Aprilaire 500M Humidifier Wiring Diagram
Aprilaire 500M 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 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.
About This Diagram
This diagram is redrawn from page 1 of the Aprilaire 500M 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 500M Humidifier Wiring Diagram, we suggest checking the Frequently Asked Questions (FAQs) section.
Printable Aprilaire 500M Wiring Diagrams
Wiring Instructions
Before you start
First, find the tab that matches your humidifier. Choose Aprilaire 500M · Manual if your unit came with a knob-type Manual humidifier control, which can mount on the return duct or on a wall. Choose 500A · Digital if it came with the Automatic Digital Humidifier Control and an outdoor temperature sensor. The current Model 500 is the same as the earlier 500A, so it also uses the Digital variant.
This humidifier should be installed by a qualified heating and air conditioning contractor, following all local, state and federal codes. The warranty is void if it is not. Have these ready:
- Low voltage wire, which is not supplied. The sources don't give a gauge. Common practice is 18 AWG thermostat cable, with two conductors for each run and a spare conductor where you can.
- A screwdriver, wire strippers, a multimeter that reads AC volts, and wire nuts.
- The supplied 24 VAC transformer and humidifier control. You also need a Model 50 current sensing relay if your furnace has no 24 V accessory terminal that turns on with the blower.
- The furnace manual. It tells you the voltage and how the accessory terminals behave. They may be labeled HUM, ACC or EAC.
Turn off power to the furnace at its service switch or breaker, and confirm with your meter that it is off. If you will add a 120 VAC feed for the transformer, that is line-voltage work. Many areas require a licensed electrician or a permit for it.
Step-by-step wiring
- Finish mounting the humidifier, bypass duct, water line and drain before you run any wire. Then turn the control knob to OFF.
- Pick a 24 V source. On the 500M, you can use the furnace's 24 VAC accessory terminals or a transformer rated at least 10 VA. On the 500A, the 24 V transformer must power the control all the time, so it can't come from a terminal that only turns on with a heat call.
- Power the transformer's 120 VAC primary from a circuit other than the furnace motor circuit. You may tap the hot 120 VAC line before it enters the furnace. Never connect the transformer to the blower motor wiring, because this can make components fail early.
- 500M: Run one wire from the 24 V source to Terminal 1 of the Manual humidifier control. Connect Terminal 2 to one of the yellow solenoid valve wires.
- 500M: Connect the other yellow wire back to the 24 V source. If you use a Model 50, wire its contact into this return path. Keep the Model 50 in the 24 VAC humidifier control circuit only, never in the transformer primary.
- 500M with Model 50: Pass the furnace blower motor common wire through the sensor on the Model 50. This makes the humidifier run only while the blower runs.
- 500A: Connect the transformer's 24 V secondary to R and C on the Digital control.
- 500A: Connect W/G to the furnace's 24 V accessory terminal and Cf to furnace common. If that terminal supplies 120 V instead of 24 V, add a 24 V transformer rather than connecting it directly.
- 500A: Mount the outdoor temperature sensor on the north, east or west side of the home, out of direct sun and above the expected snow line. Run two wires to the ODT terminals. Polarity doesn't matter.
- 500A: Connect the two H terminals to the yellow solenoid valve wires. Leave A and B empty for this wiring.
- Tighten every terminal screw and wire nut, tug each wire gently, and secure the cables away from hot surfaces and sharp duct edges.
Testing and start-up
Fully open the saddle valve and set the bypass damper handle to WINTER. Then turn the furnace power back on.
- 500M: Turn the control to its highest setting and start a heat call. When the blower starts, you should hear the solenoid click. Water should then fill the distribution tray and soon run steadily from the drain. While the valve is on, the yellow wires should read about 24 VAC. If they read near zero, nothing is reaching the valve.
- 500A: Check that R to C reads about 24 VAC even when the furnace is idle, and that W/G to Cf reads 24 VAC during a heat call. With the blower running, turn the knob to Test/Reset. The humidifier should run for one minute. Don't leave the knob there: move it back to the setting you want.
- Let the humidifier run until water drains. Check that all plumbing joints are watertight and all electrical connections are secure.
- Set the control to the recommended humidity for the current outdoor temperature.
Safety notes
- The humidifier, the controls and the Model 50 contact work at 24 VAC, and the valve is rated 24 VAC, 60 Hz, 0.5 A. Connecting 120 VAC to any of them will destroy the valve or the control. It is also a shock and fire hazard.
- The transformer primary and the furnace both carry 120 V. Turn off power and confirm it is off before touching anything inside the furnace cabinet.
- On a downflow furnace, set the blower to keep running after the heat call ends. Otherwise heat can damage the humidifier control.
- If you tap a hot water line, shut off the hot water supply first. Water above 125°F can scald instantly.
- Keep the humidifier and its water line where they cannot freeze.
Troubleshooting
| Symptom | Probable Cause | What to Check |
|---|---|---|
| Humidifier never turns on and the valve never clicks | No 24 V reaching the solenoid valve, from a dead transformer, a loose terminal or the wrong source | During a call for humidity with the blower running, measure across the yellow wires. You should see about 24 VAC. If not, check the transformer secondary, then Terminal 1 and 2 on the 500M or the H terminals on the 500A. |
| Digital control display is dark or only lights up while the furnace runs | R and C are powered from a heat-call terminal instead of a continuous 24 V source | Measure R to C with the furnace idle. It must read about 24 VAC. Move R and C to the supplied transformer, which stays powered all the time. |
| Digital control is on but never runs the humidifier during heating | W/G or Cf is not connected, or the accessory terminal is not 24 V | During a heat call, measure W/G to Cf. You should see 24 VAC. If the furnace terminal supplies 120 V, add a 24 V transformer. Check the furnace manual for when the terminal turns on. |
| Water runs to the drain even when the blower is off | The valve is powered without a blower interlock: the Model 50 contact is bypassed, or the 24 V source stays on | Make sure the Model 50 contact is wired in series in the 24 V loop. Also make sure the accessory terminal you used only turns on with the blower. |
| With the Model 50 installed, the humidifier never runs | The Model 50 sensor is on the wrong wire, or the relay is in the transformer primary | Pass the blower motor common wire through the sensor, not a speed lead or a 24 V wire. Move the Model 50 contact into the 24 VAC control circuit. |
| Transformer hums, runs hot or fails after a short time | The transformer primary is connected to the blower motor wiring | Rewire the 120 VAC primary to a separate source, or to the hot line before it enters the furnace, never the blower circuit. Replace the transformer if it is damaged. |
| Furnace low-voltage fuse blows or the control board resets when the humidifier turns on | A short in the valve wiring, or the accessory output can't supply enough power | With power off, look for pinched or bare yellow wires. Check that the 24 V source is rated at least 10 VA, or use the separate supplied transformer. |
| Humidifier runs constantly and won't shut off when the knob is turned to OFF | The control is bypassed, with the two control wires or the H terminals joined together | Turn the knob to OFF and measure across the yellow wires. You should see 0 V. If 24 V is still there, look for wires tied together that skip the control. |
| Digital control won't lower humidity in cold weather, or shows the red Service Required light after you install it | The outdoor sensor is not connected, has a broken wire or is in a bad location | Check both ODT wires for a broken or loose connection. Move the sensor to a shaded north, east or west wall above the snow line. Note any error code and contact support if it stays on. |
| Valve clicks but no water reaches the humidifier | The saddle valve is closed, or the strainer or orifice is clogged | Open the saddle valve fully. With power and water off, clean the inline strainer and check that the small opening in the orifice is clear. |
Frequently Asked Questions (FAQs)
How do I know whether I have the 500M or the 500A variant?
Look at the humidity control that came with your unit. A 500M uses a Manual humidifier control: a simple knob with two wire terminals that can mount on the return duct or on a wall. A 500A uses the Automatic Digital Humidifier Control: it mounts on the return duct, has a digital display and indicator lights, and works with an outdoor temperature sensor. The current Model 500 is the same as the earlier 500A, so it uses the Digital variant.
Can I connect the humidifier or its control directly to 120 V?
No. The solenoid valve and both controls run on 24 VAC only, and the valve is rated 24 VAC, 60 Hz, 0.5 A. Line voltage would burn out the valve coil or the control right away and create a shock hazard. Only the primary side of the supplied transformer connects to 120 VAC. Power it from a circuit other than the furnace motor circuit, or from the hot line before it enters the furnace.
Do I need the Model 50 current sensing relay?
You only need it when you can't get a 24 V signal that turns on with the blower. If your furnace has a 24 VAC accessory terminal, often labeled HUM, ACC or EAC, that powers up while the blower runs, you can usually skip it. With a separate transformer, the Model 50 senses current in the blower common wire and keeps the humidifier off whenever the blower is off.
What size wire should I use for the low voltage connections?
The sources don't specify a gauge, and low voltage wire is not supplied. Common practice is 18 AWG solid copper thermostat cable for runs between the furnace, transformer, control and valve, because it handles the valve's 0.5 A easily. Use two-conductor cable for each run, and consider a cable with spare conductors for future options. On long runs, check the voltage at the valve while it is running.
Do the yellow solenoid valve wires have polarity?
No. The two yellow wires connect to a 24 VAC coil, and you can connect either wire to either side of the circuit. On the 500M, one goes to the Manual control and the other to the 24 V source or the Model 50. On the 500A, they go to the two H terminals. Just make sure both connections are tight and insulated so they can't short together.
Can I power the Digital control from the furnace accessory terminal instead of the supplied transformer?
Not with the wiring shown. The Digital control needs 24 V on R and C all the time so it can keep reading humidity and outdoor temperature. A furnace accessory terminal that only turns on with a heat call would turn the control off between cycles. Use the supplied 24 V transformer for R and C, and connect the accessory terminal only to W/G as the signal that the furnace is running.
What are terminals A and B on the Digital control?
They are not used in the recommended wiring shown here, so leave them empty. The Digital control supports other wiring setups that are covered in its own instructions, and these extra terminals belong to those setups. The Blower Activation feature, which lets the control turn on the HVAC fan, also needs the humidifier connected to hot water. Follow the control's instructions if you want to use it.
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. Avoid the south wall and spots near dryer vents, exhausts or lights, because they would make the reading too warm. The control uses this reading to lower indoor humidity as the outdoor temperature drops, which prevents condensation on windows. You connect it with two wires to the ODT terminals, and polarity doesn't matter.
Can I run the Digital control without the outdoor sensor?
Yes. The ODT terminals accept either the outdoor temperature sensor or a manual mode resistor. In Manual mode, with the internal switch in the MAN position, an M appears on the display. The control then holds whatever humidity you set and won't adjust for outdoor temperature. You will need to lower the setting yourself before cold weather to avoid condensation.
Can I install the 500M on a heat pump system?
Yes, but connect it to hot water. A heat pump delivers cooler supply air than a furnace. Hot supply water up to 140°F gives the extra heat needed for evaporation. The wiring itself is the same: the humidifier still needs a 24 V signal tied to blower operation, either through a suitable accessory terminal or a Model 50 relay on the air handler's blower common wire.
Can a homeowner install this humidifier?
It is meant to be installed by a qualified heating and air conditioning contractor. The warranty is void if it isn't installed that way, because of the risk of misapplication. The job involves cutting the plenum, plumbing, 120 VAC transformer wiring and the furnace's low voltage circuits. An experienced DIYer may follow the steps safely, but warranty coverage and local code requirements usually make professional installation the better choice.
