DIY Fermentation Chamber Build

The build

Five steps, one afternoon

1. Source the appliance. A used 5 to 7 cubic foot chest freezer, or an upright fridge if your fermenter is tall. Measure the interior height first and compare it against your vessel plus its stand plus an inch of airlock clearance. This is the step people skip and regret.
2. Set the appliance thermostat to its coldest. You are not using it. The external controller will do the switching.
3. Plug the appliance into the controller’s cooling outlet, and the wrap heater into the heating outlet.
4. Route the probe inside -- under the lid seal on a chest freezer, or through the door gasket on a fridge. Both seals tolerate a thin probe lead without leaking noticeably.
5. Tape the probe to the fermenter and cover it with a scrap of bubble wrap or foam.

Settings

Four numbers to get right

• Setpoint: 65 to 68°F for clean ales. See fermentation temperature control for other yeast types.
• Differential: 1 to 2°F. Tighter means constant cycling and more compressor wear for no benefit.
• Compressor delay: 3 minutes or more. Restarting a compressor before its refrigerant pressure equalizes damages it.
• Calibration offset: check the probe in an ice-water slurry. It should read 32°F; if it reads 34, set the offset to -2.

The details nobody mentions

Three practical problems

Condensation

A cold box in a humid garage collects water. Every opening admits warm moist air that condenses on cold surfaces, and chest freezer floors have no drain. Over weeks you get a puddle, then rust, then a smell.

• A towel on the floor of the box, wrung out periodically. Crude and effective.
• A tub of desiccant in one corner.
• Wipe the interior down with sanitizer between batches -- mold grows readily in there.

Air circulation

Cold air stratifies, so the bottom of a chest freezer can be several degrees colder than the top. With a tall fermenter that means the beer is not at one temperature. A small computer fan running continuously inside the box evens it out, costs almost nothing to run, and also helps surfaces dry between openings.

Vibration and noise

A compressor cycling in a garage at 3am is audible through a wall. A foam mat under the appliance and a sensible differential (so it cycles less often) both help. Worth thinking about before you put it on a shared wall.

Upgrades

What to add later

• A keezer collar, if you later want the same freezer to serve beer. Note this is one or the other -- a collared keezer with taps through it is no longer available as a fermentation chamber. See keezer build guide.
• A programmable controller, if you brew lagers and want timed diacetyl rests and lagering ramps without being present.
• A floating hydrometer, so you can see what the fermentation is doing rather than inferring it -- Tilt hydrometer review.
• A second controller and chamber, rather than trying to hold two temperatures in one box. One chamber is one temperature.