Exhaust duct and ventilation equipment in EVAP system outside of industrial building against blue sky background; role of solenoid valves in indoor air quality

How Solenoids Help Improve Indoor Air Quality

Solenoid valves are electronic devices used to control the flow of gases or liquid through a system. The electromagnetic device replaces manual valves and allows users to automate their system. Solenoid valves can be used in indoor condenser systems to control the temperature while cooking. More advanced solenoid valve systems provide reliable airflow while reducing the humidity levels. Solenoids are used to control the indoor air quality in other spaces such as bathroom exhaust fans and specific electronics in designer kitchens. All these applications leverage the ability of solenoid valves to work in a wide range of condenser systems.

Temperature Control in Evaporator Systems

Solenoid valves are used to control the flow of liquid in evaporator systems. They work together with a differential thermostat to control the temperature in indoor environments. The system works by placing a thermostat bulb where it controls the discharge or supply of air in the evaporator. The thermostat controls temperature swings through a differential. This type of solenoid valve can be used to control the temperature in single and multiple evaporator systems. It is especially useful in complex systems where evaporators operate at different temperatures.

Liquid Line Shut-Off

Sometimes, engineers may attempt to increase indoor air conditioning systems’ efficiency by installing a normally closed solenoid valve inside the liquid line adjacent to the furnace or air handler. The solenoid valve is parallel to the contractor circuit and often requires a large transformer to power the valve. An alternative method is placing a normally closed solenoid valve along the liquid line adjacent to the condensing unit and connecting it to the compressor motor terminal box. This installation improves system efficiency and keeps the refrigerant charge inside the condenser coil instead of allowing it to migrate. The solenoid valve is a fail-safe device that ensures the system runs all the time.

Split Evaporators

Split evaporators are used in systems where the air temperature in an indoor space is satisfactory, but the humidity amount is too high. The situation is corrected by using one half of the evaporator to remove moisture from the air without causing excessive cooling or adding extra heat. This objective is achieved by installing a normally open solenoid valve on the side of the evaporator that is controlled by the humidistat.

Heat Reclaim Systems

Heat reclaim systems come in two forms of parallel and series systems. In series systems, the discharge gas is cooled to a low temperature inside the condenser during normal operation. During the heating phase, the solenoid valve that is normally open closes to block gas flow from the condenser while the solenoid valve that is normally closed opens to allow the flow of gas towards the heat reclaim coil. In some heat reclaim systems, complete condensation may occur inside the heat reclaim coil. However, most manufacturers prefer to utilize all the heat available and some of the latent heat for complete condensation.

In contrast, parallel systems have two separate condensers. During the normal operation, the condenser condenses the discharge gas completely. During the heat reclaim phase, the heat reclaim coil condenses the discharge gas completely. This approach allows the system to make maximum use of both latent and sensible heat.

Solenoids in Defrost Pump Systems

Pump down solenoid valves are used in condenser units installed in low ambient environments such as rooftops in cold climates. In this case, the evaporator operates at a temperature that is slightly above the ambient temperature. The arrangement enables the system to maintain pressure control at a cut-out level of 1 or 2 psi. In contrast, the cut-in pressure is maintained below the corresponding pressure of the ambient temperature. The setup ensures that the condenser will restart after cooling down during the defrosting phase. Systems with a pump down solenoid valve require the use of a thermostat in a series while the defrost time clock controls the ambient temperature of the indoor air space.

Hot Gas Defrost Systems

Hot gas defrost systems are often used in place of air or electric defrost systems. In such systems, the hot gas discharged by the compressor is directed towards the evaporator’s outlet. The hot gas heats the evaporator and thaws all the frost that may have accumulated. The gas condenses into a liquid and joins the common liquid line to supply other evaporators. Solenoid check valves help the system to work correctly when they are installed to facilitate the flow of liquid around the expansion valves. Pressure reducing valves are used along the liquid line to create a pressure differential in the condensed refrigerant coming from the defrosting evaporator and the common liquid line.

Cool Gas Defrost Systems

Cool gas defrost systems work on the same principles as the hot gas defrost systems. However, the cool gas defrost system uses gas coming from the top of the receiver system to defrost the evaporators. Since the cool gas defrost system operates at a low temperature, the heat-induced expansion of the refrigeration lines is minimized. This setup eliminates the need for condensers to have special pipes. It also stops leaks that are caused by line connections due to excessive thermal stretching.

Capacity Control Systems

Solenoid valves are used to provide condenser unloading by linking the suction lines of the compressor with the discharge. The solenoid valve is controlled through a pressure control system, which reacts to suction pressure. When the switch is closed, the normally closed solenoid valve opens so that the discharge gas is directed towards the suction side of the compressor. The system prevents the compressor from overheating by installing a thermostatic expansion valve to cool the compressor suction gas.


Solenoid valves are advanced flow control devices that enhance the effectiveness of indoor air management systems. They help control the humidity levels in kitchens and bathrooms through liquid injection and gas-by-pass mechanisms.

Johnson Electric is a leader in making custom solenoids for engineering and manufacturing firms in the U.S. Contact us for more discussion about your solenoid valve requirements.

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