13
Maximum Heatflow Adjustment (MaxHTPos)
Many installers will assume that this parameter sets the Maximum airflow of the VAV damper when the
RTU is delivering hot air. This is not the case. Both the maximum amount of cold AND hot air delivered to
the zone is set by the (Max Pos) zone damper parameter. Please see section above for more details.
The value set by this parameter will open the damper to a maximum heating position and will maximize hot
air flow when heat is requested with cold primary air using the duct reheat output.
The Max Heat flow function is only used if the local reheat configuration (RehtConf) is set to any value
except None. None = No local reheat. An example of this is a local reheat configuration using a duct
mounted reheat coil device.
Type of reheat
configured (RehtConf)
BO5 reheat output time
base (BO5 Time
MaxHTPos value function and adjustment
0 = None
N/A
Leave default of 30% or any adjustment. MaxHTPos is not used in scenario
1 = Analog Duct Rht
Only
N/A
Set to any value superior to the current selected minimum position. Ex. If the
minimum airflow is set at 25% and Max heat is set at 75%.
If primary is cold air; when the PI heating loop (and analog output ) goes
from 0 to 100%, the damper linearly move from 25% to 75% opening
2 = On/Off Duct Rht
Only
0= 15 minutes
Set to any value superior to the current selected minimum position. Ex. If the
minimum airflow is set at 25% and Max heat is set at 75%.
If primary is cold air; when the BO5 output is energized on a call for heat, the
damper will directly move from 25% to a 75% position. As soon as BO5 is
de-energized, the damper will move back to 25% opening
1= 10 seconds for Solid
state relays
Set to any value superior to the current selected minimum position. Ex. If the
minimum airflow is set at 25% and Max heat is set at 75%.
If primary is cold air; when the PI heating loop (and pulsed BO5 output )
goes from 0 to 100%, the damper linearly moves from 25% to 75% opening
3 = On/Off Peri Rht Only
0= 15 minutes
Leave default of 30% or any adjustment. MaxHTPos is not used in scenario
1= 10 seconds for Solid
state relays
Leave default of 30% or any adjustment. MaxHTPos is not used in scenario
4 = Analog Duct Rht &
On/Off Peri Rht
Set to any value superior to the current selected minimum position. Ex. If the
minimum airflow is set at 25% and Max heat is set at 75%.
If primary is cold air; when the PI heating loop (and analog output ) goes
from 0 to 100%, the damper linearly moves from 25% to 75% opening
-
The selected zone dampers minimum position has a direct impact on the temperature stability for
certain zones. Having a minimum position selected may produce an over cooling or over heating
effect. This effect is created when the primary air temperature is in the inverse mode than that
which the zone currently requires. An example of this is when an internal zone is requesting
cooling during winter while the RTU is supplying hot air for the external zones.
Adjusting the minimum position of a zone damper is mandatory by NA standards. It is however the
choice of the installer to decide if in some cases removing it or lowering it to a value below
standard may solves a system design issue. A good example of this would be an internal zone with
a grossly oversized VAV unit.
