Fermentation Temperature Control with iSpindel + a Glycol Chiller (2026)

Ferment ales at 65–72°F (18–22°C) and lagers at 48–55°F (9–13°C), then ramp for a diacetyl rest and drop to a cold crash — and the cleanest way to run that whole curve is closed-loop: an iSpindel reporting gravity and temperature over WiFi, driving a glycol chiller or heat source through an edge controller. This walkthrough covers the setpoints by phase and how to wire the control loop so the ferment holds its profile on its own.

Why fermentation temperature is the control variable

Yeast makes the beer, and yeast is exquisitely temperature-sensitive. Too warm and ale yeast throws fusel alcohols and harsh esters; too cold and it stalls or drops out before attenuating. The classic mistake is controlling the room or the fridge air instead of the beer, which lags and overshoots. And fermentation is exothermic — an active ale can run several degrees above ambient at high krausen — so a setup with no active cooling drifts warm exactly when it matters most. Closed-loop control on the fermenter is what keeps the yeast in its happy band.

Setpoints by phase

• Pitch / primary (ale): 65–72°F (18–22°C). Many clean ales ferment around 66–68°F; hold it steady through the active phase.
• Pitch / primary (lager): 48–55°F (9–13°C). Cooler, slower, cleaner — active cooling is essentially mandatory.
• Free rise / diacetyl rest: near terminal gravity, let the temperature rise a few degrees (ales to ~70–72°F, lagers up to ~60–65°F) so the yeast reabsorbs diacetyl before it flocculates.
• Cold crash: once gravity is stable, drop to 32–38°F (0–3°C) to clarify — covered in depth in cold crash & conditioning automation.

Notice that two of those transitions are gravity-triggered, not time-triggered — which is exactly why measuring gravity inline matters.

What the iSpindel gives you

An iSpindel is an open-hardware floating hydrometer that sits in the fermenter and reports specific gravity, temperature, and battery over WiFi (via MQTT) on an interval. That means you can see attenuation in real time — watch gravity fall and flatten — and use "gravity is stable" as the trigger to start a diacetyl rest or a cold crash, instead of guessing from the calendar. It's the de-facto open sensor for connected fermentation, and LoopString supports it natively.

Building the control loop

The pieces:

1. Sensor: an iSpindel publishing gravity + temperature over MQTT to the Pi. (A DS18B20 thermowell probe in the beer is a good second temperature source for the control loop, since it responds faster than the floating iSpindel.)
2. Actuators: a glycol chiller (or a chest-freezer/fridge as the cold source) for cooling, and a heat belt or heat pad for warming — both switched by relay or smart plug.
3. Controller: a hysteresis or PID loop on the Pi holding the beer temperature to the phase setpoint, with a deadband so the chiller and heater never fight, and a minimum run time so the compressor doesn't short-cycle.
4. Schedule: per-phase setpoints that step the target through primary → diacetyl rest → cold crash.

In LoopString, this is a Configurator flow: the iSpindel (and thermowell probe) feed a controller part that drives the chiller and heat belt, edge PID/hysteresis runs on the Raspberry Pi so control survives a network drop, scheduled setpoints move the target through the phases, and threshold alerts warn you if the beer runs away or the iSpindel battery dies. Gravity and temperature both log to a chart so every batch has a fermentation curve you can compare. The result is a fermenter that walks itself from pitch to cold crash while you watch from your phone.

Quick reference

1. Hold ales at 65–72°F, lagers at 48–55°F — and control the beer, not the air.
2. Use the iSpindel's gravity reading to trigger the diacetyl rest and cold crash, not a fixed schedule.
3. Run cooling and heating with a deadband and min run time so they don't fight or short-cycle.
4. Keep edge control on the Pi so a WiFi blip doesn't drop the loop.
5. Log gravity + temperature every batch to build a repeatable profile.