.TYP = 1,2; Operating Modes→MODE→OCC=OFF) and Space Sensor Cool Mode Selection (C.TYP=5 and 6) — The machine control types that use this type of mode selection are:
•C.TYP = 1
•C.TYP = 2
•C.TYP = 5
•C.TYP = 6
These particular control types operate differently than the VAV types in the occupied mode in that there is both a LOW COOL and a HIGH COOL mode. For both of these modes, the control offers two independent set points, Setpoints→SA.LO (for LOW COOL mode) and Setpoints→SA.HI (for HIGH COOL mode). The occupied and unoccupied cooling set points can be found under Setpoints.
ITEM | EXPANSION | RANGE | UNITS | CCN | DEFAULT | |
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The heat/cool set point offsets are found under Configura- tion→D.LV.T. See Table 48.
Operating modes are under Operating Modes→MODE.
ITEM | EXPANSION |
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Cool Mode Evaluation Logic — The first thing the control determines is whether the unit is in the occupied mode (OCC)
or is in the temperature compensated start mode (T.C.ST). If the unit is occupied or in temperature compensated start mode, the occupied cooling set point (OCSP) is used. For all other modes, the unoccupied cooling set point (UCSP) is used. For further discussion and simplification this will be referred to as the “cooling set point.” See Fig. 8.
Demand Level Low Cool On Offset (L.C.ON) — This is the cooling set point offset added to the cooling set point at which point a Low Cool mode starts.
Demand Level High Cool On Offset (H.C.ON) — This is the cooling set point offset added to the “cooling set point plus L.C.ON” at which point a High Cool mode begins.
Demand Level Low Cool Off Offset (L.C.OF) — This is the cooling set point offset subtracted from “cooling set point plus L.C.ON” at which point a Low Cool mode ends.
NOTE: The “high cool end” trip point uses the “low cool off” (L.C.OF) offset divided by 2.
To enter into a LOW COOL mode, the controlling tempera- ture must rise above the cooling set point plus L.C.ON.
To enter into a HIGH COOL mode, the controlling temper- ature must rise above the cooling set point plus L.C.ON plus H.C.ON.
To exit out of a LOW COOL mode, the controlling temper- ature must fall below the cooling set point plus L.C.ON minus L.C.OF.
To exit out of a HIGH COOL mode, the controlling temper- ature must fall below the cooling set point plus L.C.ON minus L.C.OF/2.
Comfort Trending — In addition to the set points and offsets which determine the trip points for bringing on and bringing off cool modes, there are 2 configurations which work to hold off the transitioning from a low cool to a high cool mode if the space is cooling down quickly enough. This method is referred to as Comfort Trending. The comfort trending config- urations are C.T.LV and C.T.TM.
Cool Trend Demand Level (C.T.LV) — This is the change in demand that must occur within the time period specified by C.T.TM in order to hold off a HIGH COOL mode regardless of demand. This is not applicable to VAV control types (C.TYP=1 and 2) in the occupied period. As long as a LOW COOL mode is making progress in cooling the space, the con- trol will hold off on the HIGH COOL mode. This is especially true for the space sensor machine control types (C.TYP = 5 and 6), because they may transition into the occupied mode and see an immediate large cooling demand when the set points change.
Cool Trend Time (C.T.TM) — This is the time period upon which the cool trend demand level (C.T.LV) operates and may hold off staging or a HIGH COOL mode. This is not applicable to VAV control types (C.TYP=1 and 2) in the occupied period. See the Cool Trend Demand Level section for more details.
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Fig. 8 — Cool Mode Evaluation
41
