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How to install a frost thermostat in the UK

Updated: October 2026

You have bought, or you are about to buy, a control for the first night the forecast mentions air frost. You need to know where to mount it, how low to set the dial, and when to stop before you touch gas wiring. This guide covers general UK practice for a wired frost stat and a plug-in greenhouse controller. Your boiler manufacturer’s wiring diagram overrides any summary here. Gas work you do not understand belongs with a Gas Safe registered engineer. Product choice sits on the [frost thermostat comparison](/hub/thermostats/frost-thermostat). Combi frost mode, as distinct from these fitting steps, is on the combi page linked further down.

You have bought, or you are about to buy, a control for the first night the forecast mentions air frost. You need to know where to mount it, how low to set the dial, and when to stop before you touch gas wiring.

Our top picks

T F C Group LLP TOWER volt-free frost stat

Best overall

T F C Group LLP TOWER volt-free frost stat
7.2/10about £15.29
View offer
LOWENERGIE LOWENERGIE TH-926T

Rank 2

LOWENERGIE LOWENERGIE TH-926T
8.4/10about £17.89
View offer
XIEHUZA XIEHUZA digital plug-in

Best value

XIEHUZA XIEHUZA digital plug-in
6.8/10about £14.99
View offer
On this page
  1. 1.Before you start
  2. 2.Mounting a wired frost stat
  3. 3.Wiring volt-free inputs
  4. 4.Setting up a plug-in stat
  5. 5.Test, then stop if the boiler stays silent

Before you start

Isolate the power before you open any mains wiring. For a plug-in, switch the socket off while you place the heater and probe. For a wired stat, follow the boiler shutdown, often by turning off the fused spur and confirming there is no call for heat. Keep the terminal map with the instructions. Do not follow a generic live and switched-live note if the PDF says frost, S, RT or volt-free. A wired volt-free stat is a fixed mount with a dry contact to the frost input, and it likely needs a competent person under Part P. A plug-in is a BS 1363 plug with the heater in the output. If a fault code appears after a wiring change, stop.

Mounting a wired frost stat

Mount the stat where the air represents the freezing risk, not where the cable is easiest. In a loft that often means near the boiler and the vulnerable runs, low on a wall or a joist rather than in the hottest apex. Follow the box for cable entry. Some budget bodies only have a bottom entry. On a mechanical frost stat the sensor is in the body, so the whole unit has to live in the zone you protect. Do not put it above a warm bedroom while pipes freeze beside the tank. If you cannot tell volt-free common from normally open, use the boiler diagram or an electrician for the first fix.
T F C Group LLP TOWER volt-free frost stat

Testsieger

T F C Group LLP TOWER volt-free frost stat
7.2/10about £15.29
View offer

Wiring volt-free inputs

Volt-free means the stat switches an isolated contact. When the temperature falls below the setpoint, that contact closes or opens, and the boiler’s own circuit sees the demand. The stat does not normally feed 230 V into the frost terminals. Some models need local mains for a sensor. Others are mechanical switches, so check the datasheet, then take the contacts to the frost pair on your diagram. Every combi diagram differs, so this page has no colour chart. Open the PDF, find external frost stat or volt-free, and match the terminal IDs. If it says consult an installer, stop. After wiring, restore power. If the boiler never fires, check winter mode or the terminals, then return the dial to about 3–5 °C.

Setting up a plug-in stat

Plug the stat into a UK socket that can carry the heater, then plug the heater into the switched outlet. An in-unit sensor needs the whole plug in the greenhouse air. A probe needs the tip at bench height, in the shade. Select heating mode. On a LOWENERGIE TH-926T, heating and cooling reverse whether the socket energises above or below the setpoint, and the wrong mode leaves heat off. Leave a tubular heater on maximum and let the stat cycle the socket. Check 13 A or 10 A against the label. LOWENERGIE reviewers often report the dial about 2 °C high, so compare a max/min thermometer and nudge down.

Test, then stop if the boiler stays silent

On a wired install, confirm frost or winter mode if the manual requires it before an external stat can call. If the loft is cold and the boiler stays silent, check the terminals before you raise the setpoint. On a plug-in, lower the setpoint until the heater clicks, or raise it briefly to prove switching, then return to the overnight hold. If a wired boiler still does not fire, stop. Internal frost mode versus an external stat is on frost stats for combi boilers. Book Gas Safe if a fault code remains, and leave boiler electrics on through winter if frost protection needs power.

Common questions

Where should a frost stat be mounted in a loft?
Mount it in the coldest air that stands for the vulnerable pipework and the boiler, away from flue heat and from a warm pocket that does not represent the pipes. That is not automatically the loft hatch where you stand to fit the cable.
How low should I set the dial?
Many pipe-protection installs use roughly 3–5 °C on a dedicated frost stat. Plug-ins often bottom at +5 °C. Use the marked range on your device and confirm the overnight minimum with a thermometer, not the printed dial alone, on a cold night.
How do I wire volt-free contacts?
Follow the frost terminal pair in your boiler manual. The stat closes dry contacts into that circuit and does not normally feed 230 V into the board. Wire colours vary, so use the labelled diagram for your model, not a generic chart.
Can I install a frost stat myself on a gas boiler?
A plug-in setup, yes. Boiler-linked wiring only if you are competent under Part P and you understand the manual. Otherwise use an electrician or a Gas Safe registered engineer for the part you are not sure of. Gas valves and the flue stay with Gas Safe.
How do I set up a plug-in greenhouse stat?
Put the socket end in the greenhouse air, plug the heater into the stat, select heating mode, and start near a 5 °C hold if that is your minimum. Check a max/min thermometer in the morning and adjust for any dial offset over the next cold nights.
What if the boiler still does not fire?
Check winter or frost mode, the terminal map and any fault code, then stop if you are unsure and call a Gas Safe registered engineer. Raising the dial without knowing the terminals can leave a lock-out in place. Do not keep turning the dial.
Do I leave the boiler electrics on all winter?
Yes, if you depend on frost protection that needs power at the boiler. Switching off the fused spur can disable internal frost mode and an external frost stat the boiler has to see all winter. Leave that supply on for the heating season.

Related pages

Frost thermostat buying guide

Frost thermostat

Best frost thermostat UK: boiler, greenhouse, budget

Frost stats for combi boilers in lofts and garages

Greenhouse frost thermostat for UK sheds and heaters

Sources

Public UK sources named below. amazon.co.uk prices, stars and review counts on these five listings were read on 19 October 2026. Page updated October 2026.

  1. Crawley Borough Council — Scott Summers Court boiler user guide (PDF) (retrieved 19 October 2026)
  2. Bradfords — Drayton RTS3 frost stat (retrieved 19 October 2026)
  3. Two Wests — The A–Z of greenhouse heating (retrieved 19 October 2026)
  4. RS PRO plug-in thermostat (UK spec) (retrieved 19 October 2026)
  5. amazon.co.uk listings TOWER B07D4GNW18, LOWENERGIE B0BPJT8WQ3, KETOTEK B0GQ82T5C8, XIEHUZA B0CSG6PP7N, GLOGLOW B0CWZ7TWWQ (retrieved 19 October 2026)