# Why does a heat pump go through a defrost cycle and ice up in winter?

> Why your heat pump steams, frosts and goes quiet on damp Memphis mornings, what a normal defrost cycle looks like, and when ice on the unit means a problem.

URL: https://blufflinememphisheatpumps.com/guide/heat-pump-defrost-ice/

Learn

Frost, steam, a whoosh and a pause are normal. A block of ice that stays isn't. Here is how to tell them apart.

Last Updated: November 12, 2026

The short answer

In cold, damp weather the outdoor coil runs below freezing and collects frost. Every so often the heat pump reverses into a short defrost cycle, warming the coil to melt it; you may see steam and hear a whoosh. That is normal. Ice that covers the coil for hours, builds up from the bottom, or coats the top fan is not.

Key takeaways

- Frost on the outdoor coil below about 40 degrees in damp weather is normal.
- A defrost cycle lasts a few minutes, with steam, a whoosh and the outdoor fan stopped.
- Backup heat runs during defrost so cool air doesn't blow from the registers.
- Thick ice that stays, ice on the top fan, or ice building from the base needs a look.
- A raised pad and clear drainage stop meltwater from refreezing under the unit.

## Frost is normal in Memphis winters

On a cold, damp Memphis morning, your heat pump’s outdoor coil runs colder than the air around it. That is how it pulls heat out of the air. When the coil drops below freezing, moisture in the air freezes onto the fins as frost.

[Bluffline Memphis Heat Pumps](https://blufflinememphisheatpumps.com/) gets asked about this every winter, so here is what normal looks like and when to call.

### Why Memphis is frost country

Memphis winters are mild but damp. The worst frosting happens between about 25 and 40 degrees with high humidity, a common Memphis January morning. Colder, drier air frosts the coil less.

## What a defrost cycle looks like

To clear the frost, the heat pump briefly reverses into cooling mode. Hot refrigerant flows through the outdoor coil and melts the frost.

### What you’ll see and hear

- **The outdoor fan stops**, so the heat stays in the coil.
- **A whoosh** as the reversing valve shifts the refrigerant’s direction.
- **Steam** rising from the coil as frost melts.
- **Water** running off the coil and out of the base.
- **Another whoosh** as it switches back to heating, and the fan restarts.

The whole cycle usually takes a few minutes.

### Inside the house

During defrost the indoor coil runs cold, so the thermostat runs the backup strips, or the furnace on dual fuel, to keep warm air at the registers. The [aux and emergency heat guide](https://blufflinememphisheatpumps.com/guide/auxiliary-emergency-heat/) explains that staging.

## How the heat pump decides when

Older heat pumps defrosted on a timer, every 30, 60 or 90 minutes of run time whether they needed it or not.

Modern units use temperature sensors on the coil and outdoor air, and some measure the pressure difference, so they defrost only when frost has built up.

### Why that matters

Each defrost costs energy: the heat pump stops heating and the backup heat runs. Demand defrost saves those cycles in dry weather and makes sure they happen in wet weather.

## When ice means a problem

Some ice isn’t normal. Call for a check if you see:

- **The whole coil encased in ice** for hours, not just frost on the fins.
- **Ice on top of the unit or on the fan** blades.
- **Ice building up from the base**, layer by layer.
- **The fan running against ice**, making a scraping noise.
- **No defrost cycles at all** during a long cold, damp spell.

### Common causes

| Symptom | Usual cause |
| --- | --- |
| Never defrosts | Failed defrost board, sensor or reversing valve |
| Ice builds from the bottom | Unit sitting too low; meltwater can’t drain and refreezes |
| Ice from a gutter drip | Roof runoff landing on the unit |
| Heavy frost every cycle | Low refrigerant charge or a dirty coil |
| Ice on the top fan | Freezing rain, or water dripping from above |

## Placement that prevents ice

Where the unit sits matters as much as the controls.

### Raise it

A raised pad, several inches above the ground, lets meltwater drain away from the base before it refreezes. A unit sitting on a sunken slab in a low corner of the side yard ices up from the bottom on every cold night.

### Keep roof water off it

A gutter that overflows, or a roof edge without a gutter, can drip straight onto the unit. In freezing rain that becomes a sheet of ice on the top fan. A gutter extension or a short diverter fixes it.

### Leave room around it

Shrubs, fences and walls too close to the coil trap cold air and moisture and restrict airflow, which makes frosting worse. The [placement and noise guide](https://blufflinememphisheatpumps.com/guide/outdoor-unit-placement-noise/) covers clearances.

## Frosted or frozen?

The difference is how much and how long. Use this as a rough guide when you look at the unit on a cold morning:

| What you see | What it usually means |
| --- | --- |
| A thin white layer on the fins, clearing every hour or so | Normal frost and defrost |
| Frost on the coil but the top fan and cabinet clear | Normal |
| Steam and dripping for a few minutes, fan stopped | A defrost cycle in progress |
| Solid ice you can’t see the fins through, lasting hours | Defrost not working, or low charge |
| Ice on the top grille and fan | Freezing rain or water from above |
| A ridge of ice growing up from the ground | Poor drainage or a pad that is too low |

### Inside signs

A defrost problem often shows up indoors first: rooms slowly falling behind on a cold, damp day, aux heat running much longer than usual, or cool air from the registers for more than a few minutes at a time.

## How long a defrost should last

Most defrost cycles end within about two to ten minutes, once the coil sensor reads warm enough or a maximum time passes. A cycle that runs much longer, or one that repeats every few minutes, points to a sensor, control or charge problem.

### On dual fuel

On a [dual fuel](https://blufflinememphisheatpumps.com/dual-fuel-heat-pump/) system the furnace usually fires during defrost instead of strips. If the switchover sits above the outdoor temperature, the heat pump isn’t running and won’t frost at all; frost only matters on the days the heat pump carries the house.

## Ice storms

Freezing rain is different from frost. It coats everything, including the top fan, and the defrost cycle can’t clear ice that formed on the outside of the cabinet.

### What to do

1. If the fan is frozen or the unit is encased, switch the thermostat to emergency heat.
2. Don’t chip at the ice; the fins and coil bend easily.
3. Let it melt as the weather warms, or ask for help.
4. Switch back to normal heat once the fan turns freely.

Power often goes out in the same storms. The [ice storm and outage guide](https://blufflinememphisheatpumps.com/guide/heat-pump-ice-storm-power-outage/) covers both.

## Defrost and the bill

Defrost cycles add a little to winter costs, because the backup heat runs and the heat pump pauses.

In a typical Memphis winter that is a small part of the heating bill. Excessive defrost, from a dirty coil or low charge, costs more and is worth fixing. A thermostat that brings on more strip heat than defrost needs is a setup issue; the [thermostat setup guide](https://blufflinememphisheatpumps.com/guide/heat-pump-thermostat-setup/) shows where those settings live.

## What to check yourself

Without opening the unit, you can:

- Keep leaves, pine straw and snow clear of the base and the coil.
- Make sure nothing drips on the unit from the roof.
- Watch for steam and the fan stopping, which shows defrost is working.
- Change the indoor filter, since poor airflow indoors affects outdoor frosting too.

## On a new installation

At start-up we set the unit level on a raised pad, check the defrost sensor and force a defrost cycle to confirm it works and that the backup heat covers it. The [commissioning checklist](https://blufflinememphisheatpumps.com/guide/heat-pump-commissioning-checklist/) lists the test.

### Older units

If your heat pump is past 15 years and its defrost board or reversing valve has failed, the repair may not be worth it. The [repair or replace guide](https://blufflinememphisheatpumps.com/guide/repair-or-replace-heat-pump/) walks through the math, and [heat pump replacement](https://blufflinememphisheatpumps.com/heat-pump-replacement/) covers the new system. The [Memphis buying guide](https://blufflinememphisheatpumps.com/guide/memphis-heat-pump-buying-guide/) puts it all in order.

## Questions about this guide

### How often should a heat pump go into defrost?

It depends on temperature and humidity. In damp weather in the 20s and 30s, every 30 to 90 minutes of run time is common. In dry cold, much less often. Modern controls defrost only when the coil needs it.

### Should I pour hot water on a frozen heat pump?

No. It can damage the fins and refreezes quickly. If the unit is iced solid, switch the thermostat to emergency heat so the heat pump stops, and have the defrost controls, sensors and drainage checked.

### Why is steam coming out of my heat pump?

During defrost the outdoor coil warms up and melts its frost, and the warm, wet coil gives off steam in the cold air. It looks alarming but is normal and usually lasts a few minutes.

## Related service

- [Service**Heat pump replacement** Your heat pump is near the end and you want the next one to last. We size it from the house and match both halves.](https://blufflinememphisheatpumps.com/heat-pump-replacement/)

## More guides

- [Learn**What is the difference between auxiliary heat and emergency heat?** Two settings that both turn on the strips, why one is automatic and the other is a last resort, and how to keep the bill in check.](https://blufflinememphisheatpumps.com/guide/auxiliary-emergency-heat/)
- [Learn**What happens to a heat pump in an ice storm or a power outage?** Ice on the unit, power out for a day, and what to do before, during and after. Plus how gas, strips and generators compare.](https://blufflinememphisheatpumps.com/guide/heat-pump-ice-storm-power-outage/)
- [Learn**Where should a heat pump's outdoor unit go, and how loud will it be?** Clearances, pads, drainage and sound on narrow Memphis side yards, and when it is worth moving the unit at replacement time.](https://blufflinememphisheatpumps.com/guide/outdoor-unit-placement-noise/)
- [Decision Guide**How do you buy the right heat pump for a Memphis house?** The whole decision in the order you meet it, from the system you have now to the readings you should get at start-up.](https://blufflinememphisheatpumps.com/guide/memphis-heat-pump-buying-guide/)

## Get an estimate for your house

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