An Aprilaire 600M humidifier attached to my HVAC vent is leaking very slowly, about 2 ounces per day. I have not seen it leak but the leak is coming out near the exhaust at the bottom of the plastic insert (see first pic below).

This is what I have tried.

  1. Inspect the housing for cracks
  2. Make sure the plastic insert is seated properly
  3. Pour water into into plastic insert to see if it is leaking
  4. Pour water into the pipe where it exits the humidifier to be sure the exit piping is not clogged.
  5. Change the water panel

I noticed some white crumbly stuff at the bottom of the humidifier (see second pic below)

What else can I do to prevent the leak?



  1. I opened the humidifier cover and turned up the humidistat so that I could see what was happening when the water flowed. I saw water dripping at the bottom corner, originating about 1/2 up the side. (see third pic below)

  2. I slipped in a thin wooden shim so the panel was not leaning against the holder (the tray) and the drip stopped. (See fourth pic below.)

I do not see how to put in the panel so it does not lean against the tray. Any help would be appreciated.

Plastic insert (view from bottom)

Powder in humidifier


enter image description here

  • The only issue I see is in the 3rd pic where the little puddle of water should be running toward the drain instead of sitting in the corner. Is it possible that the housing has been knocked so that it's no longer sitting level and this corner is now the low spot instead of the drain port being the low spot?
    – FreeMan
    Nov 20 '20 at 15:43
  • @FreeMan I doubt the humidifier is designed so water leaks out of the frame and then flows into the drain. In support of that thought, the bottom of the humidifier is flat, not v-shaped and with no path to facilitate water going into the drain.
    – Yehuda_NYC
    Nov 20 '20 at 15:59

I can't quite tell whether you're seeing normal operation and mistaking it as a problem or if there's actually a problem. The white crumbly stuff is scale, also called mineral deposits. It is calcium and other minerals that are dissolved in drinking water.

I've drawn a line on the photo below to illustrate the path water should travel. It flows from the valve into the distribution tray at the top, trickles down through the evaporator pad, then collects in the tray at the bottom and exits through the drain hose. It's normal that the water that enters the humidifier will not all be evaporated - in other words, that some water will drain off when the humidifier operates. The amount of drainage will depend on how much the humidifier operates. If the indoor humidity is already close to the humidistat setting then the humidifier won't operate much and there will be little drainage.

enter image description here

The drain port at the bottom of the humidifier pad assembly should be inserted into the fitting at the bottom of the housing so that excess water goes through the drain hose. The humidifier pad assembly should sit fairly close to level and plumb in the housing so that the collection tray won't overflow. The water supply should be adjusted high enough that there's some drainage coming off the humidifier, but low enough to avoid overflowing the collection tray.

Based on the update of the question it sounds like water, as it trickles down the panel, jumps the gap to the plastic frame. A drip then forms and runs down the exterior of the frame, missing the intended drainage path.

The shim is a good idea. It seems reasonable to leave that in place and call it a fix. An alternative might be to modify the water panel. Maybe by crushing it down a bit on or near the edge you can make the gap from panel to frame large enough that rivulets of water flowing down the panel can't jump the gap.

  • I've had the HVAC system for 11 years. For 10+ years the humidifier worked perfectly. At the end of last winter it started to drip water. I put a basin under the drip. I change the Water Panel each year but have not changed the Water Distribution Tray or Inline Strainer. Perhaps they are getting clogged and water is spilling over the top of the tray. I'll try to check that out.
    – Yehuda_NYC
    Nov 20 '20 at 14:25
  • Nope. The tray is made so that even if the filter top gets clogged the water will drip down onto the water panel.
    – Yehuda_NYC
    Nov 20 '20 at 14:39
  • I updated the original posting. There has been some progress.
    – Yehuda_NYC
    Nov 20 '20 at 14:51

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