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Old 04-06-2007, 12:00 PM   #1
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Lighting and heat generation

There've been comments in several threads over the last few months about lighting and heat generation. The comments/suggestions pretty much go like this...

Aquarium low temps are in the morning obviously, but when lighting comes on temps shoot up 2-3 degrees. Many folks suggest to raise heater settings so that they're set at the highest temperature reached after the lights go on. In other words, if your tank reaches 78 at the end of the day, set your heaters to 78.

This advice confuses me, so I'm hoping folks can explain it better into my thick skull. Here's my take of it...

Don't lights pump a set amount of energy into the water? Isn't that energy quantified by how many degree increase it can cause in the water? For practical purposes (and I know there's some assumptions here) won't 200W of lighting over a 40 gallon tank raise the tank temperature a certain number of degrees - regardless of the original temperature the water is at? If the original tank temp is 76, it's going to end up at 78. If the original tank temp is 82, it's going to end up at 84. Correct?

Since spring/summer time temps are coming, I figured this would be a good time to get this straight in my head!
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Old 04-06-2007, 12:12 PM   #2
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Regardless of the rooms temperatures the PC/VHO/MH lights are going to contribute excess heat and setting the heater to the same temperature that the tank reaches during mid-day with the lights on keeps the temperature from falling too drastically at night and subsequently heating to drastically when the lights come on.

The heater remains off most of the day because the lights warm the water and the heater has it's own internal thermostat.
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Old 04-06-2007, 12:13 PM   #3
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I think you are right on this. My tank was 76 in morning 79 in evening. I put a heater in at 78 it just got even hotter in the evening over 80. I took the heater out and put a fan in the top. works much better.
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Old 04-06-2007, 12:15 PM   #4
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Quote:
Originally Posted by produceb
I put a heater in at 78 it just got even hotter in the evening over 80.
That's due to a heater problem, the internal thermostat didn't maintain the 78 degrees during the night. Either that or it was hotter at night then during the day which happens sometimes also
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Old 04-06-2007, 12:20 PM   #5
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I think my heater would heat it to 78 but then the lights and hood would heat it past during the day, when the lights were on then it would fall to the 78 that the heater maintained during the night. When I said evening I meant at the end of the day(sorry) after the lights were on all day but before they were off.
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Old 04-06-2007, 12:24 PM   #6
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Quote:
Originally Posted by produceb
but then the lights and hood would heat it past during the day
Thus the point of setting it to the highest level of 80 degrees. Although a 2 degree drop is not an issue it would be if you had AC and the drop was to 72 for example.

Any range between 76-82 is fine as long as you don't have drastic swings in temps more then 2-3 degrees per day. If your temp is over 82 then cooling fans, air conditioning, ect... need to be implemented.
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Old 04-06-2007, 12:52 PM   #7
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My point: is there a highest point? A heater heats does not cool. Lights get much hotter than 80 degrees. If a heater heats to 85 and holds it, then it becomes 90 in a room, the heater does nothing to keep it from going to 90. A heater can get all your water to 80 but the lights still may cause the 2 degree change, that is why you need some type of cooling device in my opinion a heater does nothing, unless your heat source only gets to the temp you set your heater.
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Old 04-06-2007, 01:03 PM   #8
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Highest point IMO is 82 although tanks can take short durations of heat up to 88 without too much of a problem. Things go down hill quick if temps above 82 persist IME. A lot of it also depends on the stock. Fish can tolerate higher heat more then inverts/coral for example. Even fish will have issues with heat above 90 for prolonged periods.

Quote:
Originally Posted by produceb
A heater heats does not cool.
But it does cut off when it's thermostat has reached the setting and as you said some sort of cooling i.e. fans, ac, ect... needs to be implemented if heat reaches critical levels.

Also as a side note IMO you should always use two heaters to help both alleviate slight fluctuations in thermostat differences and also in case one of them fails. i.e. the general 2-3W per gal for both heaters.

Typically heat is more of an issue in countries where it's always hot and air conditioning is not standard. With air conditioning and fans heat is really only an issue when dealing with very high MH wattage (keeping high light corals for instance) and then alternate means of cooling like chillers may be necessary.
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Old 04-06-2007, 01:05 PM   #9
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Some heaters work better than others and some heaters have a certain degree of accuracy/range.
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Old 04-06-2007, 02:42 PM   #10
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Quote:
Originally Posted by tecwzrd
Quote:
Originally Posted by produceb
I put a heater in at 78 it just got even hotter in the evening over 80.
That's due to a heater problem, the internal thermostat didn't maintain the 78 degrees during the night. Either that or it was hotter at night then during the day which happens sometimes also
See... that's what's eating at me. It's not necessarily a heater problem... it's just that the lights are pumping heat into warmer water to start with, and you'll therefore end up with even hotter water. My bet is that if you set your heaters to 80 degrees, at the end of the next day you'll end up with 82 degree water. By the "highest temperature" logic, you'd set your heater now to 82 degrees. At the end of the next day, you'd be at 84 degrees... because the heat from the lights has the capacity to increase water temperature by 2 degrees - regardless of what temperature it started at.

Now... getting really picky here - there is a theoretical problem with my thinking because the heat capacity of water is not constant - it changes depending on temperatures. So basically... given a set heat energy input, really really hot water will not increase in temp as much as really really cold water. But for the temperature ranges we're talking about for our tanks, this really doesn't come in to play.

Assuming room temps stay constant, and heater thermostats are perfect, it just seems like for a given light over a given tank, you have a given amount of heat energy to dissapate. If you don't use fans, that is going to dissapate into your tank, thus raising the temperature. And it just seems like that energy doesn't give a darn about the temperature of the water.

Can you tell I'm still not convinced!
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