Heat pumps leak water in winter because they have a defrost cycle governed by a control board. When outdoor temperatures drop toward freezing and below, the control board will call for defrost when certain conditions are met to reverse the unit into the cooling mode, heating the outdoor coil and melting frost or ice of the coil.

The melting frost or ice is the water you see under your heat pump’s outdoor unit.

It would be concerning if you never saw water dripping under your heat pump. The amount of water or ice accumulation is closely related to the outdoor temperature and humidity. The lower the outdoor temperature and the higher the humidity, the more water you should see.

Think about a window air conditioner. Cold air blowing inside. Have you ever walked by a widow unit on the outside and felt the hot air? Imagine turning that window unit around and blowing the hot air inside the house. You have just created, in your mind, a simple heat pump.

But a problem just came up. When you want that heat in your home, it’s going to be cold outside, and the temperature of the air going over the cold coil, which is now outside, is getting much colder because of the outside air temperatures.

When the outdoor ambient drops near freezing and below, the coil will begin to freeze over, and the airflow through the coil will be restricted.

Somehow, we need to defrost that ice. That’s where the water comes from under your heat pump in the winter.

Should My Outdoor AC Unit Be Leaking Water?

About 8 out of 10 of my customers don’t understand the difference between a heat pump and an air conditioning unit. By looking at the outside, you can’t tell the difference.

Only one slight difference exists between a heat pump and a cooling-only unit. Where the copper lines attach to the unit, there is a third port for the technician to attach his gauges. But, peering down inside, past the motor and fan blade, a heat pump includes much more piping and valves.

heat pumps leak water outside in winter
Heat pumps leak water in winter as a result of defrosting

Should My Window AC Unit Leak Water Outside?

This is a great question. The answer will help you understand why your heat pump outdoor unit leaks water. Whole house heat pumps, air conditioners or central HVAC systems, and window units have indoor and outdoor coils.

Basic Refrigeration Cycle

These two refrigeration coils have fans that move air through the coils. The indoor coil picks up the heat in the house, and the outdoor coil dissipates that heat to the outside.

The compressor pumps hot, high-pressure refrigerant (Freon) into the outdoor coil against a metering device where the fan blows over the coil to move the heat to the out-of-doors.

Most window units are designed to cool a room. When inside air is circulated through the indoor coil, the coil picks up the heat in the room and transfers this heat to the outdoor coil through the refrigerant medium or what most people call Freon (a brand name).

That’s why you feel the much hotter air outside.

But because the inside of your home is above freezing, the indoor coil doesn’t freeze up like a heat pump outdoor coil in the winter. Instead, it will create condensation.

That condensation is drained through tubing or channels along the bottom of the window unit to the outside. Hence, there is a constant drip from the unit onto your flower bed or whatever is under the unit.

Are You Asking How To Fix A Leaking Heat Pump?

Now you know an outdoor heat pump unit leaking water outside is a natural event. But questions have to be specific. When your heat pump is mainly in the cooling mode in the summer, is it leaking inside?

You need to fix a condensation leak at the indoor air handler or furnace. Specifically, the indoor coil drain pan and line leading to either a condensation pump or drain line somehow removes that condensation from your house.

Depending on your level of maintenance and the expertise of the technicians working on your equipment, several things could be causing the leak.

  • The drain line could be plugged.
  • The coil pan rusted out or cracked.
  • Low on refrigerant.
  • Coil dirty or plugged.
  • Filter too small or dirty.
  • Not enough airflow.

Drain pans under indoor coils tend to get dirty and fill with grime and mold. This debris collects in the pan and can plug the drain hole. Indoor coils can freeze up in the summer just like heat pump outdoor coils can freeze up in the winter.

This coil froze because the homeowner turned his thermostat to cool, set the temperature below 70 degrees on a warm spring day, and then left it running all night when the outdoor temperature dropped. The air filter was also quite dirty, limiting the amount of airflow through the coil.

iced up indoor coil
Coils can ice up due to low airflow or low refrigerant

A Pool Heat Pump Leaking Water Can Be A Different Problem

Although a pool heat pump has many of the same parts as a residential air-to-air heat pump, it has a different type of coil or heat exchanger to heat the pool water. Instead of a coil with a fan, it’s a coil with water to pick up the heat from the heat pump.

This heat exchanger can be a chamber within a chamber, one with refrigerant and the other with water.

Do a Google image search for pool heat exchangers and get a look at the types of coils that perform this function.

You have another leak source because you would now have another coil with water.

Separating now what you’ve learned about other water leaking scenarios, a heat pump leaking water outside the unit in the winter is proper for its operation. Again, it depends on temperature and humidity.

Heat pumps should be raised up off the pad to allow for drainage. The bottom pan in the outside heat pump unit has drain holes that need to be kept clear for proper drainage.


Heat pumps and air conditioners are different because of the functions they provide. Heat pumps heat and cool, and an AC unit only cools.

Having a puddle of water under a heat pump in the winter is a good thing as long as the heat pump is raised off the pad to allow for drainage.

When a heat pump is flat on the pad and drainage is limited, ice can form around the bottom of the unit, causing damage to the coil. Ice forming around the aluminum fins and copper tubing has a crushing effect that destroys the coil.

Keep your unit maintained properly with checkups from a qualified service company.

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  1. Chad,
    We replaced our 2002 Bryant HVAC ducted flex system with a new Mitsubishi heat pump in May 2021.
    We were looking to use the heat pump in the winter, as often as we could, to save on propane (our main source of heat is radiant heat in the floor) and also having the air conditioning in the summer.
    Shortly after it was installed, we noticed a musty odor, which wasn’t there with the old system.
    In August, we had it serviced and there was no water.
    In November, we turned it off and capped off all the vents due to the odor.
    Today, the flex ducts and both plenums were removed and we are having hard insulated metal ducts installed. The contractor reported that a lot of water had collected in both plenums, on the floor of the air handler, in the drain pan and also, the exterior plenum insulation was soaked.
    We live in Maine and are confused as to why so much water had accumulated since there was no water in August, we haven’t used the system since November and the vents were capped off.
    Do you have any thoughts or explanations? We appreciate any feedback.
    Thanks so much!

    1. Gail, really hard for me to know the water source. I’ve diagnosed many situations like that one and they are all very different. Just recently a guy flooded his system and ruined the circuit board to the tune of about $800, he had plumbed his condensate drain from the cooling coil into the drain of his water softener. When the water softener flushed it flooded his furnace. Another one was a condensate trap that was installed upside down and wouldn’t drain except into the duct work. It’s really hard to say without being there.

  2. Chad. I had my heat pump worked on and the tech turned the drain pipe upward and then the drain water from my heat pump backed up in the bottom of my unit. And it looks like it has collected in there causing black mold and should that be cleaned out so the unit does not rust out the metal unit sitting on the concrete pad?

    1. Dolene, I’m unclear if this is inside or outside. It sort of sounds like the outdoor unit is what you are talking about. If so I would worry too much but cleaning never hurts. If indoors definitely clean it. -Chad

  3. Hi Chad

    Appreciate this website. It is incredibly helpful.

    I have a ductless mini-split system which both cools and heats. Currently it is spring and the temperature is about 50 degrees outside so the unit is set to heating. I notice a consistent drip of water coming out from the bottom of the outdoor unit. Humidity is currently around 92%.

    From reading the above it seems like this is not something to worry about but I wanted to confirm to be safe.

    Any advice is appreciated.

    1. JS, You’re good. Condensation outside in winter and condensation inside in summer. Enjoy the coming summer, Chad

  4. Heat pump is on a pad and leaks alot of water. I’m in southern Calif and it doesn’t get that cold at night. Know other people say they don’t have this problem. Technician said he could put a drain pan underneath and PVC pipe to a drain $400. Do heat pumps come with drain pans?

    1. Bill, I have a lot of questions about your question. Is the pad a patio or an isolated pad sitting on the dirt? Is the water hurting anything? If not, just let it drain. Sounds a little strange to have a lot of water draining in So Cal. Is the night time temp getting down into the 40’s? If so that low enough to cause some defrosting and water to drain off. Maybe have another opinion on the refrigerant level in the unit. If your unit is the only one you know of in your area that is draining off water I’d get your’s checked out closer. – Chad

      1. Thanks for answering Chad! It sits on a pad on cement on the side of the house. The water collects but there is a small drain hole at the bottom of a block wall where some water drains. I’m concerned of the water just sitting there, producing algae and again why so much water. We really don’t use the heater that much. Keep at 67 degrees. It also doesn’t get alot of sun on the side of the house. Person that installed said it’s normal, but I don’t know. Thanks for answering.

  5. Chad,

    My heat pump is leaking a lot of water…maybe half a gallon overnight, possibly. At least it looks that way.
    I’m in South Carolina and the temperature only gets down to low 40’s overnight. It is very humid here, however.
    Any ideas?

    1. Mike,
      It sounds and like it’s working the way it should. That night time temperature can drop across your outdoor coin into the freezing range for all that glorious humidity you guy have down there. You probably don’t need a 30 minute defrost cycle like we do here in the high desert.

  6. Heat pump is rests on a small platform above the back door so when it defrost in winter the dripping water freezes on the steps creating a hazzard. Can I install a pan under the unit and divert the water away?

    1. Robert,
      That sounds like an odd place, but yes you can do most anything imaginable to divert the the draining water. Could I use your email to send you mine to get a picture of that?

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