Frost Stats and Boiler Frost Protection Explained

A frost stat is a thermostat that switches your heating on when the air around it drops close to freezing, usually around 5°C, even when the timer says the heating should be off. It isn't the same as the frost protection built into most modern boilers, which only protects the boiler. If your boiler or heating pipes sit in a garage, loft or outbuilding, a frost stat is what stops them freezing on the first hard night. If you live in a flat with an indoor boiler, you probably don't need one, but you do need to set the heating correctly before you go away.

Built-in boiler frost protection

Most modern gas boilers have a built-in frost protection function. A sensor inside the boiler watches the temperature of the water in it. When that water gets down to around 5°C, the boiler fires and runs its pump for a short time to warm it back up, then stops. The exact start and stop temperatures vary by make; Viessmann's guidance describes heating the water to about 10°C, while Alpha's describes running until it's much warmer. Your user manual will give the figures for your model.

The limitation is that it protects the boiler, not the house. Viessmann's explanation is clear that the water heated this way isn't sent round your heating system. So a boiler in a warm kitchen will rarely trigger it, and a pipe in a cold loft can freeze while the boiler's own frost protection never switches on.

It also only works if the boiler can run:

• the electrical supply must be on, so don't switch it off at the fused spur
• the gas must be on
• the system pressure must be high enough, or the boiler will lock out instead. See how to repressurise a boiler if the gauge is low.

What a frost stat does

A frost stat is a separate thermostat, usually a small box on a wall or a sensor clipped to a pipe, wired into the heating controls. It measures the temperature of the space it's in, not the water in the boiler. When that space nears freezing, it calls for heat and runs the whole heating system regardless of the programmer, then switches off once the temperature has risen.

Because it overrides the timer, it's often paired with a pipe thermostat. The frost stat starts the heating when the air is cold, and the pipe stat stops it once the water in the pipes is warm enough, so the boiler doesn't run for hours heating a freezing garage. The two are sometimes combined in one unit.

Many modern programmable and smart room thermostats also have a frost setting, sometimes called frost protection or holiday mode, that holds the house at a low minimum temperature. That works in a similar way for the rooms where the thermostat is, but not for a boiler in a separate cold space. Our guide to how thermostats work covers the different controls.

Do you need a frost stat?

You're likely to need one if:

• the boiler is in a garage, a loft, an outbuilding or a lean-to
• heating pipes run through an unheated loft, under a suspended timber floor or through an outside wall
• the property sits empty for spells over winter, such as a rental between tenants or a second home

You probably don't need one if the boiler is inside the heated part of a flat or house and the pipework stays within heated rooms. In that case, built-in frost protection, a low heating setting when you're away and some lagging on any exposed pipes are enough. Our guide to preventing frozen pipes covers the lagging side, and our pipe lagging service handles the awkward runs.

Watch the condensate pipe too. On a condensing boiler, the thin plastic pipe that carries condensate water outside can freeze in a hard frost and lock the boiler out, and neither kind of frost protection prevents that. See frozen flue and condensate problems or our condensate pipe repairs.

Fitting a frost stat means wiring it into the boiler or the system's wiring centre, so it's a job for a heating engineer. We fit and wire them as part of our heating controls work.

How to test a frost stat

It's worth checking before the first cold snap, while you can still get an engineer in easily:

If nothing happens, check the boiler has power and pressure, then call an engineer. The frost stat, its wiring or the wiring centre may have failed.

Going away in winter

This is the question that matters most once the nights turn cold. Before you leave:

• leave the boiler switched on at the wall and the gas on
• set the room thermostat low, or use its frost or holiday mode. Our frozen pipes guide suggests a minimum of around 12°C for an empty property.
• check the pressure gauge is in the normal range so the boiler doesn't lock out while you're gone
• know where your stopcock is and make sure it turns, so whoever checks the house can shut the water off quickly

Home insurance policies often have conditions about homes left unoccupied in winter, such as keeping the heating on or draining the water down. Check yours before a long trip.

For the wider start-of-season checks, see when to turn the heating on, and what to check first.

Frost stat problems

The two common faults are opposites.

• The heating comes on by itself. A frost stat set too high, fitted on a cold outside wall or failing can call for heat on nights that aren't freezing. If the heating runs outside its timed periods, work through our guide to heating that won't turn off, which covers frost protection alongside valve and programmer faults.
• The heating doesn't come on in a cold snap. The stat has failed, the boiler has locked out on low pressure or a frozen condensate pipe, or the power is off. Frozen pipes are the result. If that happens, see what to do about frozen pipes or call our frozen pipe team.

Labour is £95 an hour, the £60 call-out is waived on jobs over 2 hours, and we confirm the price before starting.

Frequently Asked Questions