Automatic Emptying of a Dehumidifier

Basements in humid summer climates get incredibly damp and this results in mold and insect infestation, damage to paper and wood, and an unpleasant odor. A dehumidifier will rectify this problem but condensate is produced in copious quantities and unless a person empties the condensate bucket once a day or more or there is a convenient place to drain the water by gravity, the bucket will fill up and the dehumidifier will cease to function and moisture will take over once again.

Having tried a gravity drain system with the dehumidifier elevated and the water draining into a toilet bowl, I experienced problems with the water overcoming the pressure drop of the thin tubing that I was using and the siphon would not always work and the bucket would fill. I then saw a car windshield washer fluid pump at a junk yard with the exact size tubing connections as I had used in the dehumidifier drain. I bought it for $3 and proceeded to figure out a way to control it.

SYSTEM OVERVIEW

Pump and Plumbing

The pump is a car windshield washer fluid pump. It has a 12 volt DC motor and pumps the dehumidifier water against approximately 6.5 feet of head (vertical distance) through 9 feet of 1/4 inch inner diameter polyethylene plastic “ice maker” tubing. The water exits to the atmosphere where it acts to water the grass outside the basement window. The tubing is connected to the pump simply by sliding it over the hose barbs on the pump inlet and outlet. The suction tubing is connected to the dehumidifier using a "garden hose" style female adapter manufactured by Orbit Irrigation. The adapter has a female hose coupling on one end which mates with the male fitting on the bottom of the dehumidifier bucket and a barb-type connection on the other end into which the 1/4 inch tubing is pushed and held in place by a barb. Adapters that will do this are available in plumbing sections of hardware stores, or as in my case, garden supply sections where drip irrigation equipment is sold.

The pump draws 4.8 amperes at 12 volts which equates to 58 watts, however a 1.5 ohm ceramic power resistor has been placed in series with the pump in order to reduce its power draw otherwise it occasionally will trip out the power supply upon startup. The pump also sounds as if it is cavitating at full power and the reduction in power eliminates this effect. The resistor wastes about 9 watts of power when the pump is on but considering the amount of electrical power used by the dehumidifier itself (around 500 watts!) this waste is irrelevant.

Power Supply

The switching power supply from a Peltier Effect dehumidifier (how ironic) supplies electrical energy to the system at 13 volts DC. An LM317 voltage regulator provides the 5 volt supply for the control system from this 13 volt supply. The motor and relay coils are driven directly from the 13 volts supply. The power supply can provide 65 watts of power and will trip out if this is exceeded; the pump will exceed this upon startup necessitating the power resistor to cut back on the current draw.

A computer (ATX) power supply can be used to run the pump at full power as most ATX supplies can provide 8 or more amps on the 12 volt rail. I did not want to have an ATX supply (with fan and all) running constantly as this 13 volts supply is fanless and quite compact. The electricity wasted by running an ATX supply fan and its necessary minimum load will far exceed the electricity wasted by the 1.5 ohm dropping resistor! With some tinkering the ATX fan could be made to turn on with the pump, or eliminated altogether if the pumping cycles were kept short enough.

Control System

Two float switches in the bucket control the status of the pump. They are mounted on waxed wood and consist of two micro switches salvaged from, ironically, a Peltier-effect dehumidifier that I purchased at a thrift store solely for the Peltier module it contained. 14 gauge solid copper wire connects between the levers of the micro switches and Styrofoam floats. They are positioned in the tank to send “on” and “off” logic signals to the control circuit at specified water levels.

 
VIEW OF PUMPING AND CONTROL SYSTEMS

A “brain” is required to empty the dehumidifier bucket. It would be possible to build the system with a single float switch that turns the pump on and off, but this will result in a very short pumping cycle. In my case the water that flows back down the tube when the pump shuts off will probably turn the pump back on and the pump will just cycle, pushing water up the pipe, turning off, water falling back into the bucket, pumping turning on, etc. That's not cool. Therefore a dual float switch system is needed with the pump moving a significant amount of water each time it turns on.

DEHUMIDIFIER EMPTIER SCHEMATIC

The control circuit uses TTL logic chips (two "AND" gates and two "OR" gates, specifically) with four inputs: The low limit float switch, high limit float switch, pump "on" signal, and manual override switch.

If the high limit switch is activated (turned "on"), it is presumed that the low limit switch is also "on". At this point the pump will be turned on and the water level in the bucket will drop. At some point the high limit switch will turn off, but the controls will sense that the pump is already running and therefore will continue to run it until the low limit switch is turned "off". Once the water drops and turns off the low limit switch, the pump shuts off.

If for some reason water needs to be pumped out without the guidance of the float switches, a manual override button can be pushed and will drain the tank until the low limit switch turns off. Once the low limit switch turns off, however, the pump is shut down regardless of the status of the override switch. This protects the pump from running dry.

CREATED/WRITTEN: 2010-06-07 00:00