Frost Stat: What It Does and Whether Your Boiler Needs One

What a frost stat actually does

A frost stat is a thermostat with one job. It sits in the cold space you want to protect, and when the temperature there falls to its setting it closes a contact that tells the boiler and pump to run, overriding the timer. When the temperature rises again it switches off and the normal programme takes over.

Many frost stats are fixed at around 5 degrees; others can be adjusted, typically somewhere between 0 and 10 degrees. They look like a small plain room thermostat, often with no display, and are usually fitted on a wall in the garage, loft or plant room, or next to the boiler if it is in an outbuilding.

A frost stat is often paired with a pipe thermostat, strapped to a pipe near the boiler. The frost stat starts the heating when the air is cold, and the pipe stat stops it again once the water in the pipes has warmed up, usually to around 25 degrees. Without the pipe stat, a frost stat in a cold garage can keep the whole heating system running for hours to warm a space that will never get warm.

Frost stat vs built in boiler frost protection

This is the part that catches people out. Most modern combi and system boilers have a frost protection function in their software. If the temperature of the water inside the boiler drops to a few degrees above freezing, the boiler fires and runs its pump until it has warmed up again. Some show a snowflake symbol when this is active.

That protects the boiler. It does not protect the pipes running across a loft, the cylinder in a cold cupboard, or the radiators in an unheated extension. The boiler only knows how cold its own water is. By the time the water inside the boiler is near freezing, a pipe on an outside wall may already have frozen.

Built in frost protection also only works while the boiler has power, gas, and enough water pressure to fire. Switch the boiler off at the fused spur for the winter, or let the pressure fall away, and it is not protecting anything. Our guide on how to turn off a boiler explains the difference between switching the heating off and switching the boiler off, and why that matters in cold weather.

The frost or snowflake setting on a room thermostat or smart control is different again. It keeps the rooms above a minimum, often 5 to 7 degrees, but it measures the temperature where the thermostat is, not in the loft or garage.

Do you need a frost stat?

You probably need one if any of these apply:

• The boiler is in a garage, a loft, an outbuilding, a basement or a cupboard on an outside wall.
• Heating or hot water pipes run through a loft, under a suspended floor or along an outside wall where they are exposed to cold air.
• The hot water cylinder is in the loft or another unheated space. Our guide on hot water cylinder in the loft covers what that position needs.
• The property is left empty for long periods in winter, such as a second home or a flat between tenants.
• You have an older boiler without built in frost protection.

Where it goes and how it is wired

The frost stat should sit in the coldest part of the space it is protecting, away from the boiler casing and any warm pipes, so it reads the real air temperature. In a garage, that usually means a wall near the pipework, at a height where it will not be knocked.

It is wired into the heating controls so that it can bring on the boiler and pump, and on an S plan or Y plan system it also needs to open a motorised valve so there is somewhere for the heat to go. Our guide on Y plan and S plan heating systems explains those valves. Many wiring centres and programmers have a dedicated frost stat terminal. Getting this wrong is common: a frost stat wired only to the boiler, with the heating valve left shut, can fire the boiler with nowhere for the water to circulate.

Fitting a frost stat is a small job on most systems, and it is often done at the same time as other work in autumn. It should be done by someone competent with heating controls wiring, and the system tested afterwards by cooling the stat to prove it brings the heating on.

Other ways to stop your system freezing this winter

A frost stat is one layer. The others are cheap and worth doing before the first cold snap.

• Lag pipes in lofts, garages and under floors. Our guide on insulating hot water pipes covers which pipes matter and how to do it.
• Insulate the cold water tank in the loft, but leave the area underneath it open so a little warmth can reach it. See cold water tank in the loft.
• Keep the boiler powered and the pressure at its normal level over winter, even if the heating is off.
• If you go away, leave the heating on a frost or low setting rather than switching it off. How to set a heating timer explains how to do that on most controls.
• On a condensing boiler, the plastic condensate pipe that runs outside can freeze in a hard frost and lock the boiler out. Larger or insulated pipe on the outside run helps.
• Know where your stopcock is, so you can shut the water off quickly if a pipe does freeze and split.

If the pipes have already frozen

If the taps run slowly or not at all on a cold morning, a pipe may have frozen. Turn off the heating and the boiler, open the affected tap so water has somewhere to go as it thaws, and warm the pipe gently from the tap end with a hairdryer on a low setting or a hot water bottle. Never use a naked flame. Our guide on no water coming out of hot tap goes through the checks, including frozen feeds.

If a pipe has split, or water is coming through a ceiling, turn the water off at the stopcock and call us on 020 4652 5111. We have 20 engineers across London, and we can fit a frost stat, lag exposed pipework and put right the damage from a freeze. Our rate is on the pricing page and agreed before we attend.