Does an Air Conditioner Need Running in Winter to Keep It Working?

This question comes up every autumn, usually from someone who’s read conflicting advice. The confusion is real, and it comes from applying split-system guidance to portable units.

Portable units: storage matters, running doesn’t

A portable air conditioner is a sealed appliance that spends most of the year switched off. It’s designed for that.

What actually determines whether it works next June isn’t whether you ran it in January — it’s whether moisture was left inside it in October.

A portable unit condenses water onto an internal coil every time it runs. Put it away damp and you’ve created a sealed, dark, humid box for eight months. Mould grows, and the smell that greets you next summer is the result. That’s the failure mode, and it’s a storage problem rather than a disuse problem.

Our winter storage guide covers the routine: run it on fan-only for two to four hours to dry the coil, drain the tank fully, clean and fully dry the filter, disconnect and loosely coil the hose, and store it upright somewhere dry.

Fixed split systems: different answer

Here’s where the “run it monthly” advice comes from, and for split systems it has more foundation.

A split system is a permanently installed refrigerant circuit with a compressor sitting outside. Common reasoning for occasional winter operation:

• Compressor oil circulation. Oil circulates with the refrigerant; long total dormancy is generally considered less good for it than periodic operation.
• Seal condition. Rubber seals and gaskets can dry out or stiffen over extended periods without use.
• Early fault detection. Running it briefly tells you it works, months before you need it, when an engineer is actually available.
• Moving parts. Fans and louvre motors that never move can seize.

Many split systems are also reverse-cycle, meaning they heat — in which case you’re using it through winter anyway and the question doesn’t arise.

If yours is cooling-only: follow your manufacturer’s guidance rather than general advice. Some specify a minimum outdoor operating temperature below which cooling mode shouldn’t be run. Where periodic running is recommended, it’s usually a short run on a milder day rather than in a hard frost.

Heat pump portables: use them, that’s the point

If your portable unit is reverse-cycle, the whole question dissolves. You’re running it through winter as a heater, so it isn’t sitting idle at all.

This is genuinely one of the better arguments for buying a heat pump model — it works most of the year rather than sitting in a cupboard for eight months. Our guide to using an air conditioner as a heater covers how to check whether yours can, and note the window kit stays fitted in heating mode.

What genuinely helps a stored portable unit

1. Dry it out before storage. The single most important step, and the one most people skip.
2. Store it upright. Most manufacturers specify this so refrigerant and compressor oil settle correctly.
3. Somewhere dry and temperature-stable. A spare room or dry loft beats a damp garage or shed that swings between wet and frozen.
4. Loosely coiled hose. Folded tightly for months, the accordion material creases permanently and restricts airflow.
5. Clean filter, fully dry, refitted. Storing a damp filter is a direct route to mould.
6. Cover it loosely against dust if you like — a bag or sheet is fine, provided it went in dry.

Bringing it back into service

Do this in March or April, not June. Our spring reset guide covers the full changeover, but the essentials:

• Stand it upright for a few hours before switching on, particularly if it’s been moved or tilted.
• Check the filter and clean if dusty.
• Inspect the hose for creases or splits.
• Run it for twenty minutes on a mild day. Confirm it powers up, the compressor engages, and the air is genuinely cold.
• Smell it. Musty means moisture got in — our smell guide covers fixing it.

Finding a problem in spring means you have options. Finding it in a July heatwave means competing with everyone else for stock and engineers.

What about the window kit?

A practical winter question people ask alongside this one.

If the unit is cooling-only and stored away, take the kit out and store it with the unit — leaving it in place all winter is an unnecessary gap in your window, working against the draught-proofing covered in our draught-proofing guide.

If the unit is reverse-cycle and heating your room, the kit stays fitted — heating mode still needs to exhaust air outside, and a poorly sealed kit undermines heating exactly as it undermines cooling.

Frequently asked questions

Sources

• GOV.UK — Fluorinated gases (F-gases): rules (refrigerant circuit work and certification)
• Energy Saving Trust — Home appliances and energy use
• Energy Saving Trust — Air source heat pumps (reverse-cycle operation)

Related reading: our winter storage guide and spring reset guide.