Frost setting on a radiator valve (✱ or snowflake)
The lowest working setting on a thermostatic radiator valve head: the valve stays shut until the room falls to a few degrees above freezing, then opens so the radiator and its pipes do not freeze.
What the manual says to do
Use it for rooms you do not heat but must not freeze, or for a house left empty in winter, with the boiler and its controls left on so there is hot water to let through. Heimeier suggests it for basement and cellar rooms.
Turning the head to a number is yours to do. Swapping a valve or head on a filled system, balancing the radiators or draining the system is a job for a qualified heating engineer.
What it does not mean
It is not ‘off’: the valve still opens when the room gets cold. It does nothing if the boiler is switched off, because the valve only lets hot water in. Honeywell Home warns that heating can freeze when the head is set to 0.
- What it looks like
- An asterisk ✱ or a snowflake at the low end of the scale on the valve head, just above 0 or the closed position where there is one.
- Names used
- Frost protection setting (Danfoss, Heimeier); * Frost protection (Drayton); Frost (Honeywell Home)
Where makers differ
Danfoss marks it ✱ at about 8 °C; Honeywell Home VT15 about 6 °C; Heimeier K a snowflake at about 6 °C; Drayton TRV4 frost protection below 8 °C. Danfoss’s RAS-C² also has a ‘positive shut-off’ below the frost mark that closes the valve fully.
Other symbols with the same shapes
Sources
The settings are each valve maker’s own published figures, from the data sheets below, each fetched and read on the date shown:
- Danfoss, RAS-C thermostatic sensor (013G5015, 013G5016): Data sheet Thermostatic sensor RAS-C, VDFNQ302 (05/2015) (PDF) — “✱ = Frost protection setting”, 8 °C on the scale at Xp = 2 K; “All RAS-C sensors incorporates a Frost Protection Setting.” Checked 2026-09-27.
- Danfoss, RAS-C² thermostatic sensor (013G6040): Data sheet Thermostatic sensor RAS-C² with RA-N presetting valve, AI087886402550en-010402 (12/2025) (PDF) — RAS-C² “has a temperature range from 8 to 28° C and is equipped with frost protection setting and positive shut-off”. Checked 2026-09-27.
- Honeywell Home (Resideo), VT15 / VTL15 thermostatic radiator valve: VT15, VTL15 radiator valves product specification sheet, EN0H-2041GE23 R0121 (PDF) — TRV head settings: frost 6 °C; “Heating can freeze when radiator thermostats are set to positon ‘0’. Zero-position is also thermostatically controlled – when temperature falls, the TRV may open.” Checked 2026-09-27.
- IMI Heimeier, Thermostatic head K: Thermostatic head K data sheet, EN ed.3 10.2017 (PDF) — Snowflake ≈ 6 °C: “Basement/cellar rooms (frost protection setting)”. Checked 2026-09-27.
- Drayton, TRV4 thermostatic radiator valve: TRV4 thermostatic radiator valve information sheet (PDF) — “* Frost protection — Integral and Remote sensor below 8°C”. Checked 2026-09-27.
Written in our own words; quotations are the makers’ wording.