Snowflake: boiler frost protection
The boiler’s built-in frost protection: if the water inside it gets close to freezing, it fires by itself to warm it, even with the heating switched off, as long as it has power and gas.
What the manual says to do
In cold weather leave the boiler’s power and gas on. Vaillant: if the flow temperature falls below 5 °C, it heats the circulating water to about 30 °C. Ideal Logic Combi2: ‘FP’ shows while it runs, from below 5 °C until it reaches 19 °C. Viessmann (with an outside sensor): below 5 °C outside, the boiler water is held at 20 °C. For a house left empty in winter, Worcester Bosch says leave the programmer on constant and the room thermostat at 12 °C.
Only your boiler’s own manual says exactly what a symbol means on your model. The manuals let the householder reset a fault once, top up the pressure through the filling loop and change the settings; anything else on a gas boiler is a job for a qualified heating engineer (Gas Safe registered in the UK), as the manuals themselves say.
What it does not mean
It does not protect the whole house or every pipe: Vaillant says it “cannot guarantee flow through the entire heating installation”, and Ideal that it “does not guarantee that all other parts of the system will be protected”. It does not work with the power or gas off.
- What it looks like
- Our drawing is a snowflake, the usual sign for frost. Ideal’s display shows the letters FP; Baxi has a display symbol for ‘OFF (frost protection still enabled)’.
- Names used
- Frost protection function (Vaillant); FP, boiler frost protection (Ideal); OFF, frost protection still enabled (Baxi); Frost protection (Viessmann, Worcester Bosch)
Where makers differ
Worcester Bosch Greenstar 2000 has no frost protection in summer mode (“Frost protection is not available in summer mode”), while Ideal’s “operates in all modes”. Viessmann’s only works when an outside temperature sensor is connected. The trigger and target temperatures are each maker’s own.
Other symbols with the same shapes
Sources
No public standard defines this boiler symbol, so its meaning is taken from the manuals of 5 different maker groups (Vaillant, Ideal Heating, Baxi, Viessmann, Worcester Bosch), each fetched and read on the date shown:
- Vaillant, ecoTEC plus VU, VUW, VUI ..6/5-5: Operating instructions ecoTEC plus, 0020245033_02 (26.05.2021) GB, IE (PDF) — “If the heating flow temperature falls below 5 °C when the on/off button is on, the product comes into operation and heats the circulating water to approx. 30 °C”; “The frost protection function cannot guarantee flow through the entire heating installation”. Checked 2026-09-27.
- Ideal Heating, Logic Combi2 C24, C30, C35: Logic Combi2 user guide, UIN 237664 A01 (10/2023) (PDF) — Display code FP: “Boiler is active for boiler frost protection: operates if the ambient temperature is below 5°C until it reaches 19°C”; frost protection “operates in all modes, provided the power supply to the boiler is always turned on … does not guarantee that all other parts of the system will be protected”. Checked 2026-09-27.
- Baxi, Baxi Compact range (quick guide 7849761): Quick start guide to your boiler, 7849761 Baxi Compact quick guide user instructions (PDF) — Display symbols: “OFF (frost protection still enabled)”; “In cold weather, your boiler has ‘frost protection’ so it may run for a short time even with the heating turned off”. Checked 2026-09-27.
- Viessmann, Vitodens 100-W/111-W/111-F/141-F (black/white screen): Operating instructions for the system user, 6167584 GB (12/2020) (PDF) — “The frost protection function is only possible when an outside temperature sensor is connected. Frost protection function is active at outside temperatures below 5 °C. The burner is switched on and the boiler water temperature is held at 20 °C.” Checked 2026-09-27.
- Worcester Bosch, Greenstar 2000 combi (GR2301iW): Greenstar 2000 user instructions, 6 721 839 343 (2023/04) UK/IE (PDF) — “Frost protection is not available in summer mode”; if leaving the property unoccupied in cold weather, set the time control to constant ON and the room thermostat to 12 °C. Checked 2026-09-27.
Written in our own words; quotations are the makers’ wording.