Aprilaire 500 Humidifier Wiring Diagram
Aprilaire 500 wiring
Choose the control type; tap any terminal or component to see how it connects.
Terminals and components · 5
- 24 V AC
- Control
- Outdoor sensor
- Furnace signal
- Humidifier
- Model 50
Terminals
Terminal Goes to R · C Humidifier 24 V transformer ODT Outdoor sensor (or manual mode resistor) W/G · Cf Furnace 24 V accessory terminal and common H · H The yellow valve wires Other options
The control instructions include other wiring options.
Caution
Do not wire the transformer primary into the furnace blower circuit.
Location
On the north, east or west side of the home, above the expected snow line.
Manual mode
Without an outdoor sensor, install the manual mode resistor on ODT.
Furnace terminals
They may be labeled “HUM”, “ACC” or “EAC”. Check the furnace manufacturer for voltage and operation.
Caution
If the accessory terminal is 120 V, install a 24 V transformer.
How to connect it
Connect each H terminal to one of the two yellow valve wires.
Caution
Do not use an external 24 V or 120 V source on these wires: it can damage the humidifier.
Setting
The dial shows the recommended humidity for the outdoor temperature.
Caution
Power the transformer from a 120 V AC circuit other than the blower motor circuit; it can be fed from the hot 120 V line before it enters the furnace.
Without Model 50
Some furnaces have accessory terminals that are powered only while the blower runs; then the Model 50 may not be needed. Check with the furnace manufacturer.
How to connect it
Connect one of the yellow valve wires to the second control terminal and the other to the Model 50 (or straight to the source if not used).
Caution
Do not use an external 24 V or 120 V source on these wires: it can damage the humidifier.
How to connect it
- Run the blower motor common wire through the Model 50 sensor.
- Connect its two leads in the 24 V loop, between the source and the humidifier.
Caution
Install it only in the 24 V humidifier control circuit, never in the transformer primary circuit.
About This Diagram
This diagram is redrawn from page 1 of the Aprilaire 500 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 500 Humidifier Wiring Diagram, we suggest checking the Frequently Asked Questions (FAQs) section.
Printable Aprilaire 500 Wiring Diagrams
Wiring Instructions
Before you start
First, find out which variant you have. The Aprilaire 500 ships with the Digital humidifier control: it has a digital display, indicator lights and a knob that turns past 7 to Test/Reset. Use the 500 · Digital variant. The 500M ships with the Manual humidifier control, a plain knob control that you mount on the return duct or on a wall. Use the 500M · Manual variant. The two variants wire very differently, so don't mix them.
This humidifier is meant to be installed by a qualified heating and air conditioning contractor, in line with local, state and federal codes. The warranty is void if it isn't. You only make low-voltage connections at the humidifier, but powering the transformer means working on a 120 V circuit. Many areas require a licensed electrician or a permit for that part. Check with your local building department.
Tools and materials:
- Low-voltage wire, which is not supplied. No gauge is specified. Standard 18 AWG thermostat cable is the usual choice for runs like this.
- The supplied 24 VAC transformer (500), or a 24 VAC source rated at least 10 VA (500M).
- A wire stripper, screwdrivers and a multimeter that reads AC volts.
- A drill with a 3/4 in bit for the digital control's sensor hole.
- The Model 50 current sensing relay, only if your 500M setup needs one.
Turn off power to the furnace at its service switch or breaker. Confirm it is off before you open any panel. Set the humidifier control to OFF.
Step-by-step wiring: Model 500 (Digital)
- Mount the Digital humidifier control on the return duct, at least 6 in upstream of the bypass duct connection or the humidifier. Drill a 3/4 in hole for the RH sensor. Make sure the gasket is in place around the sensor cage, then fasten the control with two screws.
- Mount the 24 V transformer. Feed its 120 V primary from a circuit other than the furnace blower motor circuit. You may tap the hot 120 VAC line before it enters the furnace, but never wire into the blower circuit.
- Run the transformer's 24 V secondary to R and C on the control. The control must stay powered at all times.
- Mount the Outdoor temperature sensor on the north, east or west side of the home, above the expected snow line. Run two wires to the ODT terminals. Polarity doesn't matter.
- Find the furnace's 24 VAC accessory terminal, which may be labeled HUM, ACC or EAC. Connect W/G to that terminal and Cf to the furnace common. If the accessory terminal puts out 120 V, you need a 24 V transformer between the furnace and the control. Never take 120 V to W/G. Your furnace manufacturer's literature gives the voltage and timing of the accessory output.
- Connect the two H terminals to the two Yellow solenoid valve wires. Polarity doesn't matter.
- Leave A and B empty in this layout.
Step-by-step wiring: Model 500M (Manual)
- Mount the Manual humidifier control. On the return duct, place it at least 6 in upstream of the bypass duct or humidifier, with its foam gasket. On a wall, pick a spot away from drafts and heat sources, and discard the gasket.
- Pick your 24 V source. If the furnace accessory terminals only switch on while the blower runs, you can use them and you don't need the Model 50. If you use a separate transformer (10 VA minimum) that stays on all the time, add the Model 50 so the humidifier only runs while the blower runs.
- Connect one side of the 24 V source to Terminal 1 of the manual control.
- Connect Terminal 2 to one Yellow solenoid wire.
- Connect the other Yellow wire to the 24 V source. If you use the Model 50, route this wire through its contact first, then back to the source.
- Pass the blower motor's common wire through the Model 50 sensor. The Model 50 goes in the 24 VAC humidifier circuit only, never in the transformer primary.
Testing and start-up
- Check that every terminal screw is tight and no bare copper is showing. Open the saddle valve fully, then restore power to the furnace.
- Measure AC volts. On a 500, you should read about 24 VAC across R and C at all times. On a 500M, you should read 24 VAC at the source.
- Start a heat or fan call at the thermostat so the blower runs. The humidifier won't operate unless the blower is running.
- 500: turn the knob past 7 to Test/Reset. The solenoid should click, the green Humidifier On light should come on, and the humidifier runs for one minute. Then return the knob to your setting, because the humidifier won't work if you leave it in test mode. Test/Reset also resets the Water Panel timer.
- 500M: turn the knob to its highest setting. The valve should open.
- Let the humidifier run until water flows from the drain. Check that the plumbing is watertight, then set the control to the proper humidity level.
Safety notes
- The humidifier and its control run on 24 VAC. The solenoid draws 0.5 A. Connecting 120 V to H, R, C, W/G or the yellow wires will destroy the valve or the control, and it creates a shock hazard.
- Never power the transformer from the blower motor wiring. Components can fail early.
- On a downflow furnace, make sure the blower keeps running after a heat call ends, so hot air doesn't damage a duct-mounted control.
- Turn off the hot water supply before you tap a hot line. Water above 125°F scalds instantly.
- Plenum edges are sharp, so wear gloves while you work near the duct.
Troubleshooting
| Symptom | Probable Cause | What to Check |
|---|---|---|
| Digital control display is blank | No continuous 24 V at R and C, the transformer is wired into a switched circuit, or a wire is loose | Measure AC volts across R and C with the furnace idle. You should read about 24 VAC. If it only appears while the blower runs, move the transformer primary to an unswitched 120 V feed that is not the blower circuit. |
| Display works, but the humidifier never runs, even on Test/Reset | No furnace signal at W/G/Cf, or the wrong furnace terminal was used | Start a heat call and measure between W/G and Cf. You should read 24 VAC while the furnace runs. If you read nothing, find the true HUM, ACC or EAC accessory terminal and confirm its voltage in the furnace documentation. |
| Solenoid doesn't click on Test/Reset, though the blower is running | Loose H to Yellow connection or a failed solenoid coil | Measure AC volts across the two Yellow wires during the one-minute test. You should read about 24 VAC. If voltage is there and nothing clicks, the valve is faulty. If there's no voltage, recheck the H terminals. |
| Control, valve or fuse burned out right after power-up | 120 V applied to the low-voltage side, often from a 120 V accessory terminal wired straight to W/G or the valve | Measure the accessory terminal output with the furnace running. If it reads 120 V, put a 24 V transformer between it and the control. Replace damaged parts only after the wiring is corrected. |
| Humidifier runs with the blower off and drips water into the duct | 500M wired to an always-on transformer without a Model 50, or the Model 50 contact bypassed | With the blower off and the control turned up, check for 24 VAC across the Yellow wires. It should read 0 V. Add the Model 50 with the blower common passed through its sensor, or switch to a blower-tracking accessory terminal. |
| Model 50 installed, but the humidifier never comes on | Wrong blower wire through the sensor, or the Model 50 is in the transformer primary | Make sure the blower common wire is the one passing through the sensor and that the Model 50 contact is in the 24 V loop only. With the blower running, the contact should close. |
| Red Service Required light flashes with E1, E2, E3 or E4 | The control has detected an abnormal operating or wiring condition | Write down the code, recheck the ODT, W/G/Cf and H connections, and contact an HVAC contractor or Technical Support with the code. |
| Humidity doesn't drop when it gets cold outside, and windows fog up | ODT sensor wire open or shorted, sensor badly located, or the control set to manual mode | Check the two ODT wires for a cut or short. Make sure the sensor is out of direct sun and above the snow line. Check whether the display shows M, which means manual mode. |
| Humidifier won't shut off, even with the dial at OFF | Valve wires connected straight to the 24 V source so the control is bypassed, or the solenoid is stuck | Turn the power off and trace the Yellow wires. On a 500 they go only to H and H. On a 500M one goes to Terminal 2. If the wiring is correct and water still flows with no power, the valve is stuck. |
| Works on a test, then stops after about a minute | The knob was left in Test/Reset | Turn the knob back to a normal setting, such as 5. If you leave it at Test/Reset, the display flashes the software revision, for example r3, and the humidifier won't operate. |
Frequently Asked Questions (FAQs)
How do I tell whether I have a 500 or a 500M?
The 500 comes with a Digital Humidifier Control, and the 500M comes with a Manual Humidifier Control. The digital control has a display, indicator lights, a knob that goes past 7 to Test/Reset, and an outdoor temperature sensor. The manual control is a simple knob with no display, so you turn it slowly and listen for clicks to find the indoor RH. Choose the 500 · Digital or 500M · Manual variant to match, because the terminals and wiring differ completely.
What voltage and current does the Aprilaire 500 use?
The Aprilaire 500 runs on 24 VAC at 60 Hz and draws 0.5 A. That applies to both the 500 and the 500M. The solenoid valve and the controls are low-voltage parts, so never connect them to 120 V. If your 24 V comes from a separate transformer on a 500M, it must be rated at least 10 VA. The 500 includes its own 24 VAC transformer for the digital control.
Can I power the transformer from the furnace blower circuit?
No, never wire the humidifier transformer into the furnace blower motor circuit. That can cause premature component failure, and on the 500 it would cut power to the control whenever the blower stops. Use a 120 VAC source other than the blower circuit. You may tap the hot 120 VAC line before it enters the furnace. Either way, the 500's digital control needs a continuous 24 V supply at R and C.
Do I need the Model 50 current sensing relay?
You only need the Model 50 when your 24 V source doesn't tell the humidifier when the blower is running. This is usually a 500M powered by a separate, always-on transformer. The relay senses current in the blower common wire and closes its contact only while the blower runs, so water never flows into still air. If your furnace has an accessory terminal that is live only while the blower runs, you can usually skip it.
What if my furnace accessory terminal is 120 V instead of 24 V?
Add a 24 V transformer between the accessory terminal and the humidifier circuit. The W/G input on the digital control and the 500M valve loop expect 24 VAC, and 120 V will damage them. Accessory terminals may be labeled HUM, ACC or EAC, and their voltage and timing vary by brand. Check your furnace manufacturer's documentation, and measure the terminal with a meter while the blower runs before you connect anything.
Does polarity matter on the solenoid valve or the outdoor sensor?
No, polarity doesn't matter for either one. The two Yellow solenoid valve wires can go to either H terminal on a 500. On a 500M, either Yellow wire can go to Terminal 2. The two outdoor temperature sensor wires can go to the ODT terminals in either order. What matters is tight connections, no nicked insulation, and no contact between the bare conductors.
Where should I mount the outdoor temperature sensor?
Mount it on the north, east or west side of the home, above the expected snow line. Those sides avoid direct afternoon sun, which would make the sensor read too warm. The control uses the outdoor reading to lower indoor humidity as it gets colder, by about 1% for every 2°F, which keeps condensation off windows. A badly placed sensor means humidity stays too high in cold weather.
Can I run the digital control without the outdoor sensor?
Yes, you can run it in manual mode. The ODT terminals accept either the outdoor temperature sensor or a manual mode resistor, and the control has an internal switch with a MAN position. When the control is in manual mode, the display shows M. You then have to lower the setting yourself as outdoor temperatures fall, for example to about 35% RH at 20°F and 25% RH at 0°F.
What do the A and B terminals do on the digital control?
A and B aren't used in the standard Aprilaire 500 wiring, so leave them empty. The recommended hookup uses only R and C for continuous 24 V power, ODT for the outdoor sensor, W/G and Cf for the furnace signal, and the two H terminals for the solenoid valve. The control's own instructions cover other wiring options. Don't connect A or B unless you are following one of those layouts.
What is the Blower Activation switch, and can I use it?
Blower Activation lets the digital control turn on the HVAC fan by itself to add humidity. You can use it only if the 500 is plumbed to hot water. With the switch ON, the control samples the air after one hour of idle time and keeps the fan running while humidity is needed, for up to 2 hours. With the switch OFF, the humidifier runs only while the fan or heat is on.
Where should the humidifier control go on the ductwork?
Put the control on the return duct, at least 6 in upstream of the bypass duct connection or the humidifier. That way it reads house air, not air that has just been humidified. The digital control needs a 3/4 in hole for its RH sensor and a gasket around the sensor cage. The 500M manual control can go on the return duct or on an interior wall away from drafts and heat sources.
