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Old 08-15-2014, 10:16 AM   #1
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Heater: So how much heat do I really need? Square vs Cube rule

I'm confused by heater sizing. I've got a 45G tank and a 100W heater and all is good, but frankly have no idea if it's running 95% of the time or 2%. Overkill is easy and cheap in that size.

I'm getting a 220G tall tank (72x24x30). I started looking at advice, and most advice seems volume related (e.g. I've heard 5 W per gallon = 1100W). While proportional to volume at initial fill, heat loss is proportional to the surface area not volume for loss into the room, so any guidance that is proportional to gallon-age seems wrong to me.

So I found this nice calculator:

Aquarium Heater Calculator

That says with even a whopping 15 degree difference I need 80W to break even; hugely different from 1100W (and at the more appropriate 5 degree difference break even is 26W).

When I look at Hydro (I liked their canister heater look at least) they don't quote anything larger than 80G and suggest 300W.

I've read several threads recommending as many as three 1000W heaters in similar size tanks.

I looked at Cobalt's sizing guide, and plotted gallons against wattage, and it is nearly completely linear, which just doesn't make sense with the square vs cube relationship of heat loss. Unless they are assuming people dump cold water in at water changes and it needs to heat fast? (Do people really do that?)

So what am I missing here? Do I really need a big honking heater for a (non-sump) 220G tank? Or is something like the Hydor ETH 300 (300w) much more than enough?
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Old 08-15-2014, 10:27 AM   #2
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Tank Heating

Hello Lin...

Figure on 5 watts for every gallon of tank size and you'll have enough.

Pretty simple.

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Old 08-15-2014, 10:36 AM   #3
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Hello Lin...

Figure on 5 watts for every gallon of tank size and you'll have enough.

Pretty simple.

B
Can you comment on why the advice is proportional to gallons rather than surface area of the tank?

I agree I'll have enough -- I don't want to have wildly too much. Besides cost, it seems safer to me to have a safety margin, but not have a heater so big that with a failure it will cook the fish (or at least not quickly).
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Old 08-15-2014, 10:52 AM   #4
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The way to avoid cooking fish is to have two or more heaters of lower wattage. I would say that 1000w seems excessive for a 220. Check the suggested wattage for Jagger heaters. They seem lower than other brands, which makes me think most ratings, such as 5 watts per gallon are too high. Unless the tank is in an exceptionally cold room, you may actually need less than you think. Ideally, a heater should never turn off. It is the constant on/off cycling that shortens their life. Sadly, it seems from reading all the cooked fish posts, heaters seem to fail in the on position more than in the off position.
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Old 08-15-2014, 11:09 AM   #5
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I run two 300 watt heaters in my 225 but my house never gets below 60 degrees. They are run by a temperature controller that will sound a alarm if the temperature goes above or below the set temperature by a few degrees. if your house gets colder then three heaters may be needed. Having two or more is a safeguard in case one fails. IME don't skimp on your heaters, it could fail and cook your tank. Good luck!


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Old 08-15-2014, 11:18 AM   #6
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I run two 300 watt heaters in my 225 but my house never gets below 60 degrees. ....

IME don't skimp on your heaters, it could fail and cook your tank. Good luck!
We are in SW Florida. Our house never gets below 74 degrees or so except a few days a year if my wife opens the door in really cool weather maybe 68. It's actually a bigger risk getting hot (AC failure).

The latter is my concern. Largely the fish would be OK if the heater failed off, but failing ON is an issue.

-----

In looking at that heat calculator I referenced above, I do not think it considers evaporation, which makes it pretty useless IMO.

I dug up this article

Feature Article: Heat Transfer in Aquariums Part 1 - Basic Theory ‚€” Advanced Aquarist | Aquarist Magazine and Blog

Near the bottom it gives a formula for heat loss due to evaporation that's parameterized by temps (well, indirectly through the tables to derive humidity ratio). If I plug in my house and tank, and assume 1 ft/sec air movement across the top (quite high) I get 276 watts from evaporative heat loss. That presumes the entire tank is uncovered. If I (as I do now) uncover about a quarter over surface agitation, it comes down proportionally to about 70 Watts.

I think this evaporative loss would be additive to the values in the calculator above.

It also means an open top tank is primarily losing heat from evaporation, which makes intuitive sense.

Still reading... thanks for input to day.
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Old 08-15-2014, 12:32 PM   #7
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You didn't mention if this is for a freshwater or saltwater tank. For a FW, you really want to try to keep the water temp as constant as possible. For marine inverts and most fish, a little fluctuation is not going to do as much damage. Having said that however, being where you are, a chiller may be more important than heater depending on what you are keeping. What's the average high temp you keep the house?
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Old 08-15-2014, 01:04 PM   #8
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Tank Heating

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Originally Posted by Linwood View Post
Can you comment on why the advice is proportional to gallons rather than surface area of the tank?

I agree I'll have enough -- I don't want to have wildly too much. Besides cost, it seems safer to me to have a safety margin, but not have a heater so big that with a failure it will cook the fish (or at least not quickly).
Hello again Lin...

This wattage will maintain a proper water temperature and not overwork the heater. This is the easiest way I know to explain it. A bit larger heater and multiple heaters will ensure the heat is distributed evenly and you'll have a backup if one fails. A bit larger heater won't come on as often, so isn't worked too much. It will last longer.

The heaters have dials on them, so a specific temperature can be set and the heater will maintain that setting until you change it. There's little or no chance of the heater cooking the fish.

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Old 08-15-2014, 02:09 PM   #9
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You didn't mention if this is for a freshwater or saltwater tank. For a FW, you really want to try to keep the water temp as constant as possible. ....Having said that however, being where you are, a chiller may be more important than heater depending on what you are keeping. What's the average high temp you keep the house?
Freshwater, low tech, planted. Haven't decided on stock.

Except for rare exceptions, our house stays at 74 degrees year round, 24x7. The exceptions are things like parties where one side of the great room is opened, but that usually only happens if the temp is within about 5 degrees of 74 anyway. Plus it doesn't stay open long.

I hate heat (don't ask why we are in SW Florida), and it's just so sunny here even when it's milder in winter that we use AC all the time.

The biggest heat risk is if the power is out for extended periods of time, but if ambient even went to 90 or so, it would take a while for the tank to heat. And I just did an Ich treatment at 88 degrees and everyone was healthy and happy (in fact I swear they were happier -- more active certainly). So I'm less worried about heat.
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Old 08-15-2014, 02:22 PM   #10
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Hello again Lin...

This wattage will maintain a proper water temperature and not overwork the heater. This is the easiest way I know to explain it. A bit larger heater and multiple heaters will ensure the heat is distributed evenly and you'll have a backup if one fails. A bit larger heater won't come on as often, so isn't worked too much. It will last longer.

The heaters have dials on them, so a specific temperature can be set and the heater will maintain that setting until you change it. There's little or no chance of the heater cooking the fish.

B
What a concept. You mean you can tell it the temperature, and it tries to maintain it? I assumed I would have to run around plugging it in when the fish were cold, and unplugging when they got hot.

To your point, however, I realize there may be little chance, but I've read too many posted horror stories. Could be fabricated (after all it's the internet), but it also makes sense.

In most if not all heaters there's going to be a relay, in fact that might be the only moving part in most (some might incorporate a bent metal thermostat). That relay arcs when it turns on and off. Going off the momentum is away from the contact, but going toward (turning on) it's a bit like a tiny arc welder. This is why the most common failure mode for relays is "stuck on"; the arc sort of welds the contact and the spring can't pull it away later.

And stuck on, absent some kind of fail safe, just runs all the time. Maybe they have fail safes, maybe they don't, not sure.

Anyway... I'm new to this hobby, so perhaps I shouldn't sound like I am arguing with good advice; sorry. But I am not new to engineering. To me the reason for trying not to get too large a heater are:

- One sized correctly runs at a higher duty cycle, which means more even heating reducing hot spots (think cooking on a stove on low heat for 20 minutes, vs cooking on a stove on high heat for 5 seconds out of every minute). (I realize you suggest the opposite is an advantage, a lower duty cycle; I just do not see why that's good).

- A stock-on heater will take longer to do damage and might be noticed before it does.

Obviously getting one too small with either allow tank fluctuations, or will take too long to make tank changes (e.g. if you did have to add water significantly colder).

Trust me, I tend to be a "more power is better" type, a la Tim Taylor in the old Tool Time show. But sometimes it isn't. Consider Heat pumps for houses -- it's quite bad actually to get one that is way over capacity, uses more energy.

This just feels like one of those cases were "way too much" may not be good. Maybe i'm wrong, will keep reading and listening, thanks for the input.
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