Comfort Zone

Thermostats and humidistats are installed in millions of homes, yet it seems that few people are aware that they can be wired in two distinct ways. Admittedly, this is a quiet concern. Thermostat wiring will never lead a political agenda or attract angry protesters. But if you are curious about managing moisture, you need to know about some important consequences.

Basic Functions

Thermostats allow selection of an optimal temperature for heating or cooling. This temperature is referred to as the “set point.” When the temperature reaches within a degree or two of the set point, a switch in the thermostat signals either the fan and compressor (cooling season) or fan and heating elements (heating season) to distribute air. In either mode, the fan setting may be positioned in the “on” or “automatic” setting. “On” means that the fan operates continuously, while “auto” means the fan fluctuates on and off simultaneously with the compressor. In cool, humid climates, the thermostat may also activate radiant heating or “reheat” in addition to dehumidification so as to remove moisture without causing the room temperature to become uncomfortably cool.

Humidistats allow for the control of relative humidity (RH) in hot or cold climates. In a hot, humid climate, the humidistat helps to dehumidify the air, designating the control as a dehumidistat. Conversly, a humidistat in a cool climate helps to humidify the air, designating the control as a humidistat. The term humidistat will be used in this article as a general term denoting the actual control and not whether it was designed for high or low humidity.

A comfortable indoor humidity, usually ranges between 45 percent and 65 percent RH. When the RH goes above the humidistat set point in a hot and humid climate, a switch signals the air conditioning system to operate, causing air to flow across cold metal fins (an evaporator coil) inside the air handler, allowing the moisture to contact the surface, condense, drip into a pan, and then discharge outside through a pipe. The wiring scheme affects two functions: temperature regulation and moisture management.

Thermostat and Humidistat Wiring

Although controls can have analog or digital settings, they really function as “on” and “off” switches. How does the wiring scheme affect their performance? That may depend on where you live.

The difference between a thermostat and humidistat is a matter of what they control. The thermostat’s control activates all three of the heating, ventilation, and air conditioning (HVAC) components: compressor (cooling), heating elements (heating), and fan (circulation). By comparison, the humidistat’s control activates the compressor (dehumidification) or humidifier (humidification) and the fan. When the temperature or RH level is lowered or raised at the control in any climate, the HVAC system remains “on” until the set points are reached.

The Difference Between Series and Parallel Wiring

Whether a thermostat and humidistat are wired in series or parallel is not immediately obvious because the wiring is hidden. Under the series wiring scheme, activation of the HVAC is dependent on attaining two set points: one for the humidistat and another for the thermostat. So, for example, if the living space is damp (70 percent RH) and cool (68 F) and the thermostat is set for air conditioning, the HVAC system may not turn on to lower the humidity unless the temperature increases to the set point (75 F). This wiring scheme could increase the moisture content of your home in a cool, damp environment.

Under a parallel wiring scheme, the humidistat and thermostat are wired separately, allowing independent operation. Only one control device is required to activate the HVAC system. Parallel wiring gives control of the HVAC to either the thermostat or humidistat, whichever reaches the activation set point first.

These differences are important because the two wiring schemes exert a profound influence on an HVAC system’s operation. In both wiring schemes, if you want a low humidity environment, then you must place the principal responsibility in the hands of the humidistat at the exclusion of the thermostat.

Installation Recommendations

Humidistat and thermostat manufacturers offer both series and parallel wiring schemes with no preference for either configuration. The only requirement is that the humidistat must be connected to ensure proper operation of humidity control. However, some manufacturers recommend the series wiring scheme to prevent the HVAC system from running continuously if the thermostat or humidistat fails in the “on” position or the set points for either control were set too low. There are pros and cons to each wiring scheme.

Pros and Cons

Both parallel and series wiring schemes offer strengths and weaknesses, depending on the desired comfort. Understanding the consequences involving both wiring schemes can assist in the selection of the appropriate configuration.

The worst-case failure scenarios for either series or parallel schemes may be serious. For example, series wiring is inherently vulnerable when the thermostat or humidistat fails to activate, causing the humidity to rise. The result could constitute a patina of microbial growth on furniture, clothing, walls, and ceilings, and possibly require the replacement of HVAC ducting.

Alternatively, a worst-case failure for a parallel scheme is the continuous operation of an HVAC system following an extended steam or water release. The resulting issues include acceleration of HVAC component wear, HVAC component failure, frozen evaporator coil, super-cooling, and excessive electricity costs. This type of failure could result in the nearly continuous operation of the HVAC unit and possible replacement of burned out HVAC components. Two case histories underscore this concern.

Series Wiring Failure Case History – A seasonal townhouse had only two exterior walls, which limited solar exposure and kept the unit cool. When the owners vacated the unit for the summer and fall seasons, the thermostat and humidistat were set to 80 F and 55 percent RH, respectively. During the unoccupied months, outdoor air infiltration and infrequent HVAC operation enabled moisture (water vapor) accumulation within the vacant residence because the thermostat’s set point temperature was reached infrequently and the humidistat could not activate the HVAC system alone without the thermostat. Upon the homeowner’s return, the townhouse supported mold growth that resulted from long-term conditions of elevated humidity.

Parallel Failure Case History – Sporadic microbial growth was attributed to a failed thermostat sensor in a home. Although the thermostat was damaged, it continued to cool the residence and raise the RH level. This condition occurred because, as the temperature dropped, there was a subsequent rise in the dew point and relative humidity. The resulting indoor environment was cold and humid. In a parallel wiring scheme, the set points for both controls (humidistat and thermostat) must be satisfied independently to shut off the HVAC system. The elevated RH levels allowed condensation and subsequent microbial growth to occur.

Knowing whether a humidistat and thermostat are wired in series or parallel provides insight as to a potential cause of damage from elevated RH and how to avoid problems when wiring your own home. The instructions for installing HVAC controls vary between manufacturers and models, so knowing which scheme is most advantageous requires additional research.

Generally, in hot or cold humid climates, parallel wiring of the humidistat and thermostat will provide the most protection from elevated RH levels. In this configuration, as long as the equipment is working properly, it will run whenever the humidity levels are out of the acceptable comfort range, regardless of temperature. In a cool humid climate, the system will activate the reheat function (if available), and in a warm climate, the system will activate the compressor regardless of the temperature. Although this configuration is less energy efficient, it makes dehumidification a priority. The greatest risk with parallel wiring involves rare circumstances of system breakdowns, such as water releases and sensor failures. Even though these situations may cause additional wear and tear on the system and temporarily high electric bills, they are relatively infrequent.  

The authors would like to thank Michael Hahn, P.E., and Warren Hahn, P.E. Hahn Engineering, Tampa, Florida, and Christopher Martinez, MSME, EIT, for providing a peer review of this manuscript.