Basics : Heaters
The 3 most common heaters are the rim mounted analog, the submersible analog and the submersible digital. Either purchase the most expensive (submersible digital) or familiarize yourself with their failure modes. Calculate your wattage requirements at 5 watts per gallon. Larger tanks (40g+) benefit from using 2 small heaters rather than one large heater. Locate the heater where there is sufficient water flow for it to heat the tank uniformly. For submersibles, I discuss orientation and the sometimes confusing waterline mark, further down this page. Protect your heater from damage (ie: falling rocks or large fish). Heater covers are available for this, and there are titanium heaters as well. For safety, use a GFI protected circuit for aquarium accessories. The larger the tank, the less susceptible it will be to temperature variations. Also discussed are some ways of hiding a heater, performance, redundancy, and just how they should be adjusted the first time.
Details...
There are other heating methods (ie: substrate heaters, heat tape, ambient heating etc) but the majority of a hobbyist's exposure will be to the basic 3 models. All three share the same basic design of an AC heating coil imbedded in ceramic. The differences between them are in the type of thermostat they use, whether they are fully submersible or not, and other more subtle features or characteristics (ie: glass thickness). Depending on the model, heaters tend to have one of 5 failure modes, cracked glass, tamper-prone, stuck ON, stuck OFF or an open heating element.
ANALOG
Also known as a hang on back (HOB) or hang on top (HOT) heater, this analog heater
is the lowest cost (entry-level) heater option. It uses an analog thermostat (a
bimetal contact similar to the spiral wire used in analog home thermostats) which
opens and closes to energize the heating coils at 120 or 240Vac (depending on
your house voltage). The dial typically has no markings. Due to it's placement,
it is the most difficult heater to hide. These heaters suffer from several failure
modes.
SUBMERSIBLE ANALOG
Submersible analogs (ie: Aquarium Pharmaceutical's Rena heater, Hagen's Thermal heater) uses the same basic design of analog thermostat as the HOB, but sealed into a waterproof assembly which can be located anywhere in the tank (usually on suction cups), where there is sufficient water movement for the heater to operate properly. Temperature may be marked on the dial with a coarse resolution. Failure modes are listed below.
DIGITAL SUBMERSIBLE
Digital submersible heaters (ie: Hagen's Tronics heater) are basically the same as the analog submersibles in terms of location in tank and operation. Temperature may be marked on dial with a finer resolution. Many models will tolerate being up out of the water, as they will detect an over temperature condition and shut themselves off. My experience with the Tronics heater is that they reset themselves and resume normal operation when completely submersed again. The Tronics do leave behind a slight burnt ozone smell to let you know that you did a silly thing, but they seem otherwise unaffected. Digital heaters are the most reliable heaters, having the fewest failure modes.
FAILURE MODES
Pick your heater:
Analog HOB 1, 2, 3, 5
Analog submersible 1, 3, 5
Digital submersible 4
1. CRACKED GLASS TUBES primarily affects analog HOB heaters
Because the heater's thermostat reacts to any cold, a water change will cause the heater to begin operating (the cooler air contacts the exposed thermostat area near the top of the glass tube). If the water level were to drop below the glass tube, then you will discover the most common failure mode with analog HOB heaters, which is having the glass tube crack when the water rises again and makes contact with the overly warm heating element. Electricity and water are a dangerous mix, for you and your fish. Immediately unplug all power to your aquarium and allow things to cool down before handling the heater, and removing the glass bits. Surprisingly, the fish usually escape this failure mode unscathed, perhaps because the electrical short travels along the water's surface, and finds no easy path to ground (until you touch the aquarium by mistake). This failure mode will also occur with submersible analogs if they are exposed to the air. It is just less likely to occur as these are lower in the tank.
2. TAMPERING almost only affects analog HOB heaters
Where children are involved, tampering is a serious consideration. If a child can reach around and turn a heater dial, they probably will, eventually. The actual failure mode comes the heater being pitched into the garbage by the hobbyist after this happens. Submersible heaters solve this by having the dial under the aquarium canopy.
3. OVERHEATING (STUCK ON) primarily affects all analog heaters
Every time an analog thermostat opens or closes its metal contacts, there is a tiny arc, whose temperature melts a tiny bit of metal. Over time, the contact points get pitted, with the ridges on one side matching the grooves on the other. One day, the heater will turn itself ON, and the arc will cause just enough of the pitted metal to melt together, effectively gluing the contacts closed. On the HOB analog heaters, the thermostat assembly could be easily removed, and these contacts filed down in a few minutes, but a submersible heaters cannot be home serviced. Realizing this limitations, manufacturers have been using special contact plating, and progressive contacts designed to delay the day that the contacts fail. As a hobbyist, your best recourse when using submersible heaters is to use new heaters, or 2 small heaters instead of a single large one. I discuss this later in heater redundancy further down this page.
4. NO HEAT (STUCK OFF) primarily affects digital heaters
Because of their solid state circuitry, digital heaters are more prone to fail OFF than ON. Use multiple heaters (redundancy) to address this problem.
5. NO HEAT (OPEN ELEMENT) primarily affects all analog heaters
When a heater turns ON, the elements get red hot and expand. They then contract
as they cool down. This continuous cycling on and off slowly fatigues the metal
wires until they become brittle and break apart (just like an incandescent light
bulb). Fortunately, they are designed for this, and it takes many years for them
to fail. I mention this as a caution for heaters purchased at auctions and garage
sales, as there is no way of knowing how much remaining life there is. I believe
digital heaters will not suffer from this as analog heaters do, because they start/stop
the power going into the elements more gradually, so there is significantly less
thermal and mechanical stress on the components.
ORIENTATION
Submersible heaters can be installed vertically, horizontally or anywhere in between, but some models work better at certain angles, so some experimentation might be in order. Like most heaters, the heating elements are in the bottom of a tube (opposite the control dial) and the thermostat is in the top or middle of the tube. Because heat rises, a vertical orientation (dial up) will cause the heater to cycle ON/OFF more (due to the heat rising off the elements), than if it were in a horizontal orientation. Some submersibles cycle too slowly in a horizontal position, as they were already designed with a lag time to compensate for rising heat (you can't win ;~). Typically, you should always install any heater where there is plenty of turbulence (ie: filter outflow) and you can safely install a submersible heater horizontally. If you see the temperature is not tracking as well as you would like, flip the heater into a diagonal position. For the purposes of hiding your heater, vertical orientation is usually the easiest to work with. Another option is to install your heater vertically but upside down (do this only after you have finished adjusting the dial). Your results should be used to guide you.
WATERLINE?
For analog HOB heaters, the waterline is straight-forward. The thermostat is located in the upper half of the heater, and for it to operate properly, this section of the tube must be underwater.
However this marking can be confusing for a fully submersible heater. As with
the the HOB, the waterline is a minimum, ensuring the thermostat is underwater.
However, some are also marked as the maximum water line. In the United States,
the responsible regulatory agency (UL) approved the use of these heaters for underwater.
As is often the case, Canada's regulatory agency (CSA) did not, demanding design
changes be implemented prior to approval for sale. Rumor has it that the design
was ULC approved (combined UL and CSA approval) and CSA balked at that agency's
decision. All the while, the manufacturer's were printing marketing materials
saying they were submersible, hoping agency approvals would arrive before they
went on sale, but glass tubes are not made as easily as cardboard wrappers. Many
assemblies were shipped with contradicting information, and the manufacturer's
responses varied by the status of their CSA docket. Once approved, the manufacturers
had to exhaust their supply of glass tubes which had a maximum water line printed
on them.
I imagine the next step was continued penetration into the markets of Europe,
Asia, Australia, Africa etc, and with every country having it's own regulatory
agency, the expense of having 2 tubes (one printed and one not), and the coordination
of sending them all to the right countries would have fatigued even the most diligent
project managers, so many more submersible heaters went out with printed maximum
water lines.
HIDING HEATERS
When in a vertical position, plants such as Vallesneria (ie: Corkscrew) are commonly used. Giant or Jungle Vals will easily reach the top of very tall aquariums. If using a submersible heater in a horizontal position, sometime the heater can be tucked behind the top layer of rockwork. When using multiple heaters, sticking them vertically into corners might work if there is enough water circulation. Uniform heating is more difficult to achieve with only one corner heater, but experiment and let your results guide you.
Eheim has canister models with integrated heater. Sump filters (wet/dry) commonly have the heater located just before the pump returning the water. Any design where the heater is not located in the tank will be at the mercy of clogged hoses, variable water flow as the filter media fills, and pump failures. When it comes to critical systems (heaters & filters), simplicity is usually the preferred path for new hobbyists to start with.
PERFORMANCE
The 3 parameters of interest to hobbyists are relative accuracy, absolute accuracy and drift (in that order of importance).
You want a heater to consistently hold the water at a set temperature. This is known as its relative accuracy, and is the most important function. Heaters will typically bounce around by 1 or 2 degrees, more if the circulation is poor.
How close the heater is to the temperature marked on the dial is known as its absolute accuracy and is far less important. It's of no consequence if your heater is off the tank temperature by several degrees. Heaters are calibrated to their dial in the air in a high volume production line, rather than taking several minutes to settle in an aquatic environment. This is why you will see an offset. Always rely on your tank's thermometer when adjusting a heater.
I consider drift as the least important as it is infrequent,
and when it does occur, it tends to happen slowly, allowing the hobbyist plenty
of time to compensate. On analog heaters, this drift is usually just a case of
ambient room air changes (for the HOBs) and metal fatigue (for the submersibles),
and is easily corrected. For digital heaters, drift is much more problematic,
indicating a progressive loss of accuracy. The adjustment range on digital models
is much smaller than on analog, so it is possible that the drift reaches a point
where it cannot be sufficiently corrected. The cause of the drift is degradation
in the circuit, either the components or more likely, the contacts between the
thermocouple and amplifiers. The heater is defective and should be replaced long
before it reaches the point where it can no longer be further adjusted. Contact
the manufacturer if you see this. Digital thermostats were in their infancy several
years ago, and are now a much more stable design, so drift is far less likely
to occur now.
ADJUSTMENT
The adjustment procedure for all types is basically the same. Install the heater is a zone where there is good water flow, and leave it there for about 15 minutes, before plugging it in. A pilot light indicates when they are heating. To adjust, start with the tank's water temperature slightly cooler than desired, and turn the heater dial until the pilot light goes on (or if the dial is numbered, set it to below the desired temperature). When the pilot light goes off, check the tank's thermometer and re-adjust the heater upwards (or downwards) as needed. It's much easier to adjust UP to the desired temperature, than DOWN.
REDUNDANCY
Above 40-50g tanks, consider using 2 smaller heaters for safety. When a heater fails to operate (digital heaters usually fail OFF and all heaters can have the heating element open), then the tank temperature will drop as a function of ambient air temperature and tank size (big tanks change temperature more slowly). Having a 2nd heater will cause the rate at which the tank cools, to slow down (giving you more time to notice the problem), and the final temperature will not be as cold as it would be if there was only the one failed heater in the tank.
When a heater fails stuck ON, (as is the case with pitted contacts in an analog heater), then the tank temperature will increase, as a function of heater power, ambient temperature, and the size of the tank. The temperature will eventually stabilize somewhere, high 90Fs or higher. High temperatures will agitate the fish, increase their metabolic rate and significantly reduce the amount of oxygen available. Death will eventually follow, if the temperature is high enough or remains high for an extended period of time. many tropical will tolerate mid-90F temperatures if only for a few days, but this is a risky experiment to try, unsupervised. Having 2 smaller heaters (ie: 2 x 150W instead of one 300W) will limit the rate at which the temperature increases, and lower the absolute temperature it stabilizes at. This makes it easier for the fish to acclimate to the change, and gives you more time to notice the problem
As our aquariums get larger, and our emotional and financial investment in them increases, improving the redundancy on critical systems is a good investment.
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