If you're staring at an aprilaire 700 wiring schematic and feeling a bit overwhelmed by all the lines and symbols, you're definitely not alone. Most homeowners find HVAC wiring to be a bit like trying to read a foreign language, but once you break it down, it's actually pretty logical. The Aprilaire 700 is a fan-powered humidifier, which makes it a bit unique compared to the bypass models you might have seen before. Because it has its own internal fan to move air through the water panel, the way it connects to your furnace and your thermostat requires a little more attention to detail.
Getting this right is important because a properly wired humidifier doesn't just make your house more comfortable; it protects your hardwood floors and keeps your skin from feeling like sandpaper during those brutal winter months. Let's walk through what's actually happening behind the scenes so you can get your system up and running without needing a degree in electrical engineering.
What Makes the Aprilaire 700 Different?
Before we dive deep into the wires, it's helpful to understand that the 700 series is a "power" humidifier. Unlike the 600 series, which relies on the furnace blower to push air through a bypass duct, the 700 has its own motor. This means your aprilaire 700 wiring schematic is going to show a connection for 120V power (to run that fan) and 24V control wiring (to tell the unit when to turn on).
The heart of the system is the solenoid valve. This is basically an electrically controlled faucet. When the humidistat decides your house is too dry, it sends a signal to open that valve, letting water drip over the evaporative pad. At the same time, the internal fan kicks on to blow air through that wet pad. If the wiring is off, you might get water with no fan, or a fan that runs but never gets any water. Neither of those does you much good when the humidity in your living room is sitting at a crisp 15%.
The Main Components in the Schematic
When you look at the diagram, you're usually looking at four main players. First, you've got the humidifier itself. Then you have the transformer, which steps your home's 120V power down to a safer 24V. Third is the humidistat—the "brain" of the operation—and finally, there's the furnace control board.
If you're using the automatic digital controller that comes with many 700 models, things get a little more interesting. That controller doesn't just sit on the wall; it often uses an outdoor temperature sensor. This sensor tells the humidifier to dial back the moisture when it's freezing outside so you don't end up with condensation and ice forming on your windows. In your aprilaire 700 wiring schematic, you'll see these as two thin wires running to the outside of your house. It's a small detail, but it's one of the most common things people skip, and then they wonder why their windows are dripping in January.
Breaking Down the 24V Control Loop
This is where most of the "work" happens in the schematic. Think of the 24V circuit as a simple loop. Power leaves the transformer, goes to the humidistat, travels to the solenoid valve on the humidifier, and then returns to the transformer to complete the circle.
If you're using a Model 60 digital controller, you'll see terminals labeled R and C. These are your power terminals. The R terminal gets the "hot" 24V wire, and the C terminal is your "common" or return wire. Without the C wire, the digital display on your controller won't even light up. Then you have the "H" terminals. These are the ones that actually switch the humidifier on. When the controller senses that the humidity has dropped below your set point, it closes a switch inside, allowing power to flow through those H terminals and out to the solenoid valve.
Integrating with Your Furnace
You don't want your humidifier running if the furnace blower isn't moving air. If the water flows but the air is stationary, you just end up with a soggy mess inside your ductwork. This is why the aprilaire 700 wiring schematic usually includes a way to sense when the furnace is actually active.
There are a couple of ways to do this. Some people use a "Model 50" current sensing relay. This is a clever little device that wraps around the furnace blower wire. When it detects electricity flowing to the blower, it "closes" the circuit for the humidifier. Other setups involve wiring directly into the furnace's "HUM" terminal. If your furnace board has a 24V HUM terminal, you're in luck—that's often the cleanest way to wire it. It only sends power when the heat cycle is active, ensuring everything stays in sync.
Handling the 120V Power Side
Since the Aprilaire 700 has its own fan, it needs to be plugged into a standard outlet or hardwired into a junction box. Most of these units come with a pre-installed power cord. It's a common mistake to think that the 24V wires from the humidistat will provide enough juice to turn the fan. They won't. The 24V signal just tells a small internal relay inside the humidifier to "close," which then allows the 120V power to reach the fan motor.
If you've hooked everything up and you hear a "click" but the fan doesn't spin, double-check that the unit is actually plugged in. It sounds silly, but in the heat of a DIY project, it's the easy things that we usually miss.
Dealing with the Outdoor Sensor
If you're looking at the aprilaire 700 wiring schematic for an automatic controller, you'll see two terminals specifically for that outdoor sensor. This sensor is a thermistor—a fancy word for a resistor that changes its resistance based on temperature.
You don't have to worry about polarity here; it doesn't matter which wire goes to which terminal. Just make sure the wires are stripped cleanly and tucked firmly into the terminals. If the controller doesn't see that sensor, it might go into a "limp mode" or display an error code, which usually results in the humidifier not running at its full potential. If you really don't want to run wires outside, most Aprilaire controllers allow you to run in "manual mode," but you'll have to remember to adjust the dial yourself whenever the weather changes.
Common Mistakes to Avoid
One of the biggest headaches people run into is "backfeeding" the furnace board. If you try to mix power from a separate transformer with power from the furnace's own R and C terminals without knowing exactly what you're doing, you can actually fry the furnace control board. That's an expensive mistake.
Always stick to the aprilaire 700 wiring schematic provided for your specific configuration. If you're using a dedicated transformer (the little box that plugs into or mounts on a junction box), keep that circuit isolated from the furnace's internal 24V system, using only the "dry contacts" or a relay to bridge the two.
Also, make sure your wire nuts are tight. HVAC systems vibrate. A loose connection might work today but shake itself loose by mid-December, leaving you wondering why the air feels so dry again. Use 18-gauge thermostat wire for your 24V runs; it's the industry standard for a reason—it's easy to work with and fits perfectly into those tiny terminal blocks.
Testing Your Handiwork
Once you've followed the schematic and everything is tucked away, it's time for the "test mode." Most Aprilaire digital controllers have a test setting. When you turn the dial to "Test," it should override the sensors and force the unit to turn on for a minute or two.
You should hear the solenoid click, see water start to flow into the drain line, and hear the internal fan start to hum. If all three of those things happen, you've nailed it. Just remember to turn the dial back to your desired humidity level (usually around 35-45%) once you're done testing. If you leave it on test, it might not shut off, and you'll end up with windows that look like they're crying from all the excess moisture.
Wiring might feel intimidating at first, but it's really just about following the path. Keep your aprilaire 700 wiring schematic handy, take your time, and always turn off the power before you start stripping wires. Your skin and your house will thank you once that first cold snap hits.