BTU (British Thermal Unit) is a unit of measure often used to describe chiller capacity. A single BTU is the amount of energy required to raise one pound of water one degree Fahrenheit. For our purposes we use BTU/hr as a measurement of work being done per hour. Our chillers are rated in BTU/hr, this is the capacity of cooling the chiller can preform in an hour.
As an example, if we wanted to cool 100 gallons by 10 Fahrenheit, we first need to know that a 100 gallons weighs about 832 lbs. 832 lbs of water x 10 Fahrenheit = 8,320 BTU. This is a straight calculation that does not take into account any heat sources trying to warm the water.
Another consideration is that with any refrigeration equipment with a BTU rating, there are certain assumptions about environmental conditions to arrive at that rating. On a typical aquarium chiller the rating will be valid at a specific water and air temperature, normally both are around 80 Fahrenheit. For example a 5,000 BTU chiller has the capability of 5,000 BTU of cooling at 80F water and air temperatures. If the water is colder or the air is hotter the chiller won’t be able to cool as effectively therefore while the chiller is rated at 5000 BTU it may only be able to provide a fraction of that. Eventually, if the water gets cold enough or the air gets hot enough, the chiller will arrive at 0 BTU/hr and won’t be able to cool any further. Like wise, in the opposite situation, if the water is warmer than 80F and the air is cooler, the chiller can actually provide more than 5,000BTU of cooling. The key point is that the chiller’s ability to cool is on a sliding scale that is dependent upon both water and air temperatures.
In order to cool a body of water the chiller needs to offset more heat than the amount of heat being put into the water. Heat can come from many sources – ambient air, pumps, lights, sun, etc. The tough part is knowing how much heat in BTU/hr is going into the water. For electrical items we can use wattage to estimate how much heat they produce. A watt is equal to 3.41 BTU. Therefore a 100 watt submersible pump has 341 BTU/hr is going into the water. For a 250 w metal halide light we can calculate it produces 852 BTU/hr, however it is a lot more difficult to figure out how much of that heat is being transferred to the water. Factors like bulb distance to water, water surface area, and even reflectivity of material in the tank would factor into how much of the 852 BTU/hr from the bulb are being transferred. As you can see, the math/science to actually assign values to the various heat inputs gets very complicated very fast.
Because the Consumer Market insists on sizing Chillers in terms of horsepower (HP), this is why we describe and name our Chillers with a horsepower (HP) rating. We need Consumers to keep in mind that a chillers BTU’s/hr is really the best way to measure the Chillers capacity. If you compare multiple brands of 1/2 HP chillers, you will likely discover a variety of different BTU/hr ratings. What this actually means is that each Chiller doesn’t really have the same capacity as we’re lead to believe by all of them having the same 1/2 HP name. For example the 1/2 HP Active Aqua chiller lists a BTU/hr rating of 4100 while our Penguin Chillers brand 1/2 HP Chiller has a BTU/hr rating of 5000. Many brands don’t advertise their BTU/hr ratings therefore its important to do your research when making your chiller selection.
Now that you have a general understanding of the basic BTU calculations and why they are so important you should now be able to arrive at a good “guesstimate” when selecting the right Chiller for your project. For additional information on selecting the right Chiller, please use the link to our article on Chiller Sizing.