Water dripping or running from the outdoor unit in heating mode is normal defrost water. The coil frosts, the heat pump defrosts, and the melt has to leave the base. On a damp cold day there can be a lot of it. An oil sheen on that water is not normal. Switch the system off and book an F-gas certified engineer. Route clear defrost water so it does not ice a path or sit under the electrics. Do not cover the unit to stop the drip. It has to breathe, including during defrost.
A cloud, a fan change, then water under the unit
In heating, the outdoor coil is the cold part. Moisture from the air freezes on it. The unit pauses its heating, melts the ice, and sends you warm air again indoors. You may see a cloud, hear a change in the fan, and then find water under the outdoor unit. That cycle can repeat.
A bedroom puddle is a different fault
The indoor unit is not usually the source of that water in heating. A puddle on the bedroom floor in heating is a different fault. A puddle under the outdoor unit, clear and only in cold damp weather, is the defrost doing its job.
Clear water, and a film that is not defrost
Route the melt so a path does not ice over. The look of the water is the check.
- Clear water in heating is normal defrost.
- It can be a surprising volume on a wet cold day.
- An oily rainbow film is not normal.
A drain, a gravel soak, or a fall off the path
Give it a drain, a gravel soakaway that actually drains, or a fall away from the path you walk. A unit on a bracket needs the water kept off the wall and off the neighbour below, not left to blow off the base pan at random. Ice on a step is the hazard to design out.
Leaves in the base pan
A blocked base pan heater outlet, where the model has one, or a pan full of leaves, can make defrost water spill in the wrong place. Clearing leaves is fair. Opening the refrigerant circuit is not. Do not cover the unit to hide the drip.
Ice that never clears is not a heavier drip
Oil sheen, a persistent oily smell, or water that is visibly contaminated means stop and call a certified company. It can mean oil from the refrigerant circuit is escaping with a leak. Do not keep running it to see if it clears. Ice that never clears, a unit that stays in defrost, or no heat indoors for long periods is a fault, not a heavier drip. Energy Saving Trust's air-to-air heat pump notes describe heating mode. They do not describe leak diagnosis. That is the engineer's visit. An F-gas engineer is who you book for an oil sheen.
Frequently asked questions
Why is my outdoor unit dripping in heating mode?
It is defrosting. Frost melts off the outdoor coil and runs out of the base. Clear water is normal. Route it so paths do not ice over.
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How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.