The cold-health alert season starts on 1 November, which is less than three weeks away. If you rely on a heat pump or reversible air conditioning to heat a home, office or shop, that date is a useful prompt to do two things: sign up for the alerts, and check that your outdoor unit is ready for frost. This guide covers both, because a cold snap is exactly when a heating system that has been quietly cooling you all summer is asked to do the opposite.

When the alert season runs

The UK Health Security Agency (UKHSA) runs the Weather-Health Alerting System in partnership with the Met Office. According to its guidance page, the system covers heat and cold, "with the core alerting season for CHA running from 1 November to 31 March". CHA is the cold-health alert. The page adds that if cold meeting the alerting criteria occurs outside that window, "an extraordinary alert will be issued", so a harsh October or April is not ignored.

What the colours mean

Alerts are impact-based. UKHSA's user guide says each alert is given a colour (yellow, amber or red) based on the impact the weather could have and the likelihood of that impact. In short:

  • Green means no alert is issued, because conditions are likely to have minimal impact on health. It is the time for planning and preparedness.
  • Yellow (response) covers periods of cold "unlikely to impact most people but could impact those who are particularly vulnerable".
  • Amber (enhanced response) means impacts "are likely to be felt across the whole health service, with potential for the whole population to be at risk", and other sectors may start to see effects.
  • Red (emergency response) indicates "significant risk to life for even the healthy population".

A yellow alert is easy to wave off, and the guide specifically warns against that, saying it is vital that response plans consider yellow alerts. The guide's own mortality example is from summer heat, but the point carries over: a quiet colour can still matter to the people least able to cope.

Who should sign up

The alerts are designed as an early warning for the health and social care sector, the responder community, the voluntary and community sector and government departments. They are sent by email to anyone who registers, and the page asks you to share the registration link with anyone who should receive them. The Adverse Weather and Health Plan, which the alerts underpin, applies to England.

That list is not the whole audience in practice. If you manage premises, look after staff or tenants, run a care or community setting, or are responsible for someone vulnerable, the alert tells you a cold spell is coming while you still have time to act. You choose which regions to receive alerts for.

What frost does to an outdoor unit

This part is general engineering, not a quote from UKHSA, and it applies to air-to-air heat pumps and reversible air conditioning running in heating mode.

Defrost cycles. In cold, damp weather, frost forms on the outdoor coil, because the coil is colder than the air around it. The unit detects this and periodically runs a defrost cycle, briefly reversing the process to melt the frost. During a cycle the fan usually stops, the indoor unit may blow cooler air or pause, and a plume of vapour can rise from the unit. That cloud looks alarming but is normally just moisture. It is part of normal operation, not a fault.

Ice build-up. What is not normal is a thick layer of ice that stays put, a coil covered in ice for hours, or a unit that keeps running without heating the room. Do not chip at it. The coil fins bend and tear easily, and a damaged coil can mean a refrigerant leak. Switch the unit off and get it looked at.

Keep the unit clear. The outdoor unit needs airflow all round. Snow drifted against it, leaves packed into the back, or items stacked beside it restrict that airflow and make icing worse. Brush loose snow away gently by hand, and keep the clearance in the manufacturer's instructions.

Condensate drains. Defrost produces water, which has to go somewhere. If it drains onto a base that is below freezing, it can refreeze into a block under the unit or in the drain line, which then traps more water. Check that drain outlets are clear and that water has a route away from the unit and away from paths where it could ice over.

Why last January matters, and what to do now

In its deep dive on 7 January, the Met Office reported that on the Monday night "Marham recording a bone-chilling -12.5°C", with several other places below -10°C and Bournemouth dropping to -8.5°C. In northern Scotland, official stations measured 52cm of snow at Oyne and 50cm at Tomintoul. It put the cold in East Anglia and the south-east down to clear skies, not snow. Check the minimum outdoor operating temperature in your unit's manual so you know what it was designed for, and do not assume that figure matches the coldest night in your area.

Before the season starts:

  • Register for cold-health alerts if you are responsible for premises or people.
  • Clear leaves and debris from around the outdoor unit and check the airflow clearances.
  • Look at the drain route from the unit and the indoor condensate outlet, and make sure nothing is blocked.
  • Run the system in heating mode now, while it is mild, so a fault shows up before the first frost.
  • Read our tips on using AC in winter, and book air conditioning repair if the unit ices up heavily, leaks water or stops heating.

Winter weather rarely stops at the cold itself. Our note on storm names and power cuts covers what happens when the power goes too.