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Aprilaire 600M Humidifier Guide: Settings & Care 

The Aprilaire 600M is a bypass whole-house humidifier with a manual dial control. Set the dial by the outdoor temperature printed on it (35% at 20°F, 25% at 0°F, 15% at −20°F), replace the Water Panel (part 35) once a year, and move the bypass damper to SUMMER when the air conditioning takes over. Air Comfort Service has serviced St. Louis homes since 1969 and services the 600M whether or not we installed it.

Read the video transcript (2 min)

Why a whole-house humidifier

Dry winter air in St. Louis means static shocks, cracked trim, and dry skin. A whole-house humidifier fixes that. This is how an Aprilaire 600M works, what to set it to, and the one part it needs every year.

How the Aprilaire 600M works

The 600M is an evaporative humidifier. Water goes in the top. A Water Panel hangs in the middle. Your furnace blower pushes warm, dry air through the wet panel, the air picks up moisture, and your ductwork carries it to every room. No tank to fill, no mist, no white dust.

The water path

When the control calls for humidity and the blower is running, the solenoid valve opens. Water passes through a small orifice to the distribution tray and spreads across the top of the Water Panel. Whatever doesn't evaporate runs to the drain and takes the minerals with it. A small, steady trickle at the drain means it's working.

The bypass damper

It's called a bypass humidifier because a short duct lets some warm air detour through the humidifier and back to the return. The damper handle on the unit controls that detour. WINTER opens it for heating season. SUMMER closes it when the air conditioning runs and you don't want added moisture.

Setting the manual control

The 600M uses a manual control, a dial marked 15 to 45 percent. The guide is printed right on it: 35 at 20 degrees outside, 25 at zero, 15 at 20 below. Set it lower before a cold snap, or you'll see fog on the inside of your windows. To find out what your indoor humidity is, turn the dial slowly until you hear a click, then back until it clicks again. Your humidity is between those two clicks.

The Water Panel

The Water Panel is the part that wears out. It traps minerals so your air stays clean, and after a heating season it's crusted. Aprilaire calls for a new one every year, part number 35. Replacing it means shutting off the water, pulling the cover and the whole evaporative assembly, cleaning the tray and the scale control insert, checking the feed tube and orifice, and flushing the drain. That's the service visit. It's the difference between a humidifier that works and one you think is broken.

How the Aprilaire 600M works

The 600M is an evaporative humidifier. Water enters at the top, spreads across a distribution tray, and runs down over a Water Panel. Your furnace blower pushes warm, dry air through the wet panel. The air picks up moisture as vapor and your ductwork carries it through the house. Nothing gets heated, nothing gets misted, and there is no tank to fill.

The "bypass" part is a short duct from the supply plenum, through the humidifier, into the return. Warm supply air takes that detour, picks up moisture, and rejoins the return air stream. A damper handle on the unit opens and closes the detour: WINTER for heating season, SUMMER for cooling season.

Water that doesn't evaporate runs to the drain and carries dissolved minerals with it. That trickle is the design working. The manual says the saddle valve on the water line should be fully open or fully closed, never used to throttle the flow; a small orifice at the feed tube sets the flow rate.

Optimal settings

The 600M's Manual Humidifier Control is a dial marked OFF, 15, 25, 35, and 45 percent, with Aprilaire's guide printed across the top:

Aprilaire manual control settings by outdoor temperature: 35% at 20°F, 25% at 0°F, 15% at minus 20°F

The number drops as it gets colder because cold windows condense moisture. If you see fog or frost on the inside of the glass, turn the dial down one step. If the house feels dry and you're getting static shocks, turn it up one step. Before a cold snap, turn it down a few hours ahead so the house has time to adjust.

Most St. Louis winter days sit between 20°F and 40°F outside, so 35% is the common setting here, dropping to 25% on the single-digit nights in January.

Is it running?

The 600M has no indicator lights. Three checks tell you what you need to know.

  1. Is the furnace blower running? The humidifier only adds moisture while the blower moves air. No heat call, no humidity.
  2. The two-click check. With the blower running, turn the dial slowly clockwise until the humidifier clicks on, then counterclockwise until it clicks off. Your indoor humidity is between those two settings. If the dial is set above the click-on point, it's calling for humidity.
  3. Look at the drain. While it runs, you should see a small, steady stream at the drain line. No stream means the water isn't getting through: a closed saddle valve, a clogged strainer or orifice, or a solenoid that isn't opening.

Check the Water Panel.

The Water Panel is a mineral-trapping evaporator pad. As water evaporates, calcium and magnesium stay behind on the panel, and after a heating season the panel is crusted and moves less air and less water. Aprilaire calls for a new Water Panel at least once a year on the 600M. The replacement is part number 35.

A worn panel is the most common reason a homeowner tells us "the humidifier stopped working." The control is calling, water is flowing, and the house stays dry because the panel can't evaporate it.

  • Dial to OFF, water supply shut off, bypass damper to SUMMER
  • Front cover off, feed tube lifted out of the distribution tray, evaporative assembly removed
  • Distribution tray unsnapped and cleaned of mineral deposits (a vinegar soak loosens the stubborn ones)
  • Old Water Panel out, scale control insert cleaned, new Water Panel 35 in with the colored spot up
  • Feed tube flexed and inspected for cracks; replaced if it's stiff or brittle
  • Drain line removed, flushed under pressure, reinstalled with a constant downward slope
  • Inline strainer and orifice inspected and cleared
  • Water back on, damper back to WINTER, dial back to your setting, operation confirmed with the furnace fan running

Every step after the first one involves the water line and the inside of the unit, which is why Air Comfort Service does this as a service visit rather than handing you the list.

Shutdown in summer

When the air conditioning takes over, do two things: turn the dial to OFF and move the bypass damper to SUMMER. Leaving the damper open all summer lets conditioned air detour through a dry humidifier for no reason. Reverse both at the start of heating season, on the same visit as the Water Panel change.

Q: What should I set my Aprilaire 600M to in winter?

Use the guide on the dial: 35% at 20°F outside, 25% at 0°F, 15% at −20°F. Turn it down one step if your windows fog on the inside.

 

Q: How do I know if my Aprilaire 600M is working?
Q: How often do I change the Water Panel?
Q: Is the Aprilaire 600M a bypass humidifier?
Q: Can I use softened water?
Q: Does Air Comfort Service work on Aprilaire humidifiers it didn't install?