Connect the whole-home humidifier to a 24-volt transformer and route the control pair through a wall humidistat before reaching the furnace control board. This configuration allows the unit to activate only while the heating blower operates and humidity demand exists.
The humidification unit uses a 24V solenoid valve and a fan assembly that distributes moisture through the duct system. Two low-voltage terminals supply power to the valve, while the internal fan typically receives 120V AC from a dedicated line connected through a relay or furnace accessory terminal.
Most installations link the humidifier control line to the furnace HUM terminal or to the W heating signal depending on the control strategy. The HUM terminal provides 24V output only while the blower runs during heating cycles, preventing unnecessary water flow when the system is idle.
A typical transformer used for this device delivers 24V at 10–20 VA. One secondary lead connects to the humidistat input, while the return lead routes to the solenoid valve. When humidity drops below the set level, the control closes the circuit and energizes the valve.
Place the humidistat on the return duct or inside the living space about 1.5 meters above floor level. Set indoor humidity between 30% and 45% during winter operation to prevent condensation on windows and cold surfaces.
Aprilaire 700 Wiring Diagram with Furnace Control Board and Thermostat Connections
Connect the humidifier solenoid valve to a 24V control line routed through a humidistat and linked to the furnace control board HUM terminal. This layout allows the moisture system to operate only during heating cycles while the blower moves air through the ductwork.
The furnace board normally provides several low-voltage terminals labeled R, C, W, G, and HUM. The R terminal supplies 24V from the transformer, while C acts as the common return. The HUM output becomes active during heating operation and is widely used to trigger whole-home humidifiers.
Route one conductor from the HUM terminal to the humidistat input. From the humidistat output, run the next conductor to one lead of the solenoid valve. The remaining valve lead connects to the C terminal on the furnace board, completing the control loop whenever humidity demand appears.
The fan assembly inside the humidifier cabinet typically operates on 120V AC. This line voltage connection often comes from a nearby service switch or dedicated outlet. Keep the low-voltage control lines separated from the 120V supply to avoid electrical interference and simplify maintenance.
Transformer rating should provide at least 10–20 VA capacity. Many furnace boards already include a transformer capable of supporting both thermostat and humidifier loads. If voltage drops below about 22V during operation, add a separate transformer for the moisture unit.
Mount the humidistat on the return air duct roughly 15–20 cm upstream of the furnace cabinet. This location measures mixed indoor air rather than direct supply air, which produces stable humidity control during heating operation.
Aprilaire 700 Terminal Identification and 24V Transformer Wiring Layout
Identify the low-voltage terminals on the humidifier control panel and connect them to a 24V step-down transformer. One secondary lead from the transformer routes to the humidistat input, while the return lead connects to the solenoid valve terminal. When the humidistat closes its contacts, current flows through the valve coil and allows water to enter the evaporative panel.
Typical furnace control boards provide terminals labeled R for 24V supply and C for common return. Run a conductor from the R terminal to the humidistat, then continue the line from the humidistat output to the valve terminal on the humidifier assembly. The second valve lead connects to C, completing the low-voltage path. Transformers rated around 24V at 10–20 VA handle the solenoid load without noticeable voltage drop during heating operation.
