AHU Psychrometric Calculator
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Client
Project Title
Project Number
Document Number
Date
Revision
AHU Reference
Prepared By
Step 1
Key Design Conditions & Plant
AHU Type
Select
Full Fresh with Run Around Coil
Full Fresh with Thermal Wheel
Full Fresh with PHX
Recirculation
% FA
%
Summer outdoor
Summer outdoor dry bulb
°C
Summer outdoor wet bulb
°C
Summer outdoor RH
%
Winter outdoor
Winter outdoor dry bulb
°C
Winter outdoor wet bulb
°C
Winter outdoor RH
%
Room condition
Room temperature
°C
Room RH
%
Room temperature tolerance
±°C
Room RH tolerance
±%
Occupancy
Occupancy
people
Chilled water
CHW flow temperature
°C
CHW return temperature
°C
CHW ΔT
K
Glycol (%)
%
LPHW
LPHW flow temperature
°C
LPHW return temperature
°C
LPHW ΔT
K
Cooling Coil
Coil bypass factor
–
Step 2
Airflows
Calculate
Supply air volume
m³/s
Fresh air volume
m³/s
FA %
%
Process/toilet exhaust
m³/s
Leakage direction
Unknown
Loses air
Gains air
Air Leakage (Pressurisation)
m³/s
AHU exhaust air
m³/s
Design margin
%
Design supply flow (incl. margin)
–
m³/s
Design fresh air (incl. margin)
–
m³/s
Design exhaust air (incl. margin)
–
m³/s
Supply air volume
m³/s
Process/toilet exhaust
m³/s
Leakage direction
Unknown
Loses air
Gains air
Air Leakage (Pressurisation)
m³/s
Fresh air volume
m³/s
FA %
%
Fresh air % = 0.
No process exhaust, leakage or occupancy entered. At least one value must be entered to enable cooling & reheat calculations.
Return air
m³/s
AHU dump air
m³/s
Design margin
%
Design supply flow (incl. margin)
–
m³/s
Design fresh air (incl. margin)
–
m³/s
Design return air (incl. margin)
–
m³/s
Design dump air (incl. margin)
–
m³/s
Step 3
Cooling Coil Sizing (Summer / Dehumidification)
Calculate
Heat recovery
Heat recovery
No
Yes
Run Around Coil
PHX
Thermal Wheel
Heat recovery effectiveness
%
System HR effectiveness
%
Outdoor Air after HR
Outdoor air temperature T
OA
°C
Mixed condition
Mixed condition temperature T
MC
°C
Mixed condition moisture content w
MC
kg/kg
Latent load
Apply latent load
No
Yes
Latent load (room)
kW
–
–
–
–
–
–
Design check
–
Total cooling duty
–
kW
CHW flowrate (incl. glycol)
–
kg/s
CHW pipe size
–
mm
Step 4
Summer Reheat Coil
Calculate
Off-cooling coil temperature
–
°C
Off-cooling coil temp after fan pick-up
–
°C
Reheat type
Select
AHU Reheat Coil
AHU Reheat & Zone HBs
Minimum Reheat off-coil temperature
–
°C
Design Reheat off-coil temperature
°C
Off-cooling coil dew point
–
°C
Summer reheat required?
YES
Resolve the Step 3 coil design warning before sizing summer reheat.
Reheat coil load
–
kW
Reheat water flowrate
–
kg/s
Reheat pipe size
–
mm
PLOT
Summer psychrometric process
Simplified T–w charts (101.325 kPa). Click Plot Chart to generate; click Replot if inputs change.
Plot Chart
Figure 1: Summer psychrometric process
Legend
Psychrometric data at plot points
Step 5
Winter Preheat Coil & Heat Recovery
Calculate
Outdoor air dry-bulb T
OA
(winter)
–
°C
Frost protection setpoint
Select
4
5
6
°C
Preheat required?
–
Preheat method
Select
Preheat Coil
Heat Recovery
Heat Recovery & Preheat Coil
Preheat & Heat Recovery
System Heat recovery effectiveness
%
Heat Recovery On-coil temp
–
°C
Heat Recovery Off-coil temp
–
°C
Design check
–
Preheat Coil On-coil temp
–
°C
Preheat Coil Off-coil temp
°C
Preheat coil load
–
kW
Preheat water flowrate
–
kg/s
Preheat pipe size
–
mm
Heat recovery
NO
YES
System Heat recovery effectiveness
%
Preheat position
N/A
After RAC / HR
Before RAC / HR
Preheat entering temp
°C
Off preheat coil temperature
°C
Winter outdoor w
–
kg/kg
Preheat coil load
–
kW
Preheat water flowrate
–
kg/s
Preheat pipe size
–
mm
Temp after preheat & RAC / HR
–
°C
Step 6
Winter reheat (upstream of humidifier)
Calculate
Reheat On-Coil temp
°C
Winter reheat method
Select
AHU Reheat Coil
AHU Reheat & Zone HBs
Minimum Reheat On-coil temp
°C
Reheat Off-coil temp
°C
Reheat Off-coil temp
°C
Winter reheat required?
YES
Terminal reheats?
Yes
No
Winter reheat required?
No
Yes
Leaving reheat coil temperature
Resolve the Step 5 frost / preheat issue before sizing winter reheat.
Winter reheat duty
–
kW
LPHW flowrate
–
kg/s
LPHW pipe size
–
mm
Step 7
Winter Humidification
Calculate
Room temperature
°C
Room RH setpoint
%
Minimum room RH in winter
%
Room Moisture Content
–
kg/kg
Outdoor air moisture content
–
kg/kg
Humidifier required?
–
–
–
Humidifier duty
–
kg/hr
PLOT
Winter psychrometric process
Simplified T–w charts (101.325 kPa). Click Plot Chart to generate; click Replot if inputs change.
Plot Chart
Figure 2: Winter psychrometric process
Legend
Psychrometric data at plot points
Output
Psychrometric Analysis & Coil Sizing Report
Generate a professional PDF report with all calculations and psychrometric charts.
🖨 Generate Report / Save PDF
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