Keeping your reservoir cool

How can I keep my reservoir cool? Maintaining a highly aerated root zone at optimum temperature is key to achieving high yields and problem-free grows. This FAQ focuses on indoor hydroponic reservoir cooling options.

Why do I need cool root temperatures?

High root zone temperatures often plague indoor growers running water culture (aero/bubbler/dwc/hydro) systems. These systems are subject to rapid heating by intense HID lighting, which increases root zone temperatures, which decreases dissolved oxygen (DO) levels. Rapid plant growth, combined with low DO levels, can cause oxygen deprivation which in turn can result in infection by opportunistic pathogens such as pythium.

The key to maximum growth is to keep the air temperature at 75-80F, but the root zone at 68F or less. Note: the reservoir should be kept slightly cooler than the rootzone – irrigation and system heating will warm the water by the time it reaches the roots.

Optimum root growth occurs at 70-75F; however, destructive root diseases also grow and reproduce rapidly at these root temperatures. Maintaining nutrient temperatures at or under 68F maximizes root growth and DO, and inhibits pythium.

Reservoir cooling options:

Warm summer temps often require aggressive cooling measures. Bubblers and dwc are difficult systems to temperature regulate due to their (usually) small volumes and lack of external reservoir.

Note: spray / drip / mist / circulate nutrients on a frequent basis to equalize reservoir and root zone temperatures. Intermittent spraying may require a slightly lower tank temp, to compensate for system heating occurring during “off” spray cycle. (ie. Keep tank temp around 64F for intermittent spray cycles, 68F for continuous spraying).

Note: submersible pumps add heat. Use an external/inline pump to minimize heat transfer. High quality digital thermometers are recommended.

Add cold water when topping up. Note: abrupt changes in temperature may shock roots.

Frozen pop bottles/milk jugs. Fill to ¾. Keep extras in the freezer to replace thawed bottles with new frozen ones, replace as necessary. Note: “Freezy packs” tend to crack and leak.

Increase size of reservoir Larger volumes are slower to warm up, pH/ppm is more stable and tank changes are less frequent.

Put reservoir/bubbling buckets onto floor, or set on concrete blocks to conduct heat away from the water.

Insulate Paint all exposed system surfaces white or use reflective material (such as mylar or reflectix). Wrap insulation around tank. Use a camping cooler for a reservoir (pre-insulated and comes with a drain!)

Swamp cooler Blow a fan directly across the surface of reservoir for excellent evaporative cooling. This method works well (expect a 10F drop in res. temp), but humidity and tds will increase, and more frequent topping up will be required.

430 scrog “?add a computer fan to a duct blowing into your tank (cut air exit holes). You can run it on a timer (1 hr on, 1 hr off). I run a float valve to keep it topped up.” Make sure lid and reservoir can be easily removed.

Blow air through the root zone Divert small amounts of cool intake air directly into the root zone.

Remote reservoir In-room reservoirs will quickly heat up to room temp. Put the reservoir (and ballasts) outside of the grow room to minimize tank heating.

Airstone / Power head / Venturi air supply should be drawn from a cool source (ie. Cool outside air).

Peltier coil (Thermoelectric chillers). (Bayou grower) “I use an Ice Probe ($125) and it works well. It uses 50 watts and pulls the temp down 4 degrees under ambient. Cools 10 gal or less.” (see for more models)

Reservoir chillers These are electric A/C units made specifically for cooling water. (Search for “Aquarium chillers”) (smokin fl) “?a heavy box with fan and compressor coils, with a 5 foot refrigeration line with a titanium coil at the end. All you do is plug it in, set the controller and put the coil in the res. Circulate nutes for the best cooling. Get a bigger model than you need.”

Cooling coil

A coil of stainless steel is put into the reservoir, cold tap water is trickled through the coil and the overflow runs down the drain. A circulation pump in thte rez makes the cooling more efficient. Adjust tap flow as necessary (Water use can reasonable). No power, unlimited cooling, quiet.

Making your own cooling coil:

Scrap yards and appliance repair shops are full of A/C and fridge coils: 4-20 loops (more surface area is better), with male garden hose connectors welded to each end (Don’t use copper or nickel coils). Available in Hydro stores (around $100 cdn).

Note: Cooling coils may not be useful for those on metered water.

4 comments to Keeping your reservoir cool

  • MyCANN

    “copper oxidizes, you know like when a penny turns green if left in water too long”

    Someone better tell the plumbing store that because the one next to my house is stocked full of copper plumbing pipe.

  • p lush

    copper oxidizes, you know like when a penny turns green if left in water too long

  • Rob

    Why do you say to not use copper coils? Does it react to the fertilizer?
    This is what I was going to use cause I have a box of 3/8 tubing laying around that would be perfect for making a coil.

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