Humidistat used to control Air Conditioning Equipment
Humidistat use on central air conditioning systems requires use of a separate thermostat setpoint temperature limit that
prevents over-cooling of the conditioned space or system freeze up that occurs when the humidistat senses humidity and
commands cooling cycle operation after the indoor temperature has dropped below the thermostats temperature setpoint.
Unless a dehumidifier is also being used, the air conditioning equipment cannot remove moisture without lowering the
temperature. AC Cooling lowers the humidity by condensing moisture out of the air as it chills the air below its dewpoint.
Without a separate thermostat used with the humidistat ; there is nothing to prevent over-cooling or system freeze-up.
ALSO SEE: Humidity Control using Air Conditioning Equipment
Humidistat Control with Central Air Conditioning Equipment
Humidistats wired in series with air conditioning system thermostats are used to limit system operation to humidity
demands, combined with, thermostat temperature setpoints. Cooling operation is halted by the Thermostat when the indoor
air temperature has dropped to reach the thermostat setpoint. Cooling operation is halted by the Humidistat when the indoor
relative humidity is below the relative humidity setpoint of the humidistat. Cooling operation is halted when either the
humidistat setpoint is reached due to low humidity or the thermostat setpoint is reached due to lowered indoor
temperatures, or both.
Humidistat use with AC Thermostats
The thermostat must be set in the "Cooling" mode in order for the Humidistat to command an AC cooling cycle.
Thermostat / Humidistat setpoints command system operation together
When the thermostat setpoint is at or above the indoor air temperature, the Humidistat cannot command a cooling cycle.
When the thermostat setpoint is below the indoor temperature, the Humidistat is allowed to command a cooling cycle.
When the Humidistat setpoint is below the indoor relative humidity, the thermostat is allowed to demand cooling on.
When the Humidistat setpoint is above the indoor relative humidity, the thermostat is not allowed to command cooling.
Both devices must command a cooling cycle simultaneously in order for the air conditioning system to operate.
Indoor Temperature and Relative Humidity ( RH ) Limitations
When the indoor temperature is at or below the thermostat setpoint ; the Humidistat cannot command a cooling cycle.
When the indoor temperature is above the thermostat setpoint ; the Humidistat is allowed to command a cooling cycle.
When the indoor Relative Humidity is above the Humidistat setpoint ; the thermostat is allowed to cycle on the cooling.
When the indoor Relative Humidity is at or below the Humidistat setpoint ; the thermostat is not allowed to call cooling.
Both indoor temperature and humidity levels must simultaneously demand cooling in order for the system to operate.
When System cycles off commanded by Thermostat Temperature Set Point
The air conditioning system will shut off automatically once indoor temperature has reached the thermostat setpoint.
If the Humidistat still senses moisture after the thermostat setpoint is reached, it cannot turn on the system.
Humidity control at temperatures below the thermostat setpoint is not possible with conventional air conditioning systems.
When System cycles off commanded by Humidistat RH Set Point
The air conditioning system will shut off automatically once indoor relative humidity has reached the Humidistat setpoint.
If the Thermostat still senses heat after the Humidistat relative humidity setpoint is reached, it cannot turn on the system.
Temperature control of indoor temperatures is cancelled once the indoor relative humidity reaches the Humidistat setpoint.
Conditioned spaces are allowed to maintain higher temperatures, so long as the relative humidity setpoint is satisfied.
Temperature Control for human comfort
Thermostats should always be used to control air conditioning systems serving conditioned spaces for human comfort.
Humidistat use for occupied spaces is not recommended since elevated temperatures will be allowed by the humidistat.
Humidistats should be set to "ON" ( 0% RH ) when using the thermostat settings for controlling human comfort.
Humidistats used to control Unoccupied Spaces
Humidistats are often used in unoccupied conditioned spaces that can be allowed to heat up to levels higher than would
be acceptable when occupied, so long as, the specified indoor relative humidity humidity level is not exceeded.
Temperatures are allowed to rise much higher than the thermostat setpoint when ever the indoor relative humidity is low.
Humidistat use for controlling unoccupied spaces is a strategy that limits energy use by limiting the system runtime to only
what is needed to control relative humidity. Systems cycle less frequently using this stategy ; reducing KWH consumption
NOTE: It is necessary to allow indoor temperatures to rise without limit to use this strategy.
Naples Air Conditioning
Products and Services
State License CMC041066
Johnson's Air Conditioning