Quote: Do heaters normally stay on continuously until the temperature is reached?
This is one of several variables that we might see in heaters. Understanding how they work and sense the temperature may help. This type is one of the most common but has what I find to be a questionable design. They don't sense the water temperature directly as there is no contact between the electrical components and the water. The temperature inside the glass tube is effected by the water temperature and this inside temperature is sensed/measured by the controls. This is where the quality f the controls and build can make a big difference. The heating element will quickly get red hot when heating (400 degrees?) The controls are very close to this extreme heat, so if there is not a really good pad of insulation between the controls and the heat, the controls will shut off. Rather than a steady flow of heat to warm the water, you may get a series of "bursts" of heat which should eventually reach your set temperature.
But that still works in most cases so there may be other things going on in your tank.
So is the temperature setting. The marks on heaters are more what I would call indicators rather than actual set temperatures. Most mean little when looking for the real temperature. I find those that just have plus and minus to be as useful as those with actual numbers. The numbers are rarely correct on the smaller, cheaper heaters. Maybe simply ignore the numbers and bump the setting higher?
Then there is sometimes a question about circulation, even in small tanks. Is it possible that the heater is sensing there is warm water around it and shutting down when the spot where the thermometer is located is still cool? More circulation to get more uniform temps?
Small tanks are tough to keep stable.
PlantedRich, thanks for the explanation of how these heaters work. I wondered how they sensed the temperature without an exterior probe.
I realized that I didn't follow the setup instructions for this new heater as closely as I should have. I unplugged it, set it to the temperature I want, plugged it back in, and will leave it for 24 hours. The heater that this one is replacing seemed to get off to a better start, but it's probably too soon to compare them.
In general heaters should be sized to the tank they control the temperature in, which it also depends upon the daily temperature hysteresis or fluctuations cycles your house goes through, night to day.
Any new heater I purchase will be put through a test with either a 5 or 40 gallon bucket/barrel to see if it can manage the temperatures without fluctuations. I hate testing with new equipment on my fish and have them get cooked or sick because of mis-functions.
I also agree with PlantedRich, most heaters are still being built to a technology standard that hasn't changed in decades. This in itself isn't a bad thing, as it's been proven the permanent magnet, assisted bimetallic controllers will slow the on-off cycles down. but smaller heaters have the problem of cycling more often because the controller is closer to the heating element.
For those who have not looked inside cheap heater, some pictures and explanation might help. These things are frequent questions on the forum as they tend to be one of the bigger headaches around.
This is a cheap heater pulled apart:
Red knob turns a screw in or out to move the bent metal blade closer with a contact to/away from another contact. When the contacts come together, the power is sent down through the few components and to the heating coil at left. The coil glows hot and there should be some way to make this heat move into the water but at the same time that high heat goes up the glass tube back to the controls.
This is what serves as a control.
When this bi-metal plate heats, it bends away from the other contact and shuts off power until it cools. When trying to maintain 78 degrees, it can't really do a fine calibrated job of sensing the temperature with a red hot coil setting two inches away! Kind of like trying to dry your hair with a propane torch? You can get the job done but there can be problems.
You might spot one of those problems that we really don't like. That little contact started out as a nice uniform shiny little spot of metal but every time the contacts close and open there is a small arc. This arc winds up burning the spot you see just the same as an arc welder. The reason this heater is now junked is because these contacts welded together and left the heater on full time. I could clean and rebuild the contacts but then the bi-metal plate is quite likely to have overheated and will be distorted to make it not a good thing to risk using.
That little flash you see when watching the heater in a dark room? That is the contacts trying to weld themselves together!
I now use temperature controllers as a backup safety on all heaters.
That's a good example of the department store variety of cheap, 1970's vintage heaters. I wouldn't even give one of those away nowadays. If you see one at a thrift store and you are tempted to purchase, for a Q-tank or other tank, please just don't.
The real reason they burnt up or welded their contacts was because they were not positively closed by an assist magnet. These things would sit in a transitional state of not being fully closed with the contacts arcing and buzzing with 110 volts AC dancing across their surfaces. With the way these things worked, and the variable quality of the stainless steel framed/asphaltum caulked aquarium tanks available back then, it was a wonder we kept any fish alive.
PlantedRich, thanks for the explanation of how heaters work. I too am surprised there weren't more failures back in those days.
It has been 24 hours, and the heater, a Hydor Theo 25W, isn't working any better. 76 degrees seems to be its maximum in my aquarium. When I checked it this morning, it was at 74. I think my last 25W heater worked better because it had a bigger heater element.
Now I need to work getting another heater into my schedule. I'm thinking I should get a full-sized heater, so I probably won't get a Theo 50W. My other options are to go back to the original heater, which is sold at an LPS five minutes away, and let the slime on the rubber parts run its course (I read that it isn't harmful and eventually goes away); or wait a few days until i can go back to the store where i bought this Hydor and exchange it for a Jager 50W or Aqueon Pro 50W (they don't carry the 25W Jager).
Finding a good heater that is small enough to not totally cook a smaller tank is still a mystery for me. When I think of the fish I've killed with heaters that stick, I really don't like even the smallest sizes too much. My last fish kill was one of the preset types, either 25 or 50 watt. I had 37 bristlenose that I had been growing out for a friend and we were taking them to auction the next morning so I caught them and put them in a ten gallon. That makes them quick and easy to catch at 7 AM? When I went to bed, all was well but about 6AM, I got the smell of dead fish! This was my third go with the heater killing the whole tank and I finally do now run the small digital controllers on all heaters.
It really takes the fun out when you have to dump the fish, and then explain to the friend as well as the person he had already swapped with!