Floating head pressure control allows the condensing pressure in an HVAC system to vary based on the ambient wet bulb temperature, rather than maintaining a fixed high pressure. This improves efficiency by lowering the condensing pressure and increasing the system's coefficient of performance (COP) when outdoor temperatures are cooler than design conditions. Field data shows floating head pressure control can reduce power consumption and increase COP by up to 22% compared to conventional fixed head pressure control. System designers must consider limitations around how far the head pressure can safely float based on components like expansion valves and condenser sizing.

1. Floating Head Pressure Control

2. • Evaporative condensers are sized for rejecting
heat at the highest ambient condition, typically the
"ASHRAE summer 1% design" wet bulb temperature.
• Conventional head pressure control maintains
condensing pressure corresponding to this fixed
wet bulb temperature condition.
• For example, a system in Toronto may have a
condenser sized for following conditions:
ASHRAE summer 1% design wet bulb: 72ºF
Condensing pressure: 180 psig (95ºF)

3. Conventional Head Pressure Control
• Head pressure typically controlled by using
pressure switches to turn condenser fan(s) and
pump(s) on and off.
• Pressure switches work on a differential, which is
the difference between cut-in and cut-out pressures.

4. Conventional Head Pressure Control
High Pressure Control
Cut-in pressure 170 psig (91ºF) 180 psig (95ºF)
Cut-out pressure 145 psig (82ºF) 160 psig (88ºF)
Condenser pump Condenser fan

5. Conventional Head Pressure Control
150 (82°F)
head pressure
(psig)
160 (88°F)
170 (91°F)
180 (95°F)
Time (minutes)
Compressor 1
Compressor 2
Compressor 3
fan on
pump on
fan on fan on
fan offfan off
on
on
on

6. Floating Head Pressure Control
• When ambient temperature falls below design
conditions, the condenser has excess capacity,
therefore head pressure can be lowered.
• Allow head pressure to float, following ambient
wet bulb temperature.

7. Average Monthly WB temperatures
for 2004 (approx.)
[www.weather.uwaterloo.ca]
FJ M A M J J A S O N D
10
20
30
40
50
60
Month
Wetbulbtemperature
(°F)
0
70
80
"ASHRAE summer 1% design " wet bulb (72°F)

8. 75
80
85
90
95
100
0 1000 2000 3000 4000 5000 6000 7000
HEAT REJECTION (MBH)
CONDENSINGTEMPERATURE(°F)
1compressor
2compressors
3compressors
50º F
W
B55º F
W
B
60º F
W
B
65º F
W
B
70º F
W
B
75º F
W
B
FAN
&
PUM
P
Floating Head Pressure
Sample Evaporative Condenser Capacity Diagram
A
B
C
COMPRESSOR HEAT REJECTION
CONDENSER CAPACITY (RUNNING FAN & PUMP) AT GIVEN WB
95° F CT
75° F WB
85° F CT
65° F WB
75° F CT
50° F WB

10. Floating Head Pressure Control
Power Consumption Comparison
CT TR HP COP kWh/Day
(°F)
95 130.1 214.4 2.86 3839
85 134.0 190.8 3.31 3416 (-11%)
75 137.9 167.9 3.87 3006 (-22%)
A
B
C

11. Measured COP, compressor power and fan power
for floating and fixed head pressure control
[ASHRAE JOURNAL]
COP
Time (minutes)
1.25
1.00
1.5
1.75
2.00
2.25
2.50
5 10 15 20 25 30 35 40
Power(kW)
10
20
30
40
50
60
70
80
0
floating head pressure fixed head pressure
Fan Power
Compressor power
COP

12. Floating Head Pressure
Design Considerations
• There are limitations to how far head pressure can
be dropped.
• System limitations include expansion valve design,
condenser water freeze protection, condenser sizing,
compressor operating parameters and waste heat
equipment performance.