Wall radiators have a valve that has this progression of settings: “❄ 1 2 3 4 5”. Naturally as you turn it a pin changes position which opens a mechanical valve a precise amount.

The question is what happens at “❄”? I always thought of that as zero, completely closed. But my central boiler seems to target 5°C/40°F even when off to protect pipes from freezing. So what happens if all the radiator valves are off/closed (❄) and the boiler is triggered to prevent freezing? Does each valve also have a thermostat to open at freezing?

I thought for years the snowflake just meant cold (closed). But I wonder if it actually means frost protection, where it does something smarter. There are times when it’s in that ❄ position and yet the radiator still heats up. I thought the valve was broken, but then other radiators occasionally did the same. Is that normal?