Heat Pump Freezing Up in Winter? Normal Frost vs. Ice That Needs a Pro

Quick answer

Thin white frost that melts off during the next defrost cycle is normal in winter. Ice is a problem when it is thicker than about 1/4 inch, forms a solid coat, or stays after defrost (Carrier), or when it covers the top or the fan (Lennox, Carrier). Don’t chip it or pour water on it: switch to emergency heat if you have it so the unit can thaw, and look for a dripping gutter, drifted snow, ice under the unit, or a clogged indoor filter. Call a licensed HVAC technician if the ice hasn’t melted after several hours or comes right back.

Frost or freeze-up? Match what you see

Light frost comes and goes; ice that keeps building is the warning sign.

What you see	Usually means	What to do
Thin white frost on the coil that melts off during defrost and comes back later	Normal (Carrier: under 1/4 inch). Goodman says frost or light ice is no cause for alarm.	Nothing. Let the defrost cycle work.
Ice thicker than 1/4 inch, a solid coat, or ice still there after defrost	A problem (Carrier). Goodman says heavy ice may mean the unit needs service.	Stop the heat pump, look for a water source, and call if it doesn’t clear.
Ice on the top grille or fan blade, or a fan that can’t turn	A problem. Lennox lists it as a freeze-up sign; Carrier warns it can cause vibration and damage.	Turn the heat pump off and use backup heat until it melts. Call if the fan won’t spin after the ice is gone (Lennox).
The whole cabinet coated, with icicles hanging from it	A freeze-up (Lennox)	Same as above. Look for a gutter or roof edge dripping onto it.
Ice building up around the base, or the unit sitting in ice or water	Melt water isn’t draining away (Lennox, Carrier)	Clear snow and debris from the base. Have the unit re-leveled or the water drained away if it keeps collecting (Carrier).
Ice on only one side, in an odd pattern, or on the refrigerant lines	Not normal. Lennox lists all three as reasons to call a dealer.	Call a technician.
Ice that comes back soon after it thaws	A refrigerant leak, airflow problem, or defrost control issue (Lennox)	Call a technician.

For central (ducted) heat pumps in heating mode. Your owner's manual may describe your model differently. Source: Carrier, Lennox, and Goodman pages and the Carrier owner's manual listed in the sources

Why a heat pump ices up at all

The outdoor coil runs 10 to 20°F colder than the outdoor air, Trane says, so moisture can freeze on it even above freezing. The defrost cycle melts that frost every so often (see our heat pump defrost cycle guide for what is normal).

A freeze-up is different. If a defrost part fails, ice keeps building, and long stretches of freezing rain, sleet, or heavy wet snow can overwhelm the defrost system (Lennox). Carrier adds that melting snow can drip onto the coil and cause a freeze-over that defrost can’t clear.

What to do when the outdoor unit is iced over

1. Light ice: wait. Trane suggests waiting a few hours, without doing anything, to see whether the unit defrosts itself.
2. Heavy ice: stop the heat pump. Switch it off at the thermostat when ice is more than 1/4 inch thick or doesn’t clear during defrost (Carrier, Lennox).
3. Stay warm on backup heat (next section).
4. Let it thaw on its own. Natural thawing typically takes 2 to 4 hours when it is above freezing outside, and longer in colder weather, according to Lennox. If the ice hasn’t melted in 6 to 8 hours, or it refreezes quickly, call a technician.
5. While it thaws, look for the cause with the checks below.

Emergency heat while it thaws

The Em Ht or Emergency Heat setting locks off the outdoor unit and heats with backup heat, typically electric resistance heat (Rheem’s older user’s guide). Manufacturers point to it when ice is the problem:

• Lennox: it lets the outdoor unit thaw while keeping your home warm.
• Carrier’s Infinity owner’s manual: in heavy snow and ice storms, it may be appropriate to turn the heat pump off and run on auxiliary heat until the ice clears, since ice on the top and fan blade can cause vibration and damage.
• Rheem’s installation instructions for its (-)P14 and (-)P15 heat pumps highly recommend it during freezing rain or sleet, so ice on the fan blade doesn’t damage the coil.

Sustained electric resistance heat raises your bill (Rheem), so switch back to Heat once the ice is gone. Goodman adds that there’s no need to turn it on by hand just because it’s cold, as long as the house stays comfortable. For what backup heat costs, see aux heat vs. emergency heat.

No emergency heat setting? Turning the system off still protects the unit, but the house will cool. Trane’s owner’s guide reminds owners to keep water pipes from freezing when the heat isn’t working; our winter thermostat guide lists the Red Cross’s frozen-pipe tips. If you use a space heater, read the safety rules in our space heater cost guide.

Find the cause: safe checks, easiest first

Do these with the system off at the thermostat, from outside the unit only.

1. Look up: gutters, downspouts, and the roof edge

Melting snow or ice can drip onto the coil and freeze, and defrost may not clear a severe freeze-over (Carrier). Find the source, such as a leaky gutter, and repair it. Gutters and downspouts shouldn’t drain onto or near the heat pump (Lennox), and Rheem recommends keeping units away from the roof’s drip edge where possible, since water pouring off a roofline without gutters can shorten fan blade or motor life.

Look for icicles or a dripping gutter right above the unit, or a downspout emptying beside it. Under a roof edge with no gutter, ask your installer about options; Lennox mentions a snow guard that covers the top only.

2. Snow drifts and snow on top

Snow and ice restrict airflow, so clear snow from around the unit as soon as possible after it piles up (Trane’s owner’s guide).

• Sides: clear snow after storms without piling it against the unit, and keep about two feet clear all around (Lennox).
• Top: gently brush light snow off with a broom, never a sharp tool (Lennox). Leave ice on top alone and let it melt with the heat pump off.
• No covers: a heat pump draws air in through the sides and blows it out the top, so Goodman says not to cover it in winter.

If snow regularly buries the bottom of the unit, the stand may be too low. Rheem says units in snowy areas should sit above the area’s average snowfall, and Carrier says they are elevated on support feet. Raising a unit is a job for your installer.

3. Is the unit sitting in water or ice?

Defrost water drains out the bottom. If that path is clogged, water pools around the unit and refreezes (Lennox).

• Clear the base. Carrier’s owner’s manual says to periodically remove debris around the base so the base pan can drain, and Rheem says not to block the drain openings. Packed snow, leaves, and mulch all count.
• Check that it’s level. Have the unit re-leveled promptly if its pad settles or shifts (Carrier); settling can cause drainage problems (Lennox).
• Water or ice underneath: arrange for it to be drained away from the unit (Carrier). Don’t chip it out.

4. Can the fan turn?

Look through the top grille without reaching in. A stopped fan with steam rising is usually a defrost cycle (Carrier’s owner’s manual calls both normal), so give it several minutes. If the fan stays stopped while the unit hums, or a blade is locked in ice, turn the unit off at the thermostat (Trane). Call if the fan still won’t spin after the ice melts (Lennox); a failed fan motor leaves little or no airflow, so the coil freezes over (Carrier).

5. Indoor airflow: the filter and the vents

Lennox lists restricted airflow among the causes of freeze-ups, including a dirty filter, closed or blocked supply vents, and collapsed or blocked ducts.

• Filter: replace it if it’s dirty (Lennox), and inspect it monthly (ENERGY STAR). Use the same size and type, with the arrows pointing in the direction of airflow (Trane’s owner’s guide).
• Vents: drapes and furniture over vents restrict airflow (Carrier’s owner’s manual).

Our guide to a heat pump not heating covers filter checks in detail. Ducts are hidden and need a technician.

6. Consider the weather

If the unit iced over in freezing rain or sleet and was fine before, the storm may be the whole story; Rheem notes that freezing moisture and sleet can ice over the cabinet early and stop the heat pump. If it freezes up again in ordinary cold, something else is wrong.

Causes that need a technician

These are explanations, not repairs. Each is work for a licensed HVAC technician.

• Defrost control, timer, or sensor. The defrost control relies on outdoor and coil temperature sensors (Trane). When the board, timer, or sensor fails, ice keeps building, one of the most common reasons for severe freeze-ups (Lennox).
• Low refrigerant from a leak. A leak makes the coil too cold, so ice forms quickly and heating drops (Lennox), and a system low on refrigerant can’t make enough heat to melt the ice (Carrier).
• Outdoor fan motor (Carrier, Lennox).
• Reversing valve or control board (Lennox).

Trane calls low refrigerant, faulty defrost controls, and a faulty sensor serious problems that likely mean a repair or replacement.

Ice in summer is a different problem

Carrier calls a frozen coil in summer suspicious, with low refrigerant the likely reason an indoor coil froze. Trane adds a dirty indoor coil as a cause, and says to shut the system down and call an HVAC professional if you see frost around the indoor coil.

Preventing the next freeze-up

• Fall tune-up. ENERGY STAR suggests having the heating system checked each fall, and Lennox recommends spring and fall tune-ups that include the defrost sensor and controls.
• Monthly filter check (ENERGY STAR), gutters that drain away from the unit, and about two feet of clear space (Lennox).
• Ductless mini-splits. Daikin’s Atmosphera manual says to take snow protection measures, such as a snow protection hood your dealer can advise on. ENERGY STAR advises checking where roof runoff falls when siting the outdoor unit.