Heater Size Head Scratcher

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redfishbluefish

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I recently replied to a post talking about heaters, and I suggested using two under-sized heaters instead of just one proper sized heater. With Eheim's being the longest heaters, I inluded this table so that they could see if the heater would fit. As I stared at this table I noticed that watts to tank size didn't add up. Let me explain with an example.

...Tank Size....Power Needed....Heater Length
7–16 gallons.........50 watt.................9"
16–26 gallons.......75 watt...............10.2"
26–40 gallons.....100 watt..............12.2"
40–53 gallons.....125 watt...............12.2"
53–79 gallons......150 watt..............13.4"
79–106 gallons....200 watt.............15.7"
106–159 gallons..250 watt..............17.3"
159-264 gallons...300 watt.............19.6"


Let's say I have 106 gallons of water. According to the table, I'd need one 200W heater. Now if I wanted to use two heaters, you'd divided 200W by 2 and you come up with 100W....so two 100W heaters. But now when I look at the recommendations, that only totals to (40G + 40G) 80 gallons? You still have a total of 200W, so why doesn't it add up. According to that table, to now get to 106 gallons I'd need two 125W heaters (53 + 53 = 106gallons), for a total of 250W. Why!

These are the little things that bother my overly anal mind! :rolleyes: :D
 
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I recently replied to a post talking about heaters, and I suggested using two under-sized heaters instead of just one proper sized heater. With Eheim's being the longest heaters, I inluded this table so that they could see if the heater would fit. As I stared at this table I noticed that watts to tank size didn't add up. Let me explain with an example.

...Tank Size....Power Needed....Heater Length
7–16 gallons.........50 watt.................9"
16–26 gallons.......75 watt...............10.2"
26–40 gallons.....100 watt..............12.2"
40–53 gallons.....125 watt...............12.2"
53–79 gallons......150 watt..............13.4"
79–106 gallons....200 watt.............15.7"
106–159 gallons..250 watt..............17.3"
159-264 gallons...300 watt.............19.6"


Let's say I have 106 gallons of water. According to the table, I'd need one 200W heater. Now if I wanted to use two heaters, you'd divided 200W by 2 and you come up with 100W....so two 100W heaters. But now when I look at the recommendations, that only totals to (40G + 40G) 80 gallons? You still have a total of 200W, so why doesn't it add up. According to that table, to now get to 106 gallons I'd need two 125W heaters (53 + 53 = 106gallons), for a total of 250W. Why!

These are the little things that bother my overly anal mind! :rolleyes: :D
Your right that’s silly
I went with BRS recommendation of no less the 2x the watt per gallon and no more then 5watts per gallon. Came out to TWO 125w Eheim s for me ;)
 

mcarroll

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These are the little things that bother my overly anal mind! :rolleyes: :D

Screen Shot 2018-02-07 at 11.56.06 AM.png


I think bigger heaters are better at heating. More surface area?
 

pluikens

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How about the fact that the charts don't include ambient temperature, desired temperature, or surface area of the tank? Not factoring in those parameters or even what solids are in the tank like rock and sand along with sea water having different properties than fresh water, I stick to guidelines like @NowGlazeIT shared.
 

NowGlazeIT

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If I were to take a guess as to why eheim made this clear error....maybe they didn’t want to market smaller heaters pushing the limits of their performance. Probably reduced the chance of total failing heaters by their numbers
 
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[Cameron]

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Larger heaters with more surface area bring the water column to heat much faster and are more efficient. Using undersized heaters IMO is poor strategy over time since that increases the failure rate of the heaters. Better to use two heaters with two separate backup controllers. In the long run, it is cheaper and safer.
 

NowGlazeIT

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I have 2 small heaters one runs most of the day and the other idles standby hooked up to a controller that will alarm me if the first has failed or started to perform poorly
The controller is set a degree below the first heater so if that heater fails to come one or produce enough heat the second heater will come on. On the flip side if the first heater fails on, then it will take a long time to heat the tank up to dangerous temperatures. A alarm will go off if temp rises one degree from normal. FWIW I use an ink bird tempature controller
 

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I noticed the same thing. Im trying to heat a 120g tank with a ~15g sump. I was going to go with a 300w neotherm but somehow thatstonly rated for up to 75g. So I decided to go with 2 200w instead. I'll find out tomorrow if it works.
 

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I mean if you look at it three 100w heaters only adds up to 120g but one 300w will do up to 264...

Obviously it's just guidelines. I always have done 2-3w/g and never had a problem, it really depends on how warm the room is and how much above room temp you need to heat the water. The Eheim heaters also add a lot of heat to the water per watt used compared to other brands.

I noticed the same thing. Im trying to heat a 120g tank with a ~15g sump. I was going to go with a 300w neotherm but somehow thatstonly rated for up to 75g. So I decided to go with 2 200w instead. I'll find out tomorrow if it works.

That's a lot of wattage for that tank size, I would just use one primary and keep the other a degree lower.
 
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Let's say I have 106 gallons of water. According to the table, I'd need one 200W heater. Now if I wanted to use two heaters, you'd divided 200W by 2 and you come up with 100W....so two 100W heaters. But now when I look at the recommendations, that only totals to (40G + 40G) 80 gallons? You still have a total of 200W, so why doesn't it add up. According to that table, to now get to 106 gallons I'd need two 125W heaters (53 + 53 = 106gallons), for a total of 250W.

I cannot say for sure that it isn't simply an error, but it may also be because heat loss is a function of the ratio of surface area to volume. While larger tanks, with similar relative dimensions, will lose more overall heat, they will lose much less heat per gallon than will smaller tanks. It is the same phenomenon that has people evolve to be shorter and squatter in cold parts of the world - reduces the surface area to volume ratio and they retain heat better.

When I upgraded my system from 265 to 450 gallons, I bought an extra finnex 500 figuring I'd need more wattage. OK, the new tank is acrylic and the old was glass, but I have actually found that I don't need the extra heating after all.
 
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ca1ore

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Larger heaters with more surface area bring the water column to heat much faster and are more efficient.

That makes no sense. 200 watts is 200 watts, regardless of how it may be distributed across different heaters. Water loses heat to the surrounding ambient at a rate that is independent of the size the heater; so while a larger heater will bring a body of water up to temperature faster than will a smaller one, it is not more efficient.

Using undersized heaters IMO is poor strategy over time since that increases the failure rate of the heaters.

Only if you do it wrong. Let's say you've determined that 300 watts of heating are required based on tank size, temperature differentials and reasonable on/off cycles. Running dual 100 waters would be a poor strategy, I agree. Running dual 150 watters would not. I require about 1,400 watts for my system. I use a single 500 finnex and three 300 watt EJ. I have them setup so that that they do not rapid cycle and failures have been zero. In fact, using this approach (and buying good quality heaters) have resulted in just a single failure in 20 years. I do replace them proactively, just not reactively.
 

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I recently replied to a post talking about heaters, and I suggested using two under-sized heaters instead of just one proper sized heater. With Eheim's being the longest heaters, I inluded this table so that they could see if the heater would fit. As I stared at this table I noticed that watts to tank size didn't add up. Let me explain with an example.

...Tank Size....Power Needed....Heater Length
7–16 gallons.........50 watt.................9"
16–26 gallons.......75 watt...............10.2"
26–40 gallons.....100 watt..............12.2"
40–53 gallons.....125 watt...............12.2"
53–79 gallons......150 watt..............13.4"
79–106 gallons....200 watt.............15.7"
106–159 gallons..250 watt..............17.3"
159-264 gallons...300 watt.............19.6"


Let's say I have 106 gallons of water. According to the table, I'd need one 200W heater. Now if I wanted to use two heaters, you'd divided 200W by 2 and you come up with 100W....so two 100W heaters. But now when I look at the recommendations, that only totals to (40G + 40G) 80 gallons? You still have a total of 200W, so why doesn't it add up. According to that table, to now get to 106 gallons I'd need two 125W heaters (53 + 53 = 106gallons), for a total of 250W. Why!

These are the little things that bother my overly anal mind! :rolleyes: :D

Maybe something to do with surface area.
 
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Has anyone else checked out the calculator at https://www.oemheaters.com/topic/immersion-wattage ? There's so many factors in determining the volume that a certain wattage heater can support. I have 500 watt Finnex titanium heater in my sump controlled by an Inkbird that keeps the DT between 78 and 79 degrees. The room gets as low as 62 in the winter when everyone is away. I know 500 watts is overkill in my 40 gallon DT with 20 gallon sump but if the Inkbird turns off the heater when the DT gets to 79 degrees, my biggest concern is the sump water getting too hot before the DT temp reaches 79 degrees during heating but I don't see that happening when I have 10x turnover.
 

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That makes no sense. 200 watts is 200 watts, regardless of how it may be distributed across different heaters. Water loses heat to the surrounding ambient at a rate that is independent of the size the heater; so while a larger heater will bring a body of water up to temperature faster than will a smaller one, it is not more efficient.
What typically happens with two undersized heaters is one of the two heaters ends up doing most of the work because its thermostat is slightly off from the other. This causes it to burn out quicker as well. The "fix" to this is to put both heaters on one controller which for some strange reason people do all the time but that means if the controller goes off or malfunctions, the tank could experience a wild temp swing.

The two undersized heaters philosophy more or less assumes that two small heaters reach max temp as fast as one large one, that both heaters are "dead on" accurate with each other, that two units uniformly heat as well as one and that all wattage consumed by a heater is transferred to the water column with the same efficiency.

In my experience, having two properly sized heaters that have built in temp controllers paired with external temp controllers is a better option. For example on a 100g tank with a target temp of 78, use two temp controllers (such as Inkbird and an Apex or RK) dialed in using the "ice water" method. Pair each with a 300w heater (depends on climate) dialed in by using a 5 gallon bucket or similar. Set the primary temp controller to 78 and its heater to 80. Set the backup temp controller to 76 and its heater to 80. For an expensive tank the backup controller and heater should be on a separate breaker preferably one with a generator hookup. I suggest when using a controller, to set alarms at 76 and 80 degrees (depending on climate and setup). This is the safest setup I know of. The only downside is $50 or so bucks more expensive. Given that a great number of tank failures track back to heaters, money well spent IMO.

Oh and on that setup, put fans on the primary controller and chiller on the backup also depending on climate and setup.
 
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Daltrey

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What typically happens with two undersized heaters is one of the two heaters ends up doing most of the work because its thermostat is slightly off from the other. This causes it to burn out quicker as well. The "fix" to this is to put both heaters on one controller which for some strange reason people do all the time but that means if the controller goes off or malfunctions, the tank could experience a wild temp swing.

The two undersized heaters philosophy more or less assumes that two small heaters reach max temp as fast as one large one, that both heaters are "dead on" accurate with each other, that two units uniformly heat as well as one and that all wattage consumed by a heater is transferred to the water column with the same efficiency.

In my experience, having two properly sized heaters that have built in temp controllers paired with external temp controllers is a better option. For example on a 100g tank with a target temp of 78, use two temp controllers (such as Inkbird and an Apex or RK) dialed in using the "ice water" method. Pair each with a 300w heater (depends on climate) dialed in by using a 5 gallon bucket or similar. Set the primary temp controller to 78 and its heater to 80. Set the backup temp controller to 76 and its heater to 80. For an expensive tank the backup controller and heater should be on a separate breaker preferably one with a generator hookup. I suggest when using a controller, to set alarms at 76 and 80 degrees (depending on climate and setup). This is the safest setup I know of. The only downside is $50 or so bucks more expensive. Given that a great number of tank failures track back to heaters, money well spent IMO.

Oh and on that setup, put fans on the primary controller and chiller on the backup also depending on climate and setup.

What happens if the return pump fails? The heaters are in the sump.
 
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