There is a dehumidifier in the basement of the house we have bought.
However, it behaves rather oddly.  I myself have very limited experience
with dehumidifiers.

When you turn it on and run it for a while, about half the coils inside
the device (aluminum-looking tubes similar to what you might see in an old
refrigerator or air conditioner) become thickly covered with frost.  The
other half (somewhat less than half) have no frost.

There is a built-in bucket below the coils to catch the water.  However,
even running the thing overnight, almost no water falls into the pan. 
Rather, the water vapor seems to condense from the air onto the coils and
become frost. 

It also makes continuous noise, mostly the fan I suppose, but similar
again to a relatively noisy refrigerator or air conditioner.

What should I do with the thing?  Is it really being any help in
dehumidifying the basement?  Does it need repair?

What I would really like is a dehumidifier which is both quiet and
powerful.  Is that an unreasonable desire?  What sort of dehumidifier
should I get, if I buy a new one?

18 responses total.

It needs a shot of coolant.  The coils with frost on them have coolant
inside, the part without the frost is empty.  Add enough coolant to
build the pressure back up and you'll see condensation dripping
from the coils, and no more frost.  

We have a unit which works like a charm when the air temperature
is maybe 78 degrees or higher and tends to behave as you 
described (except the entire coil becomes thick with frost) when
the temperature is reasonable or cool.  We mostly only use it
in the hottest weather to keep a basement less humid but it
works quite well in those conditions.  

You might try to take the cover off and clean out dust... dust can clog up
the air flow.  

The instructions on my dehumidifier mention frost forming when the unit is
used below 70 F or so. I have put mine on a timer. I let the frost build
up and then the unit turns off and the frost melts, and the cycle repeats. 
My timer is set at 3 hours on and 3 off. I leave it running this way all
year (had to replace one blower motor after several years). I also have
hosed the drain into the furnace-A/C drain, so the collected water is
automatically pumped to a house drain.

Most of the noise is the fan: the compressor is pretty quiet. 

The reason why the dehumifier sounds like a refrigerator or air
conditioner is that it is, basically the same mechanism.  The noise you
hear is likely the compressor, not the fan.  In theory, the unit should
be sucking the air past the aluminum looking tubes (which are evaporator
coils, which in a refrigerator would be either hidden, or exposed around
the freezer in older or cheaper units), then past the evaporator coils
(which would heat the air back up), then out the unit.  The water that
forms on the coils is formed when the air is cooled down, according to
the same principle that results in the need to defrost refrigerators, or
the "drip-drip" that you sometimes see from air conditioners mounted on
the transoms above the doors of stores.

And it will get *too* cold if the pressure in the
condenser coils drops.  Highschool physics.  Have
a repairperson check the pressure and fill 'er up.

You could have a repair person look at it, but it might cost as much as
purchasing a new one, especially if it's so old that it uses refrigerants that
are no longer easily available.  I usually call MasterTech when I have some
kind of appliance repair problem.  Their phone number is 662-0641.

        Buy a new one.  Make a condition of the sale that they deliver the
new one and remove the old one.

(Spring agora item #116 is linked as Science #47.)

Expanding on other comments:
 
There are two sets of coils in a dehumidifier, an evaporator and
a condenser.  The condenser is usually at the front of the machine,
hidden behind the grille.  The evaporator is at the back.  The
machine works by pumping low-pressure refrigerant vapor out of the
evaporator (which gets cold) and into the condenser.  The liquid
flows through an expansion valve or tube back into the evaporator,
where it boils to vapor again.  The fan pulls air through the
evaporator and then blows it out through the condenser.
 
If you don't have enough refrigerant, the pressure in the evaporator
goes very low.  The boiling point of the refrigerant is a function of
the pressure.  Too low a pressure causes the boiling point to fall
below the freezing point of water, and the water collects as frost so
long as there is refrigerant left to boil.  The liquid refrigerant
doesn't go very far in an under-charged system, and once it is all
vapor the cooling effect essentially stops.  This is why part of the
evaporator coil is frozen solid, and the rest is not even dewy.
 
No, the machine is probably not being any help.  It is only removing
as much water as you see frozen on the coils.  That's not significant.
 
Adding refrigerant to the system will fix it.  HOWEVER:  if this is an
older dehumidifier, it probably requires R-22 instead of one of the
newfangled ozone-friendly materials.  I have no idea what R-22 costs
these days, but if it has gone up as much as R-12, it's a lot.
 
I'd do some comparison shopping.  Given the cost of electricity, if the
difference between fixing the old machine and a new one is less than $100,
you may very well be ahead if you buy a newer, more efficient unit.  I'd
go scan Consumer Reports for information as part of the research.

A perfectly working dehumdifier freezes water on its evaporator if the
room temperature is too low (below about 70 F) This depends upon a number
of factors, but the refrigerent in a normally operating evaporator is
usually at a temperature significantly below the freezing point of water
anyway, in order to get a higher heat flux. However the temperature on the
surface of the coil is normally higher than the freezing point of water
because of the heat transfer resistances of boiling the refrigerant and
conducting heat across the aluminum, compared to that for transferring
heat and moisture from the atmosphere. When the temperature of the
surrounding air decreases, there is less heat transferred, and the
temperature of the surface of the coil also decreases, until finally it
reaches the freezing point of water. 

In other words:  Wait 'till summer and see it it works right then.  If not,
you need a new one. ;)

The fact that it was freezing water on only a fraction of the coil does
suggest that it has lost refrigerant. That means a leak, since the usual
small dehumidifiers are usually sealed systems with no place to add new
refrigerant. In that case, it is on its way to the scrap heap. 

The fraction of the coil which has frost on it is somewhat over 50%, maybe
as much as 60%.

Thanks to everyone for their knowledge about this!

Now linked to the Dwellings conference!

Fifty percent of the length of the coils, or 50% of the diameter?  If
that's just the center, you have maybe 30% of the area doing any good,
and the machine is definitely in trouble.  If it's 60% of the total area
(closer to 80% of the diameter), it's probably got life left in it.

From what you have described,the unit almost certainly has a leak. That being
the case,to try and repair it,and charge it back up,including the cost of
R-22,you're better off to go get a new unit.The noise you hear is the
compressor working extra hard to pump what little refrigerant is left.The
refrigerant also contains oil which lubes the compressor,so its just a matter
of time till it burns up.
q

Try Kiwanis for a used unit, much cheaper.  Garage has some, I think.

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