Pro and Con of multiple heaters in one tank

[StirCrazy]

Originally posted by Son Of Skyline:
Multiple heaters:
Great safety net if one fails and shuts down
Useless if one fails and stays on[/QB]

<font size="2" face="Verdana, Helvetica, sans-serif">this is just the point of having small heaters than cannot keep up by themselves. lets say you need 250 watts to just hold your tank at 82 degrees. now you go and get three 100 watt heaters.. if one of them sticks on it will not be enuf to over heat your tank.. the other two will still cycle on and off.. just not as much as when all three were working.. if two stick on.. big deal you have already determand that a 250 watt will barly get your tank to 82 so 200 will only get it to say....80. your third will still cycle on and off. the problem is that on the other hand if you are using a Tronic (solid state and fails off) the other two won't be able to keep up.. so if that is the case you have to use two heaters that are capable of heating your tank. I had a Tronic fail on me on my angel tank.. (out of the 6 tronics I have that is the only one that has failed.) luckaly I caught it wwhen it was down to 78 and that fish are more forgiving than coral hehe..

Steve

[Jack Rainville]

"Multiple heaters:
Great safety net if one fails and shuts down
Useless if one fails and stays on "

The point is, one of them is far too weak to ever over heat your tank. Get it? If one is stuck on, the others will simply not turn on as often. The only problem with the multiple heater setup is you actually have to check the heaters from time to time to make sure they're functioning. You might not even know if one is stuck on or stuck off because the temperature of the tank might not fluctuate much.

Seems to me, an expensive controller is just back to having all of your eggs in one basket again. If the controller described above fails while you're away, then you have no heat control, no lights, no fans, etc. Right?

[Delphinus]

Steve and Jack explained perfectly what I meant. The risk of "a failure of some kind" is greater but the risk of "what damage is going to be done" is mitigated (ie, limited). Thinking of process control there are concepts such as "fault-tolerance" and "fail-safe" and "single point of failure." "Fail-safe" means we recognize the possibility of a failure, but if a failure happens, it is left in a safe state. "Fault-tolerant" means that a failure might happen, but the process will be able to continue unaffected. Since a heater cannot be guaranteed "fail-safe" then you want to use a heater such that that if it suffers a worst-case scenario failure (and Murphy's Law dictates that the possibility of this is darn near 100%), then you want to be able to know that it won't cook your tank all by itself. I.e, NOT a "single point of failure." If you use TWO heaters then they both have to fail on to cook your tank, and the possibility of THAT is (theoretically) half that of a single heater failing on. Thus the multiple heater scenario is closer to a "fault tolerant" design.

This only makes sense if you're using heaters that by themselves are underpowered for the tank. If you are using fully redundantly sized heaters then indeed you are just doubling the risk of a meltdown. This is an important assumption to be pointed out.

Don't get me wrong: I'm not knocking the secondary controller idea. The reason THAT is good idea is that using a secondary controller (as well as the thermostat in the heater) is that in that scenario, you don't have "single point of failure." No one item failing is enough to shut you down.

There you go. Next lesson will be "programming in a real-time dual-redundant semi-fault-tolerant environment." Oh wait .... That's my job not my reefing hobby. Forget that! :D

[Jack]

Very well explained you guys, I am now thinking of changing to more smaller heaters than one big 250w. I am just to worried about loosing any livestock, seeing as I live on the island(no saltwater stores for miles and miles and miles...) so I want to keep what I get.

How many heaters?
2,3?
100w, 150w, ?

112gallon tank.
33 gallon tank as sump.

[Reefmaster]

hey all.
i recently had a 300W fail (off position) and am now in the market for a new one. i run 3 heaters and can't say enough about doing so. i would pay cash/trade for w.h.y. if anyone is looking to replace a big 'un with a smaller unit. actually, i also have a 50W i think that is 1.5yrs old i would trade if anyone wanted. (moderators, i know, not the trading post but fits with the discussion...)
shane