A pub landlord in Bermondsey came down to the cellar on a Monday morning and found the beer lines frozen solid. The cellar thermometer read -2°C. Two casks of real ale had been ruined — frozen, expanded, and pushed the shives out. The keg lager was borderline. He’d lost a weekend’s worth of cask stock because the cellar cooler had run all night without switching off.
A beer cellar should sit between 11–13°C for cask ales and around 3–5°C for keg products if you’re running a remote python system. When the cooler overshoots and the cellar drops below freezing, you’ve got a fault that needs fixing before it destroys more stock.
Here’s what actually causes a cellar cooler to freeze up — and what to do about it.
Failed or Miscalibrated Thermostat — The Most Common Cause
The cellar thermostat tells the cooler when to run and when to stop. It’s a simple concept: cellar gets too warm, cooler kicks in; cellar reaches target temperature, cooler switches off. When the thermostat fails, it either sticks in the “on” position — calling for cooling permanently — or drifts out of calibration so it’s reading the wrong temperature.
Mechanical thermostats with a capillary sensor are common on older cellar coolers. The capillary tube is filled with a gas that expands and contracts with temperature, operating a switch. If the tube gets kinked, the bulb corrodes, or the switch mechanism wears out, the thermostat can stick closed. The cooler runs without stopping until the cellar hits sub-zero temperatures.
Digital controllers are more reliable but not immune. A failed sensor gives the controller a false reading, and it responds accordingly. We’ve seen controllers reading 18°C when the cellar was actually at 4°C — the system was cooling aggressively against a phantom heat load that didn’t exist.
Stuck-Open Solenoid Valve
On cellar cooling systems with a remote condensing unit, a solenoid valve in the refrigerant line opens when the thermostat calls for cooling and closes when it’s satisfied. If the solenoid valve sticks open — usually from a burnt coil or debris on the valve seat — refrigerant flows continuously regardless of what the thermostat says.
The giveaway is that the thermostat clicks off (you can hear it) but the evaporator keeps running cold. The compressor eventually cycles on its own controls, but the damage is done — the cellar overcools between thermostat signal and compressor shutdown. Solenoid valve replacement is a standard repair and usually resolves the issue immediately.
Iced-Up Evaporator Coil
The evaporator coil inside the cellar is where the actual cooling happens. Under normal operation, it runs cold but shouldn’t ice up. When ice forms on the coil, it insulates the surface and blocks airflow — the system cools less effectively, so it runs longer, which creates more ice. It’s a downward spiral.
Ice on the evaporator points to one of several problems: low refrigerant charge from a leak (the coil runs colder than design temperature), a failed defrost cycle (if the system has one), a faulty evaporator fan that’s not moving enough air across the coil, or an ambient cellar temperature that’s already too cold for the system’s design parameters.
A cellar cooler with a block of ice on the evaporator needs a controlled defrost — turn it off, let the ice melt completely, then investigate the root cause before restarting. Chipping ice off the coil damages the fins and can puncture the tubing.
Poor Cellar Insulation or Ventilation
Some cellars are naturally cold — underground, stone walls, no heating. In winter, the cellar might sit at 8–10°C without any mechanical cooling. A cooler sized for summer loads will short-cycle or overshoot badly in these conditions because the heat load has dropped to almost nothing.
Conversely, a poorly ventilated cellar where the condenser is also located in the cellar space creates a heat loop — the condenser dumps heat into the cellar, the cooler removes it, the condenser dumps more. The system works harder than it should, and temperature control becomes erratic.
The condenser needs adequate ventilation to outside air. If it’s in the cellar, the cellar needs proper airflow to dissipate the rejected heat. Without this, the system fights itself.
Getting the Temperature Right
Target cellar temperatures vary depending on what you’re storing and serving:
- Cask ale: 11–13°C — too cold kills the flavour and condition; too warm and it goes off fast
- Keg beer and lager: ideally the cellar sits at 11–13°C with a python cooling system chilling the beer to 3–5°C at the point of dispense
- Wine storage: 10–14°C depending on type
A cellar at -2°C is as damaging as one at 20°C. Frozen cask ale is ruined. Frozen keg lines burst fittings. Even bottled stock can crack if the cellar drops far enough below zero.
What to Do Right Now
- Turn the cellar cooler off at the isolator — stop the overcooling immediately
- Check the thermostat setting — it should be set between 11–13°C for a standard beer cellar
- Look at the evaporator coil — if it’s a block of ice, leave the cooler off until it’s fully defrosted
- Check if the evaporator fan is running when the cooler is on
- Call an engineer to diagnose the root cause before restarting
Cellar Cooling Problems? Call ADK
We maintain and repair cellar cooling systems across London’s pubs, bars, and restaurants. Thermostat replacement, solenoid valves, evaporator defrosts, full system health checks — we carry the parts and know the systems. Call 020 3974 1419 for same-day cellar cooling repairs.