This document discusses different humidity parameters for HVAC control: - Relative humidity directly relates to human comfort but can cause oscillations when used with tight temperature control. - Dew point is unaffected by temperature changes and allows for tight control but does not directly indicate human comfort. - Wet bulb temperature indicates evaporative cooling potential for applications like cooling towers. - Enthalpy accounts for temperature and humidity changes, indicating when it is more economical to use outdoor vs return air. The best parameter depends on the control goal - whether it is human comfort, tight control, or economical air handling. Conversions between parameters can be done through calculations or transmitter outputs.

1. © Vaisala
How to choose the
right parameter for
humidity control in
HVAC

2. © Vaisala
How to choose the right parameter for humidity
control in HVAC
Presenter
Lars Stormbom
Senior product manager
Lars has over 30 years of experience with humidity
measurements in industrial and HVAC settings

3. © Vaisala
Vaisala In Brief
 We serve customers in Weather and Controlled Environment markets
 80+ years of experience providing a comprehensive range of innovative observation
and measurement products and services
 Introduced the first commercial thin film humidity sensor – the HUMICAP® – more
than 40 years ago

4. © Vaisala
Agenda
In this webinar we will go trough some
basics of humidity theory and some of the
factors influencing which humidity
parameter is the best choice in HVAC
control:
 Relative Humidity
 Dew Point
 Enthalpy
 Wet bulb temperature
2018-11-15 4

5. © Vaisala
Relative humidity

6. © Vaisala
Saturation vapor pressure
The equilibrium of water vapor
in a closed container is the
saturation vapor pressure in
that particular temperature
Condensation rate equals
evaporation rate
Air cannot hold any more
water vapor
Saturation vapor pressure is
highly temperature dependent

7. © Vaisala
Relative Humidity
The proportion of water
vapor pressure (Pw) to
the maximum vapor
pressure in the given
temperature [Pws(t)]
%RH =
𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝
𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝑆𝑆 𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 𝑎𝑎𝑎𝑎 𝑡𝑡𝑡𝑡𝑡 𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑 𝒑𝒑𝒑𝒑 𝒑𝒑𝒑𝒑 𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕

8. © Vaisala
Example
 Let’s assume we have
vapor pressure of 100 hPa
RH =
100
1013
= ~10 %
RH =
100
200
= ~50 %
RH =
100
100
= ~100 %
..And temperature 100 °C
1013
..And temperature 60 °C
200
..And temperature 45,8 °C
100

9. © Vaisala
Relative humidity as a control parameter
Directly related to human comfort
Allows changing temperature without RH change
May cause oscillation if used together with very tight
temperature control
2018-11-15 9

10. © Vaisala
Interference of temperature control with RH
2018-11-15 10
18
18.5
19
19.5
20
20.5
21
21.5
22
0 20 40 60 80 100 120
Temperature(°C)
Time (min)
T
20°C 50%RH

11. © Vaisala
Interference of temperature control with RH
2018-11-15 11
46
47
48
49
50
51
52
53
54
18
18.5
19
19.5
20
20.5
21
21.5
22
0 20 40 60 80 100 120
RelativeHumidity(%)
Temperature(°C)
Time (min)

12. © Vaisala
Interference of temperature control with RH
2018-11-15 12
46
47
48
49
50
51
52
53
54
18
18.5
19
19.5
20
20.5
21
21.5
22
0 20 40 60 80 100 120
RelativeHumidity(%)
Temperature(°C)
Time (min)
Start drying
Start humidifying

13. © Vaisala
Interference of temperature control with RH
2018-11-15 13
46
47
48
49
50
51
52
53
54
18
18.5
19
19.5
20
20.5
21
21.5
22
0 20 40 60 80 100 120
RelativeHumidity(%)
Temperature(°C)
Time (min)
Start drying
Start humidifying
Addheat
Cooldown
Addheat
Cooldown

14. © Vaisala
...result...
 Bigger oscillations of both
RH and T
 More energy used
2018-11-15 14
46
47
48
49
50
51
52
53
54
18
18.5
19
19.5
20
20.5
21
21.5
22
0 20 40 60 80 100 120
RelativeHumidity(%)
Temperature(°C)
Time (min)

15. © Vaisala
Dew Point

16. © Vaisala
Dew Point Temperature
 Dew point temperature is the
temperature, where the vapor
pressure would equal to
saturation vapor pressure
 This is the temperature, where the
given humidity level condenses by
effect of temperature
 Proportional to amount of water
vapor
 Independent of ambient
temperature

17. © Vaisala
Dew Point as a control parameter
2018-11-15 17
Unaffected by temperature changes – good for tight
control
Doesn´t indicate human comfort or effect on biological
processes like mold growth directly

18. © Vaisala
Tight control with dewpoint
2018-11-15 18
20°C 50%RH
8.6
8.8
9
9.2
9.4
9.6
9.8
10
10.2
18
18.5
19
19.5
20
20.5
21
21.5
22
0 20 40 60 80 100 120
DewPoint(°C)
Temperature(°C)
Time (min)

19. © Vaisala
Tight control with dew point
2018-11-15 19
8.6
8.8
9
9.2
9.4
9.6
9.8
10
10.2
18
18.5
19
19.5
20
20.5
21
21.5
22
0 20 40 60 80 100 120
DewPoint(°C)
Temperature(°C)
Time (min)
Control band ±0.6 °C corresponds to ±2%RH

20. © Vaisala
Wet Bulb
Temperature

21. © Vaisala
Wet Bulb Temperature
 The wet bulb temperature indicates
the temperature to which a water
surface can be cooled by evaporation.
 This cooling effect varies with the
relative humidity of the ambient air.
2018-11-15 21
Evaporation
consumes
heat
Cooling
down air
releases heat
Equilibrium =wet bulb temperature

22. © Vaisala
Wet Bulb Temperature as a control parameter
Indicates directly evaporative cooling potential of
ambient air => practical with cooling towers
Related to air energy content
Doesn´t indicate human comfort or effect biological
processes like mold growth directly

23. © Vaisala
Cooling towers and wet-bulb temperature

24. © Vaisala
Practical issues
 Maintenance
 Scaling
– Reduced performance
– Make-up water
 Pumps and fans
 Control
 Temperature set-point
– Generally optimal 2..3 °C above
the wet-bulb temperature
– Too small approach leads to
excessive evaporation and
windage loss
Cooling water in
Cooling water out
Wet bulb temperature
R
A
N
G
E
A
P
P
R
O
A
C
H
Temperature

25. © Vaisala
Enthalpy

26. © Vaisala
Enthalpy
 Enthalpy indicates how much energy needs to be expended to get to the
measured state from a reference state, usually dry air at 0°C. The most
common units are kJ/kg or BTU/lb
 Takes into account both temperature changes and humidity changes
(dominant)
2018-11-15 26
h = T ∙ (1.01 + 0.00189X) + 2.5X
Amount of water g/kg of air
kJ/kg

27. © Vaisala
Enthalpy as a control parameter
2018-11-15 27
Indicates directly if it makes sense to recondition return
air or not
Doesn´t indicate human comfort or effect on biological
processes like mold growth directly

28. © Vaisala
Outdoor air economizers
Dampers
2018-11-15[Name] 28
Conditioned airAir handling
unit
Return airExhaust air
Outdoor air
Return air
sensor
Outdoor air
sensor
Is it more economical to use outdoor air or return air?
Δ
H2O
CO2

29. © Vaisala
The Economizer logic
2018-11-15[Name] 29
1: Return air is 76°F/24.4°C,
50%RH, enthalpy is 28.7 BTU/lb
2: Cold but wet outdoor air: 72°F/
22.2°C, 90%RH, enthalpy is 33.9
BTU/lb should not be used for
cooling
3: Hot but dry outdoor air: 85°F/
29.4°C, 15.4%RH, enthalpy is 24.7
BTU/lb. Should be used instead of
return air

30. © Vaisala
How to convert between parameters?
 If you know One humidity parameter and
the temperature, all other parameters can
be calculated
 By programming into your controller
(formula document)
 Rhcalc – free conversion utility:
https://www.vaisala.com/en/lp/humidity-
calculator
 Can be selected directly as output from
many modern transmitters
2018-11-15 30
DIP-switch for selecting output parameter
Vaisala HMD62 duct humidity transmitter

31. © Vaisala
Vaisala Humidity Calculator
https://www.vaisala.com/en/lp/humidity-calculator

32. © Vaisala
Conclusion
 Use RH when human comfort is the main concern
 Use Dew Point for spaces with very tight temperature& humidity control requirements
 Use Wet Bulb temperature to control cooling towers
 Use Enthalpy to decide when to use fresh or reconditioned return air
2018-11-15 32

33. © Vaisala2018-11-15 33