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 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 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 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 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
- If the fan is frozen or the unit is encased, switch the thermostat to emergency heat.
- Don’t chip at the ice; the fins and coil bend easily.
- Let it melt as the weather warms, or ask for help.
- Switch back to normal heat once the fan turns freely.
Power often goes out in the same storms. The ice storm and outage guide 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 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 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 walks through the math, and heat pump replacement covers the new system. The Memphis buying guide puts it all in order.