Fish Heater Safety Modification
by Paul Moratto
"Perfecto" 8-inch 50-watt 115vAC Aquarium Heater
The subject fish heater, made in Taiwan, is a knock-off of the original Fish Assassin,
which was green instead of black. Both incorporate a design flaw, which results in the
heater failing to turn off when aquarium water temperature reaches intended level. Sadly,
your fish are slowly and literally cooked to death.
The reason for the failure is that the “103M” disc capacitor, which serves to
protect the bi-metal temperature-sensing switch contacts from arcing, eventually
short-circuits and bypasses the switch, keeping the heater constantly on.
To prevent this event, I installed a small fuse in series with the cap and the junction
of the AC line input and bi-metal switch contact. If the cap should ever short, the fuse
will blow, removing the cap from the circuit. Should this occur, the heater will continue
to operate, and will appear to be working properly; there may be no visible sign of
trouble unless the fuse is inspected or checked with an ohmmeter. However, do NOT continue
to operate the heater in this condition, as the switch contacts may arc and burn. Make
repairs as follows:
Replace the cap with a high quality 0.01uF disc or mylar capacitor rated for at least
1,000-volts.
Replace the fuse with any size up to but not greater than ¼ or 1/3 ampere rated for at
least 250-volts. Note: This value is calculated for a 50-watt or larger heater. Do not
replace just the fuse alone, because a replacement will surely blow immediately. If the
fuse blows, the cap IS shorted.
After reassembly, immerse the disconnected heater in water, then remove and inspect for
any sign that water can get inside the glass tube. If so, adjust or replace the rubber
O-ring seal, or add a touch of silicone sealant to it, so the tube is water-tight. Do not
plug the heater into a power source unless you are sure, and do not operate the heater out
of water either.
Crank the temperature setting control only as high as necessary, as a high or maximum
clockwise setting may create too much obstacle for the bi-metal switch strip to overcome.
This too can result in your fish being slowly and painfully poached alive. The
modification described herein does NOT address this separate problem.
When beginning use, turn the knob clockwise only until the NE-2 neon indicator lamp
ignites and remains lit long enough to heat the water to desired temperature level. As
temperature changes, the light (and heater coil element) should turn on and off as needed
to regulate water temperature. This indicates that the bi-metal strip is functioning
properly and is not “jammed on.”
The general rule for aquarium heaters is 5-watts per gallon. A heater larger than you
need will have less work-load, and will therefore save energy, as well as provide better
reliability. Equally so, two of the smaller heaters will divide the work-load of each in
half.
This modification is not recommended for those unfamiliar with risks and proper
techniques for working with electricity.
My sincere thanks to Howard Norfolk of Aquarticles.com, and Bill “Pegasus
NZ” of AquaProbs in New Zealand, for their help and suggestions.
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