Every winter, I get calls from people worried about condensation inside their homes. Windows are dripping, ceilings are sweating, and mould is starting to show up in corners. One of the first things they often say is:
“I’ve been running the dehumidify function on my split system — but it’s not helping!”
Unfortunately, not only is it not helping, it’s often making the problem worse.
Let me explain why, and what you should be doing instead.
What ‘Dry’ Mode Actually Does
Most reverse cycle air conditioners have a “Dry” or “Dehumidify” setting. It’s designed to remove humidity by cooling the air slightly, just enough to condense some moisture out, like how a cold drink collects condensation on a humid day.
This function works well in warm, humid weather. The system gently cools the air, pulls moisture out, then reheats it slightly before blowing it back into the room.
But in cold weather, especially in winter, this mode just doesn’t work the way people expect it to.
Why It Backfires in Winter
Here’s what’s happening:
- The system still tries to cool the air to remove moisture.
- But the indoor air is already cold.
- So it cools the air further — often too far.
- That colder air drops onto your windows and walls.
- Now condensation gets even worse, because the air hits cold surfaces and dumps its moisture.
And in many cases, the AC won’t even run properly in Dry mode when the outdoor temperature is too low, which means it’s just pushing air around without actually doing anything useful.
What You Should Do Instead
If you’re having condensation issues in winter, here’s the better approach:
- Use Heating Mode on Your AC
- Reverse cycle systems are very effective heaters.
- Heating the air raises the temperature of room surfaces (like windows and walls), reducing condensation.
- It also lowers the relative humidity, because warmer air can hold more moisture.
- Ventilate
- Use exhaust fans in bathrooms, laundries and kitchens.
- Open windows for short bursts when safe to do so.
- If the house is very airtight, a balanced ventilation system (ERV/HRV) is worth considering.
- Avoid Moisture Sources
- Don’t dry clothes inside unless absolutely necessary.
- Avoid unflued gas heaters, they dump water vapour into the air.
- Use a Dedicated Dehumidifier if Needed
- Especially if you’ve got cold corners or rooms that don’t heat well.
- Desiccant dehumidifiers work well in winter, unlike AC Dry mode.
Summary:
Don’t use the ‘Dry’ setting on your air conditioner in winter.
It’s designed for warm weather, and using it in cold conditions will likely make condensation worse, not better.
Instead, run your AC in heating mode, keep the home ventilated, and manage moisture sources. That’s how you stay dry and mould-free through the winter months.
