Aprilaire 400 Humidifier Wiring Diagram
Aprilaire 400 wiring
Automatic control, 24 V source, Model 50 relay, outdoor sensor and humidifier.
Terminals and components · 6
- Control
- 24 V AC
- Humidifier
- Model 50
- Outdoor sensor
Control leads
Pair Goes to Communication Communication terminals 24 V AC Accessory terminals or transformer Humidifier Valve (Model 50 in series) Sensor Outdoor sensor Manual mode
For manual mode, replace the outdoor sensor with the plastic-cased resistor and attach the manual face plate.
Caution
On a downflow furnace, make sure the blower keeps running after each heat call so high temperatures don't damage the control.
Check
The figure doesn't show where this pair connects. Check the wiring instructions supplied with the control before connecting it.
Caution
Do not connect the transformer to multi-speed blower motors or motors other than 120 V AC. With the Model 50, do not wire it into the blower circuit.
Without Model 50
Some furnaces have accessory terminals for the humidistat; then the Model 50 may not be needed. Check with the furnace manufacturer.
Caution
Do not use an external 24 V or 120 V source on these leads: it can damage the humidifier.
How to connect it
- Run the blower motor common wire through the sensor.
- Connect its two leads in series with one of the humidifier leads.
Caution
Install it only in the humidifier circuit, never in the transformer circuit.
Location
On the north, east or west side of the home, above the expected snow line.
About This Diagram
This diagram is redrawn from page 1 of the Aprilaire 400 Humidifier manual, and we offer it completely free and in English. We have also categorized it in the Humidifiers (HVAC & Water Heating) section on Diagramverse, and it has been rated by 1 user with an average rating of 5.0. If you have any specific questions about the Aprilaire 400 Humidifier Wiring Diagram, we suggest checking the Frequently Asked Questions (FAQs) section.
Printable Aprilaire 400 Wiring Diagrams
Wiring Instructions
Before you start
The Aprilaire 400 must be installed by a qualified heating and air conditioning contractor, in compliance with all local, state and federal codes. If you are not comfortable working inside a furnace cabinet or at a 120 VAC junction box, hire a licensed HVAC contractor or electrician. Also check whether your jurisdiction requires a permit for adding a transformer or tapping a furnace circuit.
- Power isolation: turn off power to the furnace at its service switch or breaker before you open any panel. Confirm it is dead with a non-contact tester or meter.
- Tools: wire stripper, small screwdrivers, a multimeter that reads AC volts, wire nuts and cable ties.
- Wire: no gauge is specified for this unit. General good practice is multi-conductor thermostat cable, typically 18 AWG, for every 24 V run, including the outdoor sensor.
- Power source: decide now whether you will use the furnace accessory terminals or a separate transformer. Either one must supply 24 V AC and at least 10 VA. Check the rating label on your transformer or the furnace manual.
- Model 50: plan to use the Model 50 current sensing relay if your furnace has no humidifier accessory terminals, or if its blower is multi-speed or not a 120 VAC motor.
The diagram has a single tab. It covers the Automatic control with the outdoor temperature sensor. If your control is set up in Manual mode, a plastic resistor case and a manual face plate replace the sensor. The rest of the wiring stays the same.
Step-by-step wiring
- Mount the humidifier and install the Automatic humidifier control in the return air duct before running any wire. That way you can measure the real cable lengths.
- Mount the Outdoor temperature sensor on the north, east or west side of the home, above the expected snow line. Run a two-conductor cable from it back to the control and connect it to the sensor leads. The sensor has no polarity.
- Prepare the 24 V source. If you use a transformer, feed its primary from the 120 VAC line at the junction box before it enters the furnace, or from the 120 VAC furnace accessory terminals. Never take it from the furnace fan motor circuit.
- Connect the control's 24 V AC pair to the 24 V source. This pair has no polarity.
- Connect the Communication terminals pair to the furnace or thermostat as the control instructions describe. This connection tells the control when the blower or heat is running, and lets it start the blower in blower activation mode.
- Run a two-conductor cable to the solenoid valve on the humidifier. Connect one lead of the Leads to humidifier pair straight to the valve.
- If you use the Model 50, route the second lead through its contact before it reaches the valve. Pass the blower common wire through the Model 50 sensor so the humidifier can run only while current flows to the blower. Put the Model 50 in the humidifier circuit only, never in the transformer circuit. Without a Model 50, take the second lead straight to the valve.
- Secure every splice with a wire nut. Strain-relieve the cables so they cannot touch the hot supply plenum or moving blower parts.
Testing and start-up
- Open the saddle valve fully, refit all furnace panels and restore power.
- With a meter, confirm about 24 V AC at the control's 24 V AC leads.
- Make a heat or fan call at the thermostat so the blower runs.
- Turn the control knob past 7 to Test/Reset. The solenoid valve should click, the green Humidifier On light should come on, and water should flow to the distribution tray for one minute. Both floats must sit at the bottom of the float chamber, or the unit will not run, even in test mode.
- Set the dial back to 5 as a starting point. If you leave it on Test/Reset, the humidifier stops after one minute.
- Check the water fittings for leaks and make sure the red Service Required light stays off.
Safety notes
- The control, valve and sensor are 24 V AC devices. Never connect any of their leads to 120 VAC. Do not use any external 24 VAC or 120 VAC source other than the furnace accessory terminals or the transformer shown, because it can damage the humidifier's components.
- The transformer primary and the furnace junction box carry 120 VAC, which can cause serious injury. Keep the furnace power off until all covers are back on.
- On a downflow furnace, make sure the blower keeps running after a heat call ends, so hot air does not damage the control.
- Sheet metal edges are sharp, and water hotter than 125°F can scald. Shut off the water before you work near the supply line.
- Before any later service, turn the control to OFF so the valve cannot open suddenly.
Troubleshooting
| Symptom | Probable Cause | What to Check |
|---|---|---|
| Nothing happens in Test/Reset: no click from the valve and no green light | No 24 V AC reaching the control, or the blower is not running | Make a heat or fan call so the blower runs. Then measure across the control's 24 V AC leads; you should read about 24 V AC. If it reads 0 V, check the transformer primary, its breaker and the accessory terminal connections. |
| Control has power and the green light comes on, but the valve never clicks | Open or loose Leads to humidifier wiring, or the Model 50 contact is not closing | With the blower running and the control in Test/Reset, measure for about 24 V AC at the solenoid valve. If the control has 24 V but the valve does not, check each splice and the Model 50 contact in the second lead. |
| Humidifier never runs even though the furnace blower is on | Blower common wire not passed through the Model 50 sensor, or a different wire was passed through it | Confirm that the blower common wire, not a hot or a control wire, passes through the Model 50 sensor. With the blower running, the Model 50 contact should close. |
| Humidifier runs when the furnace blower is off | Model 50 bypassed or wired into the transformer circuit instead of the humidifier circuit | Put the Model 50 contact in series with one of the Leads to humidifier, never in the transformer feed. Make sure the transformer primary is not taken from the blower motor circuit. |
| Transformer or control burned out shortly after installation | Transformer tied to a multi-speed or non-120 VAC blower motor, or a 120 VAC or external 24 VAC source connected to the humidifier leads | Re-feed the transformer primary from the 120 VAC line at the junction box or from the 120 VAC accessory terminals, and use the Model 50 for blower sensing. Make sure no humidifier lead touches 120 VAC. |
| Valve clicks but the humidifier drops out or chatters | 24 V source too small for the load | Check the rating label on the transformer or the accessory terminals; it must be 10 VA minimum. Measure the voltage at the control while the valve is energized; it should stay near 24 V AC. |
| Red Service Required light flashes with an E code | Sensor or wiring fault detected by the control | Note the code (E1, E2, E3 or E4). Check the Outdoor temperature sensor leads, or the resistor case in Manual mode, and the communication pair for loose or broken conductors. If the code persists, call Aprilaire Technical Support. |
| Valve clicks in test mode but no water reaches the panel | Floats not seated at the bottom of the float chamber, or saddle valve not fully open | Remove the cover and make sure both floats move freely and rest at the bottom. Then open the saddle valve completely. |
| Humidifier stopped working after the setup visit | Control knob left in Test/Reset | Turn the knob back to a normal setting such as 5. In Test/Reset the unit runs for only one minute, then the display shows the software revision. |
| Control never starts the blower to add humidity | Communication terminals not connected, or the BLOWER ACTIVATION switch is OFF | Check the communication pair connections against the control instructions and set BLOWER ACTIVATION to ON. That mode is meant for units plumbed to hot water. |
Frequently Asked Questions (FAQs)
Can I power the Aprilaire 400 from my furnace's 24 V transformer?
Yes, if the furnace has accessory terminals for a humidifier and they supply at least 10 VA at 24 V AC. Some furnaces include humidifier accessory terminals, and with them the Model 50 current sensing relay may not be needed. Check the furnace manufacturer's wiring recommendations and the rating label before you connect. If the terminals are missing or undersized, install a separate 24 VAC transformer of at least 10 VA instead.
Do I need the Model 50 current sensing relay?
You need it when the furnace has no humidifier accessory terminals, and whenever the blower is a multi-speed motor or anything other than a 120 VAC motor. The Model 50 lets the humidifier run only while current flows in the blower common wire, so water is never fed to the panel without airflow. If your furnace has dedicated accessory terminals that switch with the blower, the relay may be unnecessary.
Where should I take 120 VAC power for the humidifier transformer?
Take it from the 120 VAC line at the junction box before it enters the furnace, or from the furnace's 120 VAC accessory terminals. Never wire the transformer into the furnace fan motor circuit. Because this work involves line voltage inside or next to the furnace, have a qualified HVAC contractor or electrician do it, following local codes.
Does polarity matter on the 24 V wires?
No. The 24 V source pair, the Leads to humidifier and the Outdoor temperature sensor leads have no polarity, so either conductor can go to either side. What matters is that each pair goes to the right place. For the Communication terminals pair, follow the labels and the control instructions rather than swapping it freely.
Where should the outdoor temperature sensor be mounted?
Mount it on the north, east or west side of the home, above the expected snow line. Avoid the south side, where sunlight would make the control read a falsely warm outdoor temperature. The control uses this reading to lower indoor humidity in cold weather: about 1 percent RH for every 2°F drop in outdoor temperature. A badly placed sensor can cause window condensation or dry air.
Can I run the Aprilaire 400 without the outdoor sensor?
Yes, by setting up the control in Manual mode. You replace the Outdoor temperature sensor with the plastic resistor case, attach the manual face plate and set the internal switch to MAN; the display then shows M. In Manual mode you must lower the setting yourself when cold weather is coming, for example to 25% RH at 0°F, to avoid condensation.
Does the Aprilaire 400 need a drain connection?
The Model 400 has no continuous drain flow like the 500 and 600 series do, but it does have a safety overflow line. Connect a 1/2 inch I.D. plastic hose to the overflow connection at the base and run it with a continuous downward slope to a floor drain. Under normal operation the line stays dry. Do not attach metal tubing directly to the fitting, and do not use solvent adhesive on it.
Can I use the Blower Activation feature with my Model 400?
Yes. With the BLOWER ACTIVATION switch set to ON, the control can start the HVAC blower when humidity is needed. After one hour of idle time, it runs the fan for 3 minutes to sample the air. The humidifier should be plumbed to hot water for this mode, and the Communication terminals must be wired so the control can command the blower.
Can I install the humidifier control on a downflow furnace?
Yes, but you must make sure the blower keeps running after each heat call is satisfied. On a downflow furnace, heat can rise into the return duct where the control is mounted, and residual high temperatures can damage it. Check the furnace's fan-off delay settings and adjust them if needed.
Can I wire the Aprilaire 400 myself?
The manufacturer requires installation by a qualified heating and air conditioning contractor and voids the warranty otherwise because of possible misapplication. The low-voltage wiring is simple, but the transformer feed involves 120 VAC inside or next to the furnace, and a wrong connection can damage the humidifier or the furnace controls. If you do the low-voltage work yourself, have a professional check it.
