Steam, Drips and a Pause in the Warm Air: Making Sense of Your Air-Con's Defrost Cycle

There's a particular sort of phone call we get around this time of year. It's usually just after seven, the caller is standing at a kitchen window, and the gist is: "my air-con is on fire." Nine times out of ten it isn't. It's a reversible system doing its defrost cycle, and the outdoor unit is steaming gently into a cold Sutton morning like a cup of tea left on a windowsill.

So let's talk about what's actually happening out there, because once you understand the sequence, the panic tends to evaporate.

Why the outdoor unit frosts up in the first place

A heat pump — which is what your reversible air-con becomes in winter — doesn't make heat from nothing. It moves it. Even on a raw November morning, there's usable warmth in the outside air, and the system extracts it by pulling outdoor air across a cold coil.

The catch is that cold coil. Take the coil below the dew point and below freezing, and the moisture in the air condenses onto it and then sets solid. You get a layer of white frost building across the fins, and frost is an insulator. Left alone, it would smother the coil and the system would gradually stop being able to gather any heat at all.

So the manufacturer builds in a way to clear it. That's defrost.

What the machine does during a defrost cycle

Here's the bit that surprises people. To melt ice off the outdoor coil, your system temporarily runs the refrigeration circuit in reverse — effectively in cooling mode. That sends hot refrigerant gas out to the outdoor unit, which warms the coil, which melts the frost.

The visible side effects, in order:

• The indoor fan stops. No point blowing cold air at you while the system is busy elsewhere, so it pauses. You may notice the room go quiet, or feel a faint draught.
• The outdoor unit steams. Melted frost on a warm coil in cold air means evaporation, and evaporation looks like smoke. It's water vapour, not combustion.
• Water appears underneath. All that melted frost has to go somewhere. It runs out of the base of the unit and onto the patio, the balcony or the path.
• The outdoor fan may stop or slow. Many units do this deliberately to speed the melt.
• A gurgle or a whoosh. That's the reversing valve shifting. Perfectly normal, if a little dramatic sounding.

Then the valve shifts back, the indoor fan restarts, and warm air returns.

How long is normal?

For most domestic systems installed around Sutton, a defrost run lasts somewhere between three and fifteen minutes. In damp, near-freezing weather the system may do it quite often — perhaps every forty to ninety minutes — because damp cold air ices a coil faster than dry cold air. On a still, dry, minus-two morning you might barely notice one all day.

The useful rule of thumb: a defrost that finishes and hands back to heating is fine. A defrost that never seems to finish, or one that starts again three minutes after the last, is worth investigating.

The homeowner checklist

Before you pick up the phone, run through this. It covers the overwhelming majority of cold-morning queries.

1. Watch one full cycle from start to finish. Time it. Note whether heating comes back on its own afterwards.
2. Check the outdoor unit is clear. Leaves, hedge trimmings, a stray bin bag, or the cover someone put over it "to protect it from the weather" — all of these upset airflow and encourage icing. Never cover a unit that's in use.
3. Look at the base and the drainage route. Water should run away freely. If it's pooling and refreezing around the feet of the unit, that's worth a call.
4. Check for a solid block of ice rather than a thin white crust. A crust melts in minutes; a block doesn't.
5. Glance at the indoor unit. Filters clogged with dust reduce airflow and can make defrost behaviour worse. A quick vacuum of the filters takes five minutes.
6. Leave the system on. Switching it off at the wall overnight doesn't help — it just means the coil starts cold and ices faster tomorrow.
7. Note any error code on the display or the app. That single piece of information saves a lot of diagnostic time.

When it's time to book rather than wait

Get someone out if you see any of these:

• Ice that's still there an hour after the cycle should have cleared it.
• Defrost running every few minutes, with barely any heating in between.
• The indoor unit blowing cold for long stretches with no return to warm air.
• Water appearing inside the property rather than outside.
• A flashing error code, or a system that's shut itself down.
• The outdoor unit visibly iced at the base and sides, not just across the front fins.

Most of these point to a sensor, a reversing valve, a low refrigerant charge or a drainage issue. None of them are DIY territory, and none of them get better by being ignored until February.

A case note from a Sutton flat

Late last November we were called to a two-bedroom flat off Brighton Road. The caller was convinced the system was leaking gas, because the outdoor unit on the balcony had produced a cloud of steam and then the flat went cold for what felt like ages.

We timed it on arrival: fifty seconds of steam, eleven minutes with the indoor fan off, then back to heating. Textbook. What had actually caused the drama was a build-up of fallen leaves jammed behind the balcony unit, restricting airflow and forcing the system into more frequent defrosts than it needed. We cleared the debris, cleaned the filters, checked the drainage and left. No parts, no refrigerant, no drama — just a system that had been working harder than it should because of a few handfuls of leaf litter.

A quick word on winter servicing

Defrost problems are one of the reasons we suggest a pre-winter check rather than waiting for something to fail in January. A service won't stop frost forming — that's physics — but it will confirm the sensors are reading correctly, the drainage is clear, the refrigerant charge is where it should be, and the coil is clean enough to defrost efficiently rather than laboriously.

If your system is steaming, dripping and pausing on cold mornings and then getting back to work, that's the design doing its job. If it's doing any of those things for too long, or not coming back at all, give Sutton Cooling Services a ring and we'll take a look before the cold really settles in.