How to Choose the Right Parameter for Humidity Control in HVAC

Where human comfort is the main factor, for example in office buildings, RH is often a good choice for a control parameter. It allows flexibility in temperature settings without changing the RH control settings, allowing higher temperatures in summer and lower ones during winter, for example. It is also directly related to human comfort and many biological processes such as mold growth. A drawback is that very tight temperature and humidity control is difficult to achieve as temperature also affects relative humidity. These two controls may compete with each other, causing oscillation in the control loops.

The dew point temperature (Td) indicates at which temperature you will see the onset of condensation. A low dew point temperature indicates dry conditions while a high dew point indicates high humidity. The dew point cannot exceed the ambient temperature; when the dew point is the same as the ambient temperature, you have reached saturation and RH is 100%. The advantage of using dew point in HVAC controls is that it isn’t affected by changes in temperature, so you can control both temperature and humidity precisely and reliably. The control circuits are independent of one another, so changing the temperature does not change the dew point in the controlled space and vice versa. Dew point and temperature control are used in spaces with the highest requirements for stability, like labs, museums, and data centers.

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. When the air is saturated with water there is no evaporation and no cooling. The wet bulb temperature is used to control cooling towers, which can provide low-cost cooling especially in hot and dry climates. If the humidity is too high and the wet bulb temperature approaches the ambient temperature, the cooling effect becomes too small and it no longer makes sense to run the cooling towers.

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 unit is kJ/kg. If you know the enthalpy of return air and make-up air, you can use this information to decide whether to recondition the return air or replace it with outdoor air. This is not immediately evident just from temperature measurements as the humidity of the air impacts the enthalpy more than the temperature. Enthalpy is thus the recommended choice when your target is to maximize energy savings.
All of these humidity measurements can be calculated from RH and temperature. This can be done in your control system, but many modern humidity transmitters do the job for you.

Take a look at the Vaisala HMD62 duct humidity sensor that allows you to select the desired humidity output with a DIP switch. Also check our free online humidity calculator that makes parameter conversions easy!