Why Your Heat Pump Steams and Goes Quiet on Cold Phoenix Mornings

A heat pump that goes quiet, whooshes and sends up a cloud of steam on a chilly morning is most likely running its defrost cycle. Defrost mode is a normal, automatic routine that melts frost off the outdoor coil so the system can keep heating your home. In most cases, you don't need to do anything at all.

It still helps to know what normal defrost looks like, because a few signs point to something else. At Viking Heating & Air Conditioning, a family-owned heating and cooling company in Chandler, we'd rather you understand your system than worry about it.

What Is Heat Pump Defrost Mode?

A heat pump doesn't burn fuel to make heat. Instead, it moves heat. It pulls heat from outside into your home in winter and out of your home in summer. If you're still comparing heating systems, our complete residential heating guide for Phoenix Valley homeowners walks through the main options.

To pull heat out of winter air, the heat pump's outdoor coil can get much colder than the air around it. The coil is the grid of thin metal fins inside the outdoor unit. When moist air passes over that cold coil, frost can form on it. A layer of frost works like insulation, which blocks the coil and makes the system work harder for less heat.

Defrost mode clears that frost and ice off the outdoor coil. An automatic control reverses the system and stops the outdoor fan to defrost the coil when needed. For a short time, the heat pump switches from heating to cooling mode, which sends warm refrigerant to the outdoor coil. Once the frost has melted, the system goes back to heating your home.

Key Takeaway: Defrost mode is a built-in step, not a breakdown. Your heat pump briefly reverses itself to melt frost off the outdoor coil, then returns to heating.

What Defrost Mode Looks and Sounds Like

During defrost, the outdoor fan stops, so the steady fan noise goes away for a short time. The unit may make some strange hissing or gurgling noises. You may also hear a whooshing sound as the system shifts modes. These sounds are normal.

Then comes the steam. Our technicians explain it this way: when a heat pump goes into defrost, it heats its frozen outdoor coil, and that is what causes the steam. The cloud may rise from the outdoor unit for a short time, and that is normal and harmless. The steam is water vapor.

You may also see water running out from under the unit. That water is melted frost. It must drain away from the unit to prevent damage from ice buildup.

How often defrost happens depends on your equipment. Some units start defrost on a timer. Others use an electronic control that senses coil and air temperatures to decide when a defrost cycle is needed.

Key Takeaway: A quiet outdoor fan, a whoosh, a short burst of steam and a little water under the unit are all normal signs of defrost. None of them on its own means your heat pump is failing.

Why It Happens on Cold, Damp Phoenix Mornings

It's easy to assume a heat pump in the desert should never frost up. But the coil doesn't need freezing air to collect frost. While the refrigerant is pulling heat, the coil's surface can be much colder than the surrounding air. That's why frost can form even when the weather feels mild, and it may start to form on the outdoor coil when the outdoor air turns cool but is still above freezing.

Phoenix winter mornings can get cool enough for that. The National Weather Service's Phoenix New Year's climate page lists a normal low of 45 degrees for January 1, based on 1991–2020 normals. That's above freezing, but it's not unusual to see a heat pump defrosting in weather that mild. So an occasional defrost cycle on a cold Valley morning shouldn't surprise you.

Moisture matters too. The outdoor coil can get cold enough for moisture in the air to freeze onto it, especially when temperatures hover around freezing and humidity levels are high. That's why you're more likely to notice defrost on a cold, damp morning, such as after a winter rain, than on a warm, dry afternoon. For other ways to get ready for the cooler months, see our guide on how to prepare your HVAC system for colder weather in the Phoenix Valley.

Why Does the Air From Your Vents Feel Cooler?

You may notice a slight drop in temperature during defrost. Warm air production pauses briefly while the system melts frost outside. There are two reasons the air from your vents can feel cooler.

First, while the heat pump is melting frost outside, it isn't moving heat into your home in the usual way. To cover that gap, many heat pumps automatically switch on backup or auxiliary heat strips during defrost. Those strips keep the temperature inside from swinging.

Second, heat pump air feels different from furnace air even when everything is working. In heating mode, the air from your supply registers is only a little warmer than the air already in your home. It will feel cool if it blows directly on you. That is normal heat pump behavior and doesn't mean defrost has gone wrong.

Sometimes your home keeps losing heat through the morning, or the air stays cool long after the steam has cleared. In that case, it's worth having the system checked.

Steam or Smoke? How to Tell the Difference

Defrost steam follows a clear pattern. It shows up while the outdoor unit is defrosting, rises for a short time and fades once the frost has melted. It is water vapor, and water vapor has no burning smell.

Smoke doesn't fit that pattern. If what you see is dark, hangs around, smells like something burning, or keeps coming when the unit isn't defrosting, it isn't defrost steam. Don't wait for it to clear up on its own, and have a technician check the unit. If there is a fire, the U.S. Fire Administration says to get outside to your meeting place, stay there and call the fire department.

Key Takeaway: Defrost steam is short-lived, has no burning smell and only shows up during a defrost cycle. Anything dark or lingering, or anything that smells like burning, is not defrost and needs a technician.

Signs Something Is Not Working Right

Defrost should start, finish and hand the job back to heating on its own. Rheem's heat pump owner's manual, for one, lists reasons to call a service technician, and several of them fit here. Watch for these signs:

• Ice that keeps building. The manual says a frost-covered coil is nothing to worry about "unless it continues to build up a thicker layer with areas of hard clear ice." Excess ice buildup is also on its list of reasons to call.
• New screeching or grinding noises. A whoosh during defrost is normal. The manual lists new, unusual noises as a reason to call a service technician.
• Defrost that runs too often or too long. If your heat pump seems to enter the cycle too often or runs for unusually long periods, that may point to a problem.
• Short-cycling. The manual lists a heat pump that is short-cycling (turning on and off rapidly) and not heating or cooling properly as another reason to call.

When defrost goes wrong, the cause can be in the parts that control it. Our heating repair team works on heat pumps, and we can find out whether the problem is the defrost system or something else.

Key Takeaway: Frost that clears during defrost is normal. Ice that keeps building, new screeching or grinding, and defrost that seems stuck all call for a service visit.

What You Can Safely Check Before You Call

Some heat pump problems have simple causes. Rheem's owner's manual lists these checks a homeowner can make before calling for service:

• Make sure the power is on, and check the breakers or fuses for both the indoor and outdoor units.
• Look for anything restricting airflow, including blocked grilles and registers, dirty filters and a blocked outdoor coil.
• Confirm that your thermostat is set properly.
• Keep fences, shrubs and other obstructions at least two feet from all coil air inlets, and never use a weather cover over the outdoor unit.

Filters deserve a quick look. Heat pumps work best with clean filters, and dirty filters can restrict airflow. The same manual says to keep the coil free of grass clippings, weeds and leaves. It also says to turn the electricity off before cleaning, and never to use the heat pump as a stand for garden hoses or tools.

Past those basic checks, we recommend leaving the coil, the wiring and the refrigerant parts to a licensed technician. A seasonal visit through our preventative maintenance program is a good time to have your heat pump looked over before the coldest mornings arrive.

Key Takeaway: You can check power, filters, thermostat settings and clearance around the outdoor unit yourself. Leave ice, wiring and refrigerant work to a professional.

Frequently Asked Questions

Is it normal for my heat pump to send up steam in winter?

Yes, in most cases. During defrost, a heat pump warms its outdoor coil to melt frost. The melting frost can rise as a cloud of steam. Heat pump makers describe this steam as normal, harmless water vapor. The steam should last a short time and then clear. If what you see is dark, lingers, or smells like something burning, it is not defrost steam, and a technician should check the unit. If there is a fire, the U.S. Fire Administration says to get outside to your meeting place, stay there and call the fire department.

Why does the outdoor fan on my heat pump stop in winter?

If the outdoor fan stops for a short time on a cold day, the heat pump may be defrosting. An automatic control reverses the system and stops the outdoor fan to defrost the coil when needed. With the fan off, the steady fan noise goes away. The unit may make some strange hissing or gurgling noises, and you may hear a whooshing sound as the system shifts modes. Once the frost melts, the heat pump returns to heating. If the fan seems to stay off for unusually long periods, have the system checked.

How often does a heat pump go into defrost mode in Phoenix?

It depends on your equipment and the weather. Some heat pumps start defrost on a timer. Others use an electronic control that senses coil and air temperatures. Frost is especially likely when temperatures hover around freezing and humidity levels are high. In the Valley, that means you are most likely to notice defrost on cold, damp winter mornings. If your unit seems to defrost too often or for unusually long periods, that may point to a problem.

Can a heat pump frost over when it isn't freezing outside?

Yes. The outdoor coil can be much colder than the surrounding air while the refrigerant is pulling heat. When that combines with moisture in the air, frost can form even when the weather feels mild. Frost may start to form on the outdoor coil when the outdoor air turns cool but is still above freezing. A layer of frost that melts during defrost is normal. Rheem's owner's manual says not to worry unless the frost keeps building into a thicker layer with areas of hard, clear ice.

Is a screeching or grinding noise part of defrost mode?

Not as a normal defrost sound. During defrost, you may hear a whooshing sound as the system shifts modes, and the unit may make some strange hissing or gurgling noises. Those sounds are normal. Screeching and grinding are not among those normal defrost sounds. Rheem's owner's manual tells homeowners to call for service if they hear new, unusual noises. Treat a new screech or grind as a reason to have the heat pump checked instead of waiting for it to go away on its own.

Should I cover my heat pump's outdoor unit for winter?

No. Rheem's owner's manual says to never use a weather cover over the outdoor unit, even if it is turned off for long periods of time. The manual explains that a cover will hold moisture in the unit. That causes more rust buildup and damage to electrical parts than exposure to weather. The same manual says to keep fences, shrubs and other obstructions at least two feet from all coil air inlets. It also says to keep the coil free of grass clippings, weeds and leaves, and to turn the electricity off before cleaning.

What can I check myself before calling for heat pump service?

Rheem's owner's manual lists a few safe checks. Make sure the power is on, and check the breakers or fuses for both the indoor and outdoor units. Look for anything restricting airflow, such as blocked vents, a dirty filter or debris around the outdoor coil. Then confirm your thermostat is set properly. Heat pumps also work best with clean filters. We recommend leaving refrigerant, wiring and defrost control repairs to a licensed technician.

Normal Defrost, and When to Call

On a cold Phoenix morning, a heat pump that goes quiet, whooshes and sends up a short cloud of steam is most likely defrosting. It's melting frost off the outdoor coil so it can keep heating your home, and it goes back to normal on its own. Watch instead for ice that keeps building and for new screeching or grinding. Defrost that seems to run too often or too long also needs a look, and so does anything that looks or smells like smoke.

Still unsure whether what you're seeing is normal? Call us at (480) 689-5167 or send us a message, and we'll help you work out whether it's routine defrost or something that needs a closer look.