Frost stat: what it does and when a boiler needs one

What a frost stat actually does

A frost stat is wired into the heating controls so that it can call for heat independently of the programmer and room thermostat. When the temperature at the frost stat falls to its set point, it closes a contact, the boiler fires and the pump circulates warm water. When the space has warmed a few degrees, the frost stat opens again and the heating goes back to whatever the timer says.

Many frost stats have a fixed setting of around 5°C. Adjustable ones typically let you choose a point a few degrees either side of that. The aim is not to heat the garage or loft, only to keep the water in the pipes and boiler from freezing.

On a system with a hot water cylinder and zone valves, the frost stat is normally wired so that the heating zone opens as well, otherwise the boiler would fire with nowhere for the water to go. The exact wiring depends on the boiler and control layout, and the boiler manufacturer's instructions show where a frost thermostat should connect.

Frost stat or the boiler's built-in frost protection?

Most modern boilers have a frost protection function. It watches the temperature of the water inside the boiler and fires the burner if it gets close to freezing. That is useful, but it only protects the boiler's own heat exchanger. It does nothing for a condensate pipe running outside, for heating pipes under the floor of a cold extension, or for a boiler in a garage where the pipework around it may freeze before the water inside the boiler does.

Smart and wireless thermostats often have a frost setting as well. The catch is where their sensor is: usually in the hallway or living room, which rarely gets anywhere near freezing. It protects the house, not the garage or loft where the boiler is. Our guide to boiler frost protection in an empty home goes further into what the built-in function covers.

When a frost stat is worth fitting

Heating season has just started, which makes this a good time to look at any part of the system outside the warm envelope of the house. A frost stat is worth considering in these situations:

• The boiler is in a detached or integral garage, an outbuilding or a lean-to
• The boiler or heating pipes run through a loft or a room that is not heated
• The property is empty for weeks at a time in winter, such as a second home, a void rental or a flat between tenants
• A previous winter brought a frozen condensate pipe, a burst pipe or a boiler lockout in a cold snap
• Heating pipes run under a suspended floor or through an exposed extension

A frost stat does not replace insulation. Pipe insulation for unheated spaces is still the first line of defence, and the frost stat is the backstop when temperatures fall further than the insulation can cope with.

Frost stat and pipe thermostat together

A frost stat on its own can keep the boiler running for long periods on a very cold night, because a garage or loft may take a long time to warm up, if it warms at all. The usual answer is to pair it with a pipe thermostat, sometimes called a pipestat, clipped to the return pipe. The frost stat starts the heating when the air is cold, and the pipe thermostat stops it once the water in the pipes has warmed enough to be safe. The result is short bursts of heating instead of hours of running.

This matters for running costs, and also for wear on the boiler. A boiler that fires repeatedly in short bursts can look like boiler short cycling from another cause, so if yours is doing that in cold weather, have the frost stat setting and wiring checked.

Where to position a frost stat

Put it in the coldest part of the space you are protecting, close to the pipework at most risk, and at a height where cold air collects rather than up by a warm ceiling. Keep it away from the boiler casing, which gives off heat while the boiler runs and would make the frost stat switch off too early. Avoid direct sunlight through a garage window, which can fool it on a bright frosty morning.

If the risk is a boiler in a loft, the frost stat usually goes in the loft near the boiler and pipe runs, not in the hallway below. In a garage, mount it on an inside wall near where the pipes enter, not on an outside door that swings open.

How to check a frost stat is working

Test it before the cold arrives. On an adjustable frost stat, turn the heating timer off, then turn the frost stat dial up above the current temperature of the space. The boiler should fire and the pump should run within a minute or two. Turn the dial back to its normal setting afterwards and check the boiler stops. A fixed-setting frost stat is harder to test without cooling it, so an engineer can check it at the annual boiler service.

If the boiler does not respond, check that the boiler has power at its fused spur and that gas is on. A frost stat cannot do anything if the power is off, so do not switch off the boiler or turn off the gas at the meter before going away in winter. If both are on and the frost stat still does not bring the heating on, the wiring or the stat itself needs looking at.

Fitting or repairing a frost stat

Fitting a frost stat means working on the boiler's control wiring and sometimes on the zone valve wiring centre. If you are upgrading heating controls anyway, it is easy to add at the same time. It is a job for a heating engineer who knows how your controls are laid out, particularly on a system with a cylinder, zone valves or a wireless thermostat. A wrongly wired frost stat can fire the boiler with nowhere for the water to circulate.

Loyal Plumbers installs and repairs heating controls, including frost stats and pipe thermostats, across London and everywhere inside the M25. Gas work is done by Gas Safe registered engineers, we run 10 vans 24 hours a day, and our call-out charge is £60. Call 020 4652 8585 or send an enquiry with the boiler make, where it is, and what the system has, such as a cylinder or zone valves.