The mysterious dehumidifier leak

I have two dehumidifiers of this shape. The one I have in the workshop started leaking onto the floor at some point, and I opened it up and found that all the dust it picked up from the air formed a sludge in the drip tray, which caused it to overflow.

The other one occasionally leaks onto the floor, so I figured I should open it up and also clean out the drip tray.

When I opened it, I saw the filter, and realized I hadn't cleaned that in ages, so I washed that out while I was at it.

I didn't see any dirt in the drip tray, and pouring some water onto the cooling coils, it flowed out of the drip tray's drain normally.

A dehumidifier is basically a small air conditioning unit, but instead of cooling the inside air and pumping the heat outside, it cools down the air and immediately warms it up again. Cooling the air causes water to condense out of it, much like water condenses onto an ice cold drink.

I figured maybe this one had iced over from time to time and when the ice melted it it dripped outside the drip tray, and maybe cleaning the filter would fix that. But when I put it back in the basement, I once again found a puddle around it in the morning.

I wanted to catch this in the act, so I set it up, with the enclosure open and used a Raspberry Pi with camera module it to capture a time lapse.

I found that the cooling coils would ice over roughly every 30 minutes. After that, the unit turned off the compressor (for the heat pump), just blowing air through the unit to melt the ice. When that happened, water dripped out the bottom.

I set it up to run without the tank in place so I could see how the water came out of it. When the unit went on its de-icing cycle water dripped out of it fairly fast, and some of it, because of surface tension, went off the back of the spout, then dripped down somewhere else, even though the spout was actually the lowest point. I didn't know water would do that!

One time I checked on it I saw the coils were mostly iced over, so I knew a defrost cycle was imminent, and I stuck around to watch the water dribble out.

I took a hose cap, drilled a hole in it, and put it on the screw thread. That way the water would have to go further down before going back up, so hopefully it would drip off this cap instead of going back up and dripping down somewhere else.

Also, given how the water drips off the back of the spout, if the unit was leaning back slightly, the water would run all the way back on the plastic and go down the back wall, behind the tank.

I added a dab of caulking to give the water a bit of a low spot to dribble off.

A simple design change to make sure the spout is much lower than the plastic behind it would eliminate this type of dribbling, but I'm sure it's something the designers of this dehumidifier hadn't anticipated.

While I was at it, I noticed there was about a 1 cm gap between the coils that cool the air and the ones that warm it back up, and holding a bit of tissue paper near the gap, I could tell a fair amount of air was sucked in that gap.

So I took some black foam insulation (cut from pipe insulation) and stuffed it into the gap.

Testing it again, the dehumidifier still iced over from time to time, but only half as often as before.

I also had a power meter measuring the unit's power draw. As the unit iced up, power consumption always went up by a few tens of watts. Power consumption typically started at 300 watts, but as the cooling coil became colder and the warming coil warmer, went up to over 400 watts. With a greater temperature difference, the compressor has to overcome a greater pressure difference for the refrigerant to evaporate inside the cooling unit and condense inside the heating part, so it uses more power.

Switching the fan between high and low speed made a difference of less than ten watts. With more air flow, the temperature difference between the two heat exchangers should be lower, which should mean less power needed for the compressor, and that will more than make up for the less than ten extra watts for the fan. So, counter-intuitively, turning the fan on high makes the unit louder but consume less power.