Reservoir Temperature and Water Chillers: How Big a Chiller You Need, With the BTU Math
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Reservoir Temperature and Water Chillers: How Big a Chiller You Need, With the BTU Math
Warm reservoirs are one of those problems that hides behind other symptoms. The room is in range, the nutrients are mixed right, and the roots still look tired. Then someone puts a thermometer in the tank and it reads 80.
ModernFarms carries Active Aqua and Elemental Solutions chillers in sizes from 1/10 HP to 1 HP. This guide covers what temperature to aim for, the physics of how much heat you have to remove, and which chiller sizes the listings actually support. Where a listing gives no capacity, we say so rather than guess.
The 30-Second Answer
The target: our Bluelab Conductivity Pen listing recommends feeding at 65 to 72 degrees F. The physics: water takes about 8.34 BTU per gallon per degree F, so 100 gallons down 10 degrees is about 8,340 BTU. The units: the Active Aqua 1/2 HP lists 4,020 BTU per hour for $899.00 and the 1 HP lists 10,050 BTU per hour for $1,296.00. The value: that is about $0.22 per rated BTU per hour for the 1/2 HP and $0.13 for the 1 HP. The caution: the pulldown times below are ideal numbers, and real chillers take longer.
Why Reservoir Temperature Matters
Our Conductivity Pen listing says that water too warm or too cold exposes roots to stagnant growth or root rot, and recommends feeding at 65 to 72 degrees F. Warm water also holds less dissolved oxygen, which is a general fact of physics, not something from a listing. That is why a tank that runs 78 or 80 degrees is worth fixing, and why you should measure it before you buy anything. A thermometer, or a meter that reads temperature like the Bluelab Conductivity Pen at $99.00, is cheaper than a chiller and tells you whether you need one.
Try the free fixes first: move the reservoir out of direct warm air, keep it lidded and light-proof, and keep the pump from heating a small tank. Our grow room cooling guide covers the air side, and the VPD guide shows how room conditions tie together. If the water is still warm after that, a chiller is the answer.
The BTU Math
One gallon of water weighs about 8.34 pounds, and it takes one BTU to change one pound by one degree F. So each gallon needs 8.34 BTU for every degree you remove. Multiply by the gallons and the degrees, then divide by the chiller's rated BTU per hour for the time.
| Reservoir | BTU to remove 10 degrees F | 1/2 HP (4,020 BTU/hr) | 1 HP (10,050 BTU/hr) |
|---|---|---|---|
| 50 gal | 4,170 | 1.0 hr | 0.4 hr |
| 100 gal | 8,340 | 2.1 hr | 0.8 hr |
| 150 gal | 12,510 | 3.1 hr | 1.2 hr |
These are best-case numbers. They assume the rated figure holds at your water and room temperature, and they ignore heat leaking into the tank and heat from the pump. Treat them as a floor. The useful comparison is the ratio: the 1 HP unit does the same job in about 40 percent of the time, which matters most for a large tank or a reservoir that warms back up quickly.
The Chillers We Carry
| Chiller | What the listing says | Price |
|---|---|---|
| Active Aqua Chiller with Power Boost, 1/2 HP | 4,020 BTU per hour. Recommended volume 90 to 172 gallons. Titanium evaporator, R134a refrigerant, boost function. 19 x 14 x 19 in. Indoor use only. | $899.00 |
| Active Aqua Chiller with Power Boost, 1 HP | 10,050 BTU per hour. Recommended volume 80 to 250 gallons. Same titanium evaporator and boost function. 22 x 15 x 19 in. | $1,296.00 |
| Active Aqua Water Chiller with Power Boost, 1/10 HP | Listing describes a 1/10 HP unit with power boost. It does not publish BTU or a recommended volume. | $392.32 |
| Elemental Solutions H2O Chiller, 1/4 HP | Listing gives no capacity or volume. | $112.28 |
| Elemental Solutions O2 H2O Chiller, 1/10 HP | Listing gives no capacity or volume. | $87.03 |
The honest gap is the bottom three rows. The 1/10 HP Active Aqua and both Elemental Solutions listings don't publish a BTU rating, so we can't run the math on them from the page. Because capacity is what you are buying, send us your tank volume and the temperature you are trying to hold before you order one of those, and we'll tell you whether it is the right size or whether to step up. We won't tell you a small unit will cool a big tank.
Sizing Guide, With Margin
Both Active Aqua listings give a recommended volume range, and the ranges overlap: 90 to 172 gallons for the 1/2 HP and 80 to 250 for the 1 HP. In the overlap, the larger unit is the one that keeps up when the room is hot. If your reservoir is near the top of the 1/2 HP range, or the tank sits in a warm room, the 1 HP is the safer fit even though it costs $397 more. If you only need to hold a 100-gallon tank a few degrees below room temperature, the 1/2 HP does it for less.
Here is the price per rated BTU per hour: $899.00 divided by 4,020 is about $0.22, and $1,296.00 divided by 10,050 is about $0.13. The bigger unit is cheaper per unit of cooling, which is common with refrigeration equipment. That doesn't mean bigger is better for you. A unit that cools far more than you need costs more to buy and takes more floor space.
The listings also note that the starting temperature of your solution affects how long the first chill takes, and that the units are not rated for central or house plumbing. The 1/2 HP listing says it is approved for indoor use only. A 1/2 inch fitting kit is $11.34 and is listed for the 1/4 HP and 1/2 HP models; the larger chillers list their own fittings as included.
Power and Room
A chiller moves heat out of the water and releases it into the room, so a chiller in a sealed grow room adds load to your air conditioning. Our HVAC sizing guide shows how to count that load, and the electrical circuits guide helps you check the breaker before you plug in. For bigger projects, ModernFarms finances equipment over $25,000 for licensed facilities; our financing guide explains what we can and can't tell you about terms.
Start the conversation
Send us your reservoir volume, your current water temperature, and your target. Email info@modernfarms.store or call 818-504-8165, and we'll tell you which size fits, or that you don't need one yet.
Frequently Asked Questions
What temperature should a hydroponic reservoir be?
ModernFarms' Bluelab Conductivity Pen listing recommends feeding at 65 to 72 degrees F and warns that water that is too warm or too cold can lead to stagnant growth or root rot. Your nutrient maker may give a tighter range, so check their guidance.
Do I need a chiller or just a fan?
If the reservoir stays in range with room air and some shade, you may not need one. A chiller is for reservoirs that run warm even when the room is in range, usually from pump heat, warm lights nearby, or a hot room. Measure the water for a few days before you buy.
How do I size a water chiller?
Start with the volume and how many degrees you need to pull down. Water takes about 8.34 BTU per gallon per degree F, so 100 gallons down 10 degrees is about 8,340 BTU. Compare that with the chiller's rated BTU per hour, then allow generous margin for pump heat, ambient heat, and real-world conditions.
What chillers does ModernFarms carry?
Active Aqua chillers at 1/10 HP ($392.32), 1/2 HP ($899.00), and 1 HP ($1,296.00), plus Elemental Solutions 1/4 HP and 1/10 HP units. The 1/2 HP lists 4,020 BTU per hour and a recommended volume of 90 to 172 gallons. The 1 HP lists 10,050 BTU per hour and 80 to 250 gallons.
Why does the starting temperature matter?
The Active Aqua listings note that the starting temperature of your solution affects how long the first chill takes. A reservoir that begins at 85 degrees F takes much longer to reach range than one that begins at 75.
Can I use a chiller outdoors?
Active Aqua's 1/2 HP and 1 HP listings say the unit is not rated for central or house plumbing, and the 1/2 HP says approved for indoor use only. Keep them indoors.
Can I finance equipment for a larger facility?
Financing is available for projects over $25,000 for licensed facilities. A single chiller won't reach that, but it can be part of a larger order. Rates and terms are not published, so ask in writing.
This article is general information, not technical, financial, or legal advice. BTU figures come from the manufacturer's listings and the pulldown times are ideal calculations, not guarantees. Prices and stock reflect ModernFarms' live catalog as of October 8, 2026. We don't upsell, and if a lid and a shady corner fix your tank, we'd rather you keep the $899.