Complete Wiring Guide for Aprilaire 700 Humidifier Installation and Setup
To properly integrate the Aprilaire 700 series into your HVAC system, begin by identifying the transformer output terminals on your furnace or air handler–typically labeled R (24V hot) and C (common). Connect the unit’s red wire to the R terminal and the black wire to the C terminal, ensuring tight, corrosion-resistant crimps with a non-insulated spade connector (minimum 16 AWG thickness). Avoid splicing into existing thermostat circuits, as voltage drops can disrupt solenoid operation.
The humidistat control (if standalone) must be wired in parallel to the furnace fan circuit–use the G terminal for this. For models with an optional outdoor temperature sensor, route the blue (sensor) and white (ground) wires through a 18/2 shielded cable to prevent EMI interference from adjacent HVAC components. Secure the sensor to an exterior north-facing wall, at least 3 feet above ground, using weatherproof mounting tape. Verify continuity with a multimeter before sealing connections with heat-shrink tubing.
For installations with a bypass damper, the orange wire must connect to the W terminal (heat call) via a NC (normally closed) 24V relay to prevent backflow. If your system includes a variable-speed blower, add a current-sensing relay (e.g., Honeywell R8222) to delay humidifier activation until airflow stabilizes–this prevents mineral buildup in the distribution tray. Always label all connections with heat-resistant wire markers and avoid routing near sharp edges or high-vibration zones.
Final checks demand a low-current test with the humidistat set to 35%: the solenoid should audibly click within 2 seconds of power-up. If no response, measure voltage across the solenoid coil (expected: 22-26VAC)–any deviation indicates a faulty transformer or loose terminal. Once verified, fill the reservoir with distilled water and run a full heating cycle to confirm proper drainage through the drain hose (minimum ½-inch ID, sloped ¼-inch per foot).
Electrical Connection Guide: Model 700 Humidification System
Begin by locating the furnace control board’s 24V AC terminal strip, typically marked as “R” (power), “C” (common), and “W” (heat). The 700 series humidistat requires a dedicated 24V circuit; splice the red wire (R) from the furnace to the humidistat’s labeled “R” terminal. Secure the white wire (C) to the common terminal–failure to do so risks erratic operation or damage to the solenoid valve.
For installation requiring automatic operation, connect the humidistat’s “W” output to the furnace’s “W” terminal. Verify compatibility with multi-stage systems: some older furnaces lack sufficient amperage for the solenoid coil. In such cases, install a 24V relay between the humidistat and furnace, using contacts rated for at least 5A resistive load.
Below is the terminal assignment reference for standard installations:
| Component | Terminal | Wire Color | Voltage |
|---|---|---|---|
| Humidistat Input | R | Red | 24V AC |
| Humidistat Input | C | White | Common |
| Humidistat Output | W | Green/Yellow | 24V AC |
| Solenoid Valve | + | Black | 24V AC |
| Solenoid Valve | – | Gray | Common |
When integrating with a heat pump, bypass the humidistat’s “W” connection during defrost cycles to prevent moisture overload. Most thermostats handle this automatically, but manual configurations require a defrost termination switch wired in series with the humidistat’s output.
Use 18 AWG copper wire for all low-voltage connections, ensuring strands are tinned or terminated with insulated crimp connectors. Avoid daisy-chaining multiple devices off the same “R” terminal–voltage drop exceeds limits beyond three devices, causing solenoid chatter or failure. For systems with multiple transformers, isolate the 700 series circuit using a separate 40VA transformer.
Test continuity between the solenoid valve and humidistat before finalizing connections. Activate the furnace and observe the humidistat’s display; if codes “E2” or “E5” appear, recheck the “R” and “C” connections–these indicate power faults. Adjust the humidistat’s setpoint to 35–45% relative humidity for balanced performance in most climates.
For installations exceeding 150 feet of low-voltage wiring, increase wire gauge to 16 AWG to compensate for resistive losses. If using smart thermostats (e.g., Ecobee, Nest), verify the humidistat’s compatibility with “humidifier” terminals–some models require firmware updates to recognize non-standard humidistat outputs.
Seal all splices with heat-shrink tubing or silicone-filled wire nuts to prevent corrosion. Label each wire at both ends for future troubleshooting. Keep the solenoid valve’s drain line clear; clogs trigger safety shutdowns within 2–3 minutes of activation. Replace the water panel annually–reduced efficiency increases current draw on the solenoid, shortening its lifespan.
Key Components for Installing Your Humidification System
Start by acquiring a low-voltage thermostat cable–typically 18-gauge, 3-conductor–to link the control board to the furnace’s transformer or designated 24V power source. Verify the cable length matches the distance between units, adding 12–18 inches for slack. Avoid solid-core wire in high-vibration areas; stranded copper resists breakage better.
Install a dedicated fused disconnect or an in-line fuse rated for 3 amps on the hot side of the 120V circuit if the unit lacks built-in protection. Choose a slow-blow fuse to handle inrush currents from the solenoid valve. Check local codes–some jurisdictions require GFCI protection for humidifiers near water sources.
Solenoid and Water Supply Essentials
Secure a 24V solenoid valve compatible with the unit’s flow rate (0.5–1.5 GPM). Ensure the valve’s inlet pressure matches your home’s water supply–typically 30–80 PSI. Use a pressure-reducing valve if incoming pressure exceeds 80 PSI to prevent solenoid failure. Opt for lead-free brass valves if lead content is a concern.
A saddle valve isn’t recommended for permanent installations; instead, cut into the main water line and add a T-fitting with a shutoff valve. Use PEX or copper piping depending on your plumbing system. Include a sediment filter upstream of the solenoid to prevent clogging from mineral deposits. Test for leaks at 1.5x operating pressure before finalizing connections.
Control and Safety Mechanisms
Confirm the humidistat’s compatibility with your HVAC system–most models require a 24V control signal. If integrating with a smart thermostat, verify manufacturer support for humidifier control relays. For standalone setups, mount the humidistat at breathing height, away from direct airflow or heat sources.
A safety float switch–either mechanical or electronic–is critical for preventing overflow. Position it below the water inlet to trigger the solenoid shutoff if the drain clogs. For systems without a drain, incorporate an overflow pan with a secondary float switch wired in series with the solenoid. Label all circuits clearly to simplify future troubleshooting.
Installation Walkthrough: Linking Your Whole-Home Moisture Controller to Heating Systems
Shut off power to the furnace at the circuit breaker before handling any components. Identify the low-voltage transformer terminals on the heating unit – typically marked “R” and “C” for 24V output. Strip ¼ inch of insulation from the corresponding wires on the automatic damp controller’s harness, then secure them under the furnace’s terminal screws using a screwdriver with torque settings between 4-5 inch-pounds to prevent wire stripping or loose connections.
Positioning the Water Panel Assembly
Mount the moisture distribution module on the supply ductwork using the provided foil tape, ensuring it sits downstream of the furnace blower within 12 inches of the heating unit’s outlet. Cut a 3-inch diameter hole in the duct using aviation snips, aligning it with the intake grill on the module’s housing. Seal edges with high-temperature silicone to maintain duct integrity. Connect the ½-inch flex tubing from the water supply valve to the module’s inlet port, tightening with an adjustable wrench to 15-18 foot-pounds.
Attach the solenoid valve to the cold water line, installing a saddle tee for the ¼-inch supply line. Verify the valve’s orientation – the arrow should point toward the moisture unit. Secure the humidistat sensor near the return air plenum, away from heating elements or direct airflow, using the mounting bracket and screws. Route low-voltage wiring through ½-inch conduit along the duct path, clipping every 18 inches to prevent sagging.
Restore power and test the system by adjusting the humidistat to 35% relative humidity. Check for water flow through the bypass tube within 90 seconds of blower activation – a steady drip indicates proper operation. If flow doesn’t appear, verify solenoid activation with a multimeter (24V across terminals) and inspect tubing for kinks. Record baseline humidity levels after 24 hours to establish seasonal adjustment parameters.
Understanding the Terminal Layout on the Model 700 Humidity Controller Panel
Locate the transformer output first–it’s marked R and C on the control board. These terminals supply 24VAC power necessary for operation. Ensure the transformer’s secondary side connects directly here without intermediate splices, as voltage drop beyond 0.5V will trigger safety lockouts.
The W terminal activates the solenoid valve when the unit calls for moisture. Verify the connection wire gauge matches the run length: 18AWG for distances under 50 feet, 16AWG if exceeding. Mis-sized wires create resistive heating, prematurely wearing solenoid coils.
Adjacent to W, the H terminal links to external humidity sensors or thermostat humidity control circuits. If bypassing auto-regulation, jumper H to R to force continuous operation. This override ignores installed sensor readings entirely–confirm duct static pressure remains below 0.8″ WC to prevent condensate backup.
Critical Terminal Pairings
G connects to the furnace fan relay. In bypass installations, leave this terminal unoccupied unless the blower runs independently of heating demand. Activating G simultaneously with W during cooling cycles risks evaporator freeze-up; use a delay-on-break relay if integration is unavoidable.
The COM terminal serves as the common return path for all low-voltage circuits. Test continuity between COM and C–impedance exceeding 1 ohm indicates corroded joints or undersized grounding conductors. Replace terminal lugs annually in high-dust environments.
For multi-stage furnaces, Y interfaces with secondary heating outputs. The unit modulates solenoid pulses proportionally to stage calls; incorrect staging aligns valve cycles with furnace short-cycling, reducing electrode lifespan by 40%. Isolate Y with a 1N4007 diode if furnace logic inverts signal polarity.
Safety and Calibration Access
The Test terminal activates manual override. Pressing this button triggers a 60-second solenoid pulse regardless of sensor input. Use this feature exclusively for system confirmation–lingering moisture during testing oversaturates media pads, necessitating immediate replacement. Label this terminal clearly to prevent accidental engagement.