Aprilaire 600M Humidifier Wiring Diagram
Aprilaire 600M wiring
Choose the control type; tap any terminal or component to see how it connects.
Terminals and components · 4
- 24 V AC
- Control
- Outdoor sensor
- Furnace signal
- Humidifier
- Model 50
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.
Terminals
Terminal Goes to R · C Furnace R and C (power) ODT Outdoor sensor W Furnace W G Thermostat G H · H Transformer and valve loop Gf Furnace G (blower) Check
After the one-minute test cycle, the control shows its software revision (e.g., r1).
How to connect it
Connect control R and C to the furnace R and C terminals, together with the thermostat wires.
Location
On the north, east or west side of the home, above the expected snow line.
How to connect it
Connect control W to the furnace W terminal, where the thermostat W also lands.
How to connect it
Disconnect the thermostat G wire from the furnace and take it to control G.
How to connect it
Connect control Gf to the furnace G terminal.
How to connect it
- One H terminal goes to one side of the transformer secondary.
- The other H goes to a yellow wire; the other yellow wire returns to the transformer.
Caution
Keep the transformer continuously powered and not from the blower motor circuit.
What changes
Only the thermostat G wire goes through the control; the rest stays the same.
About This Diagram
This diagram is redrawn from page 2 of the Aprilaire 600M 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 600M Humidifier Wiring Diagram, we suggest checking the Frequently Asked Questions (FAQs) section.
Printable Aprilaire 600M Wiring Diagrams
Wiring Instructions
Before you start
Pick the tab variant that matches the control that came in your box. The Aprilaire 600M · Manual variant is for the Model 600M, which ships with a Manual Humidifier Control: a dial with a simple contact, two terminals and no display. The 600 · Digital variant is for the Model 600, which ships with the Automatic Digital Humidifier Control (ADHC) and an Outdoor Temperature Sensor. If your control has a terminal strip marked R, C, W, G, Gf, H, H, ODT, A and B, use the Digital variant.
Aprilaire voids the warranty unless a qualified heating and air conditioning contractor installs the humidifier, and the work must follow local, state and federal codes. Wiring the 120 VAC primary side of the transformer is line-voltage work. If you are not qualified for that, or your jurisdiction requires a permit, hire a licensed professional.
- Furnished: the 24 VAC transformer, the control for your model, the Outdoor Temperature Sensor (Model 600 only) and the saddle valve.
- Not furnished: low voltage wire, sheet metal screws and the Model 50 current sensing relay (only needed in some installations).
- Wire: the humidifier draws 0.5 A at 24 VAC, 60 Hz. The sources don't give a wire gauge. Standard multi-conductor thermostat cable is common practice for runs like this, so check your local code.
- Tools: a non-contact voltage tester, a multimeter that reads AC volts and continuity, wire strippers and a screwdriver.
Turn off power to the furnace at its service switch or breaker before you begin. Confirm it is dead with your tester. Turn the humidifier control to OFF.
Step-by-step wiring
600M · Manual variant
- Mount the Manual Humidifier Control on the return duct at least 6" upstream of the humidifier or bypass duct, or on an interior wall of the living space away from drafts and heat sources.
- Choose your 24 V source: either the furnace's 24 VAC accessory terminals or the supplied transformer. Either one must be rated at least 10 VA.
- Run one lead of the 24 V source to Terminal 1 of the manual control.
- Run Terminal 2 to one of the two Yellow wires on the solenoid valve. The valve has no polarity, so either yellow wire works.
- If you need the Model 50, run the blower motor's common wire through its sensor. Then wire its contact between the second yellow wire and the other lead of the 24 V source. If you don't use it, the source must be one that only gives 24 V while the blower runs. The sources don't cover this, so verify how your furnace's accessory terminals behave.
- Install the Model 50 in the 24 VAC control loop only, never in the transformer's 120 VAC primary.
600 · Digital variant
- Mount the ADHC on the return duct at least 6" upstream of the humidifier or bypass duct.
- Mount the ODT outdoors on the north, east or west side of the home, above the expected snow line. Run its two wires to the two ODT terminals. They have no polarity.
- Connect control R to furnace R and control C to furnace C.
- Connect control W to furnace W. Leave the thermostat's W in place at the furnace too.
- Move the thermostat's G wire off the furnace and land it on control G. Then run control Gf to furnace G.
- Leave the thermostat R, C, Y and W wires on the furnace as they are. C is only there if your system has one.
- Wire the humidifier loop. Connect one H to one lead of the supplied 24 VAC transformer and the other H to one Yellow valve wire. Run the second yellow wire back to the transformer's other lead.
- Leave A and B empty.
- Power the transformer primary from a continuously powered 120 VAC source that is not the furnace motor circuit. You may tap the hot 120 VAC line before it enters the furnace, but never wire the transformer into the blower circuit.
Testing and start-up
- Open the saddle valve fully and check that all plumbing joints are watertight. Turn the damper handle to WINTER.
- Restore power. With a meter, confirm about 24 VAC at the transformer secondary or the 24 V source. On the Digital variant, also confirm it between R and C on the control.
- 600M: make a heat or fan call at the thermostat so the blower runs. Turn the control knob to its highest setting. The solenoid should click and water should flow to the distribution tray and then to the drain. Then set the knob back to the RH you want.
- 600: make a heat call so the blower runs, then turn the knob past 7 to Test/Reset. The humidifier runs for one minute, and then the display shows the software revision, such as r1. Don't leave the knob in Test/Reset. Set it back to a normal setting, starting at 5.
- Check that the blower actually starts and stops on a G call through the control.
Safety notes
- The humidifier, the valve and the controls run on 24 VAC only. Putting 120 VAC on the yellow wires, the H terminals or the control will destroy them and creates a shock and fire risk.
- The transformer primary is 120 VAC. Disconnect power before you touch it, and make that connection in an approved junction box.
- Never connect the transformer to the blower motor wiring.
- If you use Blower Activation on the Digital control, plumb the humidifier to hot water. Supply water must not exceed 140°F. Shut off the hot water before tapping the line, because water above 125°F scalds instantly.
- On a downflow furnace with a return-mounted control, make sure the blower keeps running after the heat call is satisfied.
Troubleshooting
| Symptom | Probable Cause | What to Check |
|---|---|---|
| Nothing happens: the valve never clicks and no water flows | No 24 V in the humidifier loop, a broken loop, or the blower isn't running | Run the blower first. Measure about 24 VAC at the transformer secondary or 24 V source, then follow the loop: control terminals, both Yellow wires, and the Model 50 contact if fitted. With the control calling, you should read about 24 VAC across the two yellow wires. |
| The 600M works on fan-only but not during heat, or never works with the Model 50 fitted | The Model 50 is sensing the wrong wire, or its contact is in the wrong circuit | Only the blower common wire should pass through the Model 50 sensor. Its contact must be in the 24 VAC loop, not in the transformer primary. With the blower running, check for continuity across the contact. |
| The humidifier runs even when the blower is off | The 24 V source isn't interlocked with the blower (600M without a Model 50), or the transformer is wired into the blower circuit | Check where the 24 V source comes from. Either add the Model 50 on the blower common, or use an accessory output that is live only while the blower runs. Make sure the transformer primary is on a separate 120 VAC feed. |
| Thermostat fan-on no longer starts the blower (600 Digital) | The thermostat G wire was left on furnace G, or Gf isn't connected | Thermostat G must land on control G, and control Gf must go to furnace G. With a fan call, measure about 24 VAC from Gf to C. |
| The digital control display is blank | R or C is missing or loose, or there is no common at the furnace | Measure about 24 VAC between control R and C. Then check that both wires land on furnace R and furnace C, not on the thermostat side. |
| The digital control never runs the humidifier during heat calls | Control W isn't connected to furnace W | During a heat call, check for about 24 VAC between control W and C. Then run Test/Reset with the blower on to confirm the valve clicks. |
| Control powers up but the valve stays silent (600 Digital) | The H loop is open: the transformer has no power, a yellow wire is off, or the H wires went to R or C | One H should go to the transformer, the other H to a yellow wire, and the second yellow wire back to the transformer. The transformer secondary should read about 24 VAC. The H terminals must not be tied to furnace R or C. |
| The valve or control burned out right after power-up | 120 VAC was applied to the low-voltage side | With power off, check that only the transformer primary connects to 120 VAC. Before reconnecting anything, the secondary must read about 24 VAC. |
| Automatic mode gives odd humidity levels or a service light (600 Digital) | The ODT is disconnected, damaged or badly placed | Check both ODT wires for breaks and loose terminals. Mount the sensor on the north, east or west side, out of the sun and above the snow line. Note any error code (E1 to E4) and contact your HVAC contractor or Aprilaire Technical Support. |
| The furnace transformer trips or hums when the humidifier switches on | The 24 V source is undersized | Use a source rated 10 VA or more. If the furnace transformer is already heavily loaded, use the supplied 24 VAC transformer instead. |
Frequently Asked Questions (FAQs)
Which wiring variant do I use, 600M Manual or 600 Digital?
Pick the variant that matches the control supplied with your humidifier. The Model 600M ships with a Manual Humidifier Control, which has two terminals and a dial that makes a simple contact, so use 600M · Manual. The Model 600 ships with the Automatic Digital Humidifier Control and an Outdoor Temperature Sensor, so use 600 · Digital. The ADHC has a full terminal strip labeled R, C, W, G, Gf, H, H, ODT, A and B, which makes it easy to tell the two apart.
Do I need the Model 50 current sensing relay?
You only need the Model 50 when nothing else limits the humidifier to running while the blower runs. It senses current in the blower common wire and closes its contact in the 24 VAC humidifier loop only while the blower is on. This stops water from flowing over the panel with no airflow. It isn't included with the humidifier. If your 24 V source is already live only while the blower runs, you may not need it. Check how your furnace behaves before you leave it out.
Can I power the humidifier from the furnace's 24 V transformer?
Yes, on the manual 600M setup, as long as the source is rated at least 10 VA, such as the furnace's 24 VAC accessory terminals. The digital control itself takes R and C from the furnace. On the Model 600, the humidifier valve loop runs through H and H from the separate 24 VAC transformer supplied with the humidifier, which must be continuously powered. If the furnace transformer is already heavily loaded, use the supplied transformer.
Where can I connect the 120 VAC side of the supplied transformer?
Connect it to a 120 VAC source other than the furnace motor circuit. You can take power from the hot 120 VAC line before it enters the furnace, but never wire the transformer into the blower circuit or the blower motor leads. On the digital setup the transformer must stay powered at all times. Make the line-voltage connection in an approved box, and have a qualified electrician or HVAC contractor do this part if you aren't qualified.
Do the yellow solenoid valve wires have polarity?
No, the two Yellow wires on the 24 V solenoid valve have no polarity. On the 600M, one goes to the manual control's Terminal 2 and the other goes to the Model 50 or to the 24 V source. On the 600, one goes to an H terminal and the other goes back to the transformer. The valve works either way round as long as it gets 24 VAC. The humidifier is rated 24 VAC, 60 Hz, 0.5 A.
Why does the thermostat G wire move to the digital control?
The thermostat G wire moves so the digital control can run the furnace blower itself. With thermostat G on control G and control Gf on furnace G, the control passes your thermostat's fan calls through to the furnace. It can also start the blower on its own for Blower Activation, sampling the air and finishing humidification after a heat call. If you leave thermostat G on the furnace, the fan still works, but the control can't turn on the blower.
What are terminals A and B on the digital control for?
Terminals A and B aren't used in this installation, so leave them empty. The standard Model 600 hookup only uses R and C for control power, W to sense heat calls, G and Gf for the blower, H and H for the valve loop, and the two ODT terminals for the outdoor sensor. Don't land any thermostat or furnace wire on A or B to "use up" a spare conductor. Tape back unused conductors instead.
Is the Outdoor Temperature Sensor required?
The Outdoor Temperature Sensor is needed for automatic mode, where the control adjusts the humidity target to the outdoor temperature. It lowers the target by about 1% RH for every 2°F drop, to help prevent condensation on windows. In manual mode you adjust the setting yourself as the weather changes. The sensor comes with the Model 600. Mount it on the north, east or west side of the home, above the expected snow line, and run its two wires to ODT. They have no polarity.
Can I use Blower Activation with cold water?
No. Plumb the humidifier to hot water if you turn on the Blower Activation switch on the digital control. In that mode the control runs the blower without a heat call, so the air passing through the Water Panel isn't heated, and hot water provides the energy needed for evaporation. Hot water is also recommended on heat pump systems. Keep the supply at 140°F or below and the water pressure at 125 psi or below.
Where should the humidifier control be mounted?
Mount a return-duct control at least 6" upstream of the humidifier, the bypass duct connection or any fresh air intake, so it reads the true return air humidity. You can also mount the 600M's Manual Humidifier Control on an interior wall in the living space, away from drafts and heat sources. On a downflow furnace with a return-mounted control, make sure the blower keeps running after the heat call ends so the control keeps reading moving air.
