How To Choose an Aquarium Chiller and How It Works

Categories: Heaters & Chillers, Equipment

Last updated: Apr 01, 2022

How an Aquarium Chiller Works

Most aquarium chillers use a vapor-compression method to chill your aquarium water. While the science behind this process can be a little complicated, it is a good idea to understand the basic principles and components in order to properly operate and maintain the chiller.

The diagram above represents the basic parts found on common aquarium chillers and we've outlined the functions below in order to help you better understand exactly how your aquarium chiller works.

Aquarium water is pumped into the chiller and enters the heat exchange. Inside the heat exchange, aquarium water flows around a series of cool metal coils filled with refrigerant. This is where heat is transferred from your aquarium water into the refrigerant.
The heated refrigerant is compressed and changes from a liquid into a gas inside the compressor. The pressure created inside the compressor is what moves the refrigerant through the cooling system.
The gaseous refrigerant is sent into the condenser from the compressor via pressure. A fan blows air over the condenser cooling the refrigerant inside and expelling heat into the atmosphere around the chiller. This is why it is important to place your chiller in a well-ventilated area. This process is similar to what happens in the radiator in your car.
As the refrigerant passes through the condenser, it is cooled and transformed back into a liquid.
Refrigerant from the condenser passes through an expansion valve which reduces pressure inside the refrigerant line controlling the rate at which the aquarium water is cooled inside the evaporator or heat exchange.
A temperature controller monitors the temperature of your aquarium water and automatically switches the chiller ON when the temperature rises and turns it OFF when temperatures are within a suitable range. The temperature controller is built into most chillers but sometimes sold separately depending on the brand of the chiller.

Choosing The Right Chiller Using BTU

Choosing an aquarium chiller is not only about how much water your aquarium holds, you should also consider how much of a temperature drop you need to achieve and the air temperature around the chiller. A chiller that only needs to drop the water by 3-5°F does not require as much power as a chiller working to cool the water by 10-15° F. Additionally, warmer ambient air temperatures (above +/- 75° F) around the chiller itself will reduce the cooling power. If the ambient air is too hot, (above 78°F) the chiller may not be able to cool at all.

With that in mind, there is a simple calculation or general rule we can use to estimate how much energy will be required to reach the desired temperatures in our tanks using BTU - British Thermal Units. Most chiller manufacturers will provide a BTU/hr rating for their chillers which is how much heat energy (BTU) the chiller can remove per hour. Keep in mind, this formula doesn't account for additional sources of heat and assumes the chiller is placed somewhere with reasonable air temperatures around 70°F.

BTU - British Thermal Units: Unit of measure that indicates how much power is required to heat one pound of water by one degree F. One gallon of water weighs roughly 8.3 lbs.

With this information, we can estimate it will require roughly 8.3 BTU/hr to chill one gallon of aquarium water by 1° F. So if you have a 10-gallon aquarium, that will require a chiller capable of 83 BTU/hr to drop the water by 1° F or 416 BTU/hr for a drop of 5° F.

Always reference the manufacturer's recommended maximum tank size ranges but more importantly find out the BTU/hr rating for the chiller.
Estimate how much of a water temperature drop you need to achieve.*
Multiply your tank size in gallons by 8.3. Then multiply that by the number of degrees you need to drop.
- (Water volume x 8.3) x #° temperature drop = Appropriate BTU/hr rating

For example, in a 55-gallon aquarium that needs to drop by 4°F to reach the suitable temperature range of 78°F:

456.5 BTU/hr x 4-degree drop = A chiller rated for at least 1,826 BTU/hr

To make the math easy and be 100% certain you're getting a strong enough chiller, it is best to round up to 10 BTU/hr per gallon of aquarium water for a 1°F drop and use the same formula to estimate the size of the chiller. It's always better to slightly oversize your chiller as opposed to undersize because an undersized chiller will operate far less efficiently.

Using the same example, in a 55-gallon aquarium that needs to drop by 4°F to reach the suitable temperature range of 78°F:

550 BTU/hr x 4° drop = a chiller rated for at least 2200 BTU/hr

*Manufacturers usually recommend the MAXIMUM tank size but the reality is, it all depends on exactly how many degrees you need to drop the water temperature and the amount of heat your equipment is adding. In a real-world scenario, most tanks that actually need a chiller won't operate any hotter than 83-84° F with the lighting ON and the pumps running. That means a maximum drop of 6° F to reach the suitable tropical water temperatures of 78°.

Do I Really Need a Chiller?

With modern-day LED lighting and DC-powered water pumps, most aquariums placed in a temperature-controlled household probably won't require a chiller. Keeping ambient air temperatures around the tank somewhere around 70°-72° F year-round tends to be sufficient.

AC-powered pumps, metal halide, and high-output fluorescent lighting transfer heat into the aquarium water and used to be the biggest reason home aquarists found the need to use chillers; all of which are dated technologies in modern aquaria. So thanks to modern aquarium technology using less electricity and operating at cooler temperatures, most tropical reef aquariums won't require the use of a chiller. If they do, the necessary drop in temperature is minimal.

If you plan to keep cold water animals at water temperatures in the low 70s or 60s, a much stronger chiller would be appropriate.

Quick Tips

ALWAYS use a water pump that can provide the proper flow rate through your chiller as recommended by the chiller manufacturer.
Verify your chiller is getting the appropriate water flow after you have it plumbed. Simply time how long it takes to fill up a 5-gallon bucket to estimate Gallons Per Hour.
Always place your chiller in a well-ventilated area.
Never let your chiller run dry; this can quickly damage internal components.
Chillers use a substantial amount of power; be sure to allow for the necessary electricity consumption in addition to all of the other equipment on your aquarium.
Clean the fan and condenser fins regularly to remove dust and increase airflow.
Calcium build-up inside the heat exchange can reduce chilling efficiency; regular back-flushing with a freshwater/vinegar solution can help remove build-up and keep your chiller performing optimally.
A properly sized chiller is important for longevity and proper function. Your chiller should only be ON for roughly 15-20 minutes per hour. If you find it running much more often or for longer periods, it's undersized for the amount of heat in your aquarium or just not chilling properly.