Sears Kenmore Model 33 dehumidifier won't turn off

This thing keeps my basement from being dank and dungeon-like.
Occasionally this dehumidifier would refuse to shut off and would turn the coils into a solid block of ice. I would unplug it, let it thaw, plug it back in, and it would work fine again for weeks or months more.
But now it refuses to turn off at all. Plug it in and it runs forever and goes all Ice-Agey on me every time.
What's wrong?
--
Tegger

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wrote:

This isn't a direct answer to your question but can you just plug it into a timer and have the timer shut it down?
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What normally shuts it off, a humidity sensor or a timer or what? The external timer Tegger suggests sounds like a good quick and dirty solution to the problem.
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wrote:

Oops, I meant the solution suggested by Doug.
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A built-in dial that goes from one raindrop to three raindrops. I guess that's internationalese for "dry to wet". I keep it set in the middle, which is what the Owner's Manual recommends.
--
Tegger

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Does turning that control do anything???? It might be clogged with dust after all these years, it can't hurt to take it apart and see (after unplugging the device, of course).
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I realize I haven't been very clear about this issue.
To clarify: This machine runs on the duty-cycle principle. It turns on when the humidity sensor (controlled by the aforementioned dial) says the air is too wet, and turns off when the air is sufficiently dry.
According the Owner's Manual, one raindrop means "take out only a little water", and three raindrops means "take out a lot of water".
So how come it won't turn off anymore? I understand that a dehumidifier is basically an air-conditioner. Any A/C experts out there?
--
Tegger

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It looks like this model comes with an icing sensor. When ice forms on the coils, the sensor is supposed to switch off the compressor while keeping the fan running, allowing the ice to run off as water.
There is a foam filter in front of the coils, and it's pretty dirty. Working from gregz's premise, I removed the filter to see what difference better airflow made. It turns out that the coils ice over 100%, right over the icing sensor. The icing sensor is about the size and shape of a soda- bottle cap, is potted, and has two thick electrical wires. Maybe it's stuck and is not switching, like gregz's example.
When I get time, I will remove this sensor and test it. Since I don't know if it's normally closed or normally open, I'm guessing I should see a state-change as the temperature crosses the freezing mark.
--
Tegger

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It sure sounds like a stuck contact problem, either in the deicer sensor or in the 3-position switch,
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wrote:

Just for fun, I just pulled the icing sensor and took it apart, which required care, and judicious use of a Dremel.
The soda-bottle cap's (see above) closed-end sits against a turn of the cooling coil, held there by a clip. Just inside the cap, where a washer would be in a real bottle cap, is a disc-shaped bimetallic spring. This spring says "LI-LI-LI; TOD-TOD-TOD" repeatedly on one side, and is blank on the other. The blank side faces the closed-end of the bottle cap. Against the printed side sits a tiny piston that bears against a set of contacts.
At room temperature, the spring is slightly domed towards the blank side, and the piston does not touch the contacts. Below freezing, the dome suddenly pops so that it is domed towards the printed side. This action pushes the tiny piston so as to open the contacts. This breaks the current, and the compressor would stop running.
This means the contacts are normally closed. A VOM shows me that the contacts are in fact closed (zero ohms) above freezing and open (infinite) below that, which is what I found before I dissected it.
I think I'm going to have to wait for some really hot, humid weather before I can gather more information.
--
Tegger

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I have one that the contacts stick on. Have to use external control to be certain it shuts off. If it ices needs more airflow. If it ices in one spot, needs refrigerant.
Greg
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If you have the skills, put a thermostat on the coils that get icy. When the coils get cold, the thermostat is satisfied, and opens the circuit to the compressor.
More air flow is good. . Christopher A. Young Learn more about Jesus www.lds.org . .
wrote:

I have one that the contacts stick on. Have to use external control to be certain it shuts off. If it ices needs more airflow. If it ices in one spot, needs refrigerant.
Greg
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More than likely dirty contacts that are sticking.
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On 6/3/2013 7:03 PM, Tegger wrote:

I would suggest getting someone to check the refrigerant charge. If you take it to a shop with a feller who knows what he/she/it is doing when it involves refrigeration and air conditioning, H/S/I could probably tell by feel and thermometer measuring the differential temperature of the evaporator and condenser without having to instal service valves for gauges. Most small units have no service connections on the sealed system. If the unit is old, there could be a tiny leak caused by vibration over the years. ^_^
TDD
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Thanks for the suggestion. I don't think this dehumidifier lacks refrigerant, I thought it was having trouble shutting off when it should.
A careful reading of the instructions tells me that the unit is only meant to operate under conditions of certain intersecting temperature/humidity vectors. Apparently if the unit is operated outside of those vectors, it can ice up. It seemed that the unit was icing up randomly, but now I think the icing may actually be tied to the temp/humidity level.
I didn't touch any of the parts that have refrigerant in them. I did disassemble the clip-on de-icer unit, which was fun; I now know exactly how it works. I'm a techy, engineery type, and such discoveries bring a happy glow to my troubled mind.
Everything's back together again and working fine, but it's neither warm enough nor humid enough to make the thing start running unless I crank the dial way up there, so I need to wait for the jungle weather to commence.
--
Tegger

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If the temperature sensor does open the contacts when the temp gets down to freezing, then why were you having the original problem of the unit not shutting off/ cycling on and off before it iced up? I'm confused (happens more frequently the older I get)<g>.
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I've worked on more than a couple residential dehum which do not have a freeze stat.
And, one time I worked on one (hanging from the ceiling of a commercial property's cellar) where the freeze stat was unclipped from the line, and hanging free. They had been turning the unit off every couple days to thaw. I clipped the freeze stat back on, and put on some tape and tie straps to help keep it in place. I'm guessing some ignorant person unclipped the freeze stat.
A creative person could use a thermostat of the proper temp range, to act as a freeze stat. Set for about 40F, it would allow the coils to be cold, but not frozen. . Christopher A. Young Learn more about Jesus www.lds.org . .
If the temperature sensor does open the contacts when the temp gets down to freezing, then why were you having the original problem of the unit not shutting off/ cycling on and off before it iced up? I'm confused (happens more frequently the older I get)<g>.
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Me too. The instructions are generic to many models, not all of which have icing sensors; it's not clear that the icing warning is meant for the models with sensors.
Anyway, I solved the problem by turing the knob a tiny bit towards the "wet" side. See my other reply.
--
Tegger

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On 6/5/2013 4:24 PM, Tegger wrote:

The dehumidifier pumps heat away from the coil. The air blowing over the coil adds heat to the coil in 2 ways. One is by the temperature of the air. The other, likely more important, is by the humidity of the air. When water condenses out it releases heat that it took away when the water evaporated. If you are not adding enough heat to the coil the coil temperature will fall. When it falls to the freezing point the coil ices up and becomes a large ice cube.

Yes, a true techie.
I have a dehumidifier that works great for a while, then always ices up for a while. The freeze stat apparently doesn't always work. If there is a freeze stat and it works a dehumidifier shouldn't ice up.

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In theory, yes. I didn't take the time to determine the exact temperature at which the little disc "pops", so it's entirely possible that the "pop" temperature is lower than that encountered during an icing episode. Would that be a defect? a malfunction? a design flaw? I don't know.
In any case, with the control knob turned a little more towards the "wetter" side, icing seems to have stopped.
--
Tegger

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