Heat Pump Frozen? Normal Frost and Defrost vs. Ice That Needs a Technician

Frost on a heat pump's outdoor unit in winter usually means it's working. Trane says the outdoor coil runs 10 to 20°F colder than the air, so moisture can freeze on it even in the low 40s, and the heat pump melts it off in a defrost cycle that can send up a cloud of steam. A coil buried in solid ice is different: Carrier says ice thicker than 1/4 inch, or ice that stays after defrost, may point to restricted airflow, low refrigerant, or a defrost problem.

This page covers the difference and what to do about it. For cool air from the registers, see heat pump blowing cold air; for ice on the lines or indoor coil in summer, see AC frozen coil.

Normal Frost and Defrost

Light frost is normal. Lennox calls a light layer of frost on the outdoor coils “completely normal, especially in cold, humid weather,” and Carrier puts normal frost at generally under 1/4 inch. Goodman says the defrost control senses frost or light ice and starts a defrost cycle on its own.

How defrost works. The reversing valve switches the heat pump into cooling for a few minutes to warm the outdoor coil, Trane explains. Some units defrost on demand when sensors call for it (Trane); others run on a 30-, 60-, or 90-minute timer (Lennox). Lennox says a heat pump should defrost itself every 30 to 90 minutes.

What you'll see and hear. The outdoor fan stops and steam or fog rises as the melted frost evaporates. Carrier's and Bryant's manuals and Trane call this normal, Rheem's manual describes the same sequence, and Daikin says the unit can give off water or steam. Carrier's and Bryant's manuals add a “whoosh” at the start and end. Carrier traces it to the reversing valve and says to call only if it turns excessively loud or constant.

Inside, a few minutes of cooler air. Bryant and Lennox say the air may run cooler during defrost. Most systems bring on backup heat, and the thermostat may show AUX (Trane, Goodman); Trane says homes without backup heat, and some dual-fuel systems, may feel cool air, which is also normal.

How long. About 5 to 15 minutes (Lennox, Bryant), 5 to 10 (Carrier), or 3 to 10 (Daikin); Trane's demand-defrost systems usually take 3 to 5. Then, Rheem's manual says, the fan restarts and heating resumes.

Signs It's Not Normal

• Ice thicker than about 1/4 inch, a solid coat, or ice that doesn't clear during defrost (Carrier).
• Thick ice over the coils and cabinet, ice on the fan blades or stopping them, icicles, or ice built up at the base (Lennox).
• Snow and ice on the top and fan blade, which Carrier's manual says can cause damaging vibration.
• Ice on the refrigerant lines, or on only one side of the unit (Lennox).
• Defrosts that come constantly, or run longer as ice spreads (Trane).

Most Likely Causes

Check them in this order. The first four are yours to check; the last three are a technician's.

1. Snow, leaves, or a cover blocking airflow. Goodman says anything that keeps air from reaching the outdoor coil hurts performance, and Lennox lists debris, leaves, and snow among the airflow problems that let the coil ice over. Snow drifted or shoveled against the sides counts, and so does any cover or wrap (see the FAQ below).
2. A dirty filter or blocked vents indoors. Lennox's airflow list also includes a clogged filter, closed or blocked supply vents, and blocked ducts, and it calls a filter change the simple kind of fix.
3. Water dripping onto the unit. Carrier says meltwater from snow or ice can leak onto the coil and freeze, more than defrost may handle, and names a leaky gutter as a source to repair. Lennox adds that long spells of freezing rain, sleet, or heavy wet snow can overwhelm defrost.
4. A unit that sits too low or can't drain. Defrost meltwater has to drain before it refreezes, which is why Trane's installer's guide raises units in snowy areas (see Prevent It). Lennox says a clogged drain pan or line lets water pool and refreeze, and a settling pad can cause drainage problems; Carrier's and Bryant's manuals say to clear debris from around the base so the base pan drains.
5. A failed defrost control, timer, or sensor. Lennox calls it one of the most common causes of severe freeze-ups: defrost never starts, and ice keeps building. Trane and American Standard point to faulty defrost controls or sensors when ice won't go away or keeps growing.
6. Low refrigerant from a leak. The coils get too cold and ice forms quickly, Lennox says, and the compressor can be damaged; Carrier says a system low on refrigerant can't make enough heat to melt the ice.
7. A failed outdoor fan motor. Carrier says a defective motor means little or no airflow through the coil, so it freezes over. Lennox's failed-parts list adds the reversing valve and control board.

What You Can Fix Yourself

1. Watch one defrost. Give it the 5 to 15 minutes Lennox and Bryant describe. If the ice is thicker than 1/4 inch or doesn't clear, Carrier says to turn the system off and call a qualified HVAC professional; the steps below are for while you wait.
2. Stop the heat pump, and stay warm. Lennox says to turn it off at the thermostat, or switch to emergency heat so the outdoor unit thaws while the house stays warm. Rheem reserves EM heat for a suspected outdoor-unit problem and says long use raises electric costs; Trane says to call for service once you use it. In heavy snow and ice storms, Carrier's manual says running on auxiliary heat with the heat pump off may be appropriate.
3. Cut the power at the disconnect box near the unit before you clear anything (Lennox).
4. Clear snow and debris. Brush light snow off the top with a broom, never a sharp tool, and clear the sides (Lennox, Trane). Don't pile snow against it (Lennox), and remove any cover. Keep about two feet clear all around, Carrier's and Lennox's general guide.
5. Check the filter and vents. Hold the filter up to the light; if you can't see through it, Lennox says replace it. Open closed registers and move furniture off vents (Lennox, Carrier's manual). Our filter guide walks through the swap.

6. Find the water. Fix or redirect a gutter or downspout that drains onto the unit (Carrier, Lennox); our gutter guide covers doing it from the ground. If icicles line the eave above it, see ice dam on the roof.
7. Check underneath. Clear debris from around the base so the base pan drains (Carrier, Bryant). If the pad has settled and the unit tilts, or water or ice collects under it, Carrier's and Bryant's manuals say to re-level it promptly and get the water drained away.
8. Let it thaw. Lennox says that takes 2 to 4 hours above freezing, longer in colder weather. Then switch the disconnect back on, give the system a few minutes to restart, and set HEAT (Lennox). Not melted in 6 to 8 hours, or refreezing quickly? Call (Lennox).

What Not to Do

• Don't chip, pry, or hit the ice. Bryant and Carrier say not to chip ice from the coil, and Lennox warns that tools can damage coils, fins, and fan blades: “Even a small dent can reduce efficiency and lead to expensive repairs.”
• No hot water or heaters. Lennox says to “never pour hot water on the unit, as the sudden temperature change can crack components and cause serious damage,” and warns that hot water and high heat sources can cause damage a warranty may not cover.
• Don't open panels or wrap the unit. Carrier says not to remove panels or work on internal components; Goodman says running it with a cover in place could damage the system.

When to Call a Pro

Bryant leaves problems with refrigerant, electrical parts, controls, the reversing valve, and the defrost system to a qualified technician, and Carrier calls a defective fan motor another job for a pro. EPA rules under Section 608 of the Clean Air Act require anyone handling refrigerant to be certified and prohibit venting it. Call when:

• Ice is over 1/4 inch or won't clear in defrost (Carrier), or coats the unit heavily (Goodman).
• It hasn't thawed in 6 to 8 hours, or refreezes quickly (Lennox).
• Ice covers the refrigerant lines or only one side, or the fan won't spin once the ice melts (Lennox).
• You hear grinding, squealing, or banging, or hissing with weak heat, a low-refrigerant sign (Lennox).
• Defrost never starts after several hours (Lennox), or comes constantly and runs longer (Trane).
• A breaker trips again after one reset; Bryant says leave it off and call. See breaker keeps tripping.
• You're running on EM heat (Trane).

Lennox warns that DIY repairs on these parts can void the warranty. If you'll be away while you wait, keep the heat no lower than 55°F, the American Red Cross minimum, so the wait doesn't end in frozen pipes.

Prevent It

• Keep it clear. Trim plants two feet back on all sides and clear snow after storms (Lennox). Carrier's and Bryant's manuals set a 12-inch minimum from tall grass, vines, and shrubs, and the Department of Energy says to clear weeds and debris so air circulates freely.
• Raise it where snow piles up. Carrier's and Bryant's manuals say units where snow is expected sit on support feet. Trane's installer's guide calls for 3 to 12 inches above the pad, depending on local weather, so meltwater drains before it refreezes, with clear base pan drain holes and a snow-drift barrier where drifts build. Mitsubishi Electric says contractors should mount it above the snow line. If yours sits low in a snowy yard, ask an HVAC contractor about a stand.
• Keep roof water off it. Trane's guide says roof runoff and icicles shouldn't fall directly on the unit, and Lennox says gutters and downspouts shouldn't drain onto or near it.
• Book maintenance. Lennox recommends tune-ups in spring and fall, with the defrost sensor and controls inspected; ENERGY STAR recommends annual pre-season check-ups, heating in the fall. Check the filter monthly (ENERGY STAR) and replace it every 1 to 3 months (Lennox). The winter guide schedules it.

Frequently Asked Questions

Is it normal for a heat pump to have frost on it?

Yes. Lennox says light frost is completely normal in cold, humid weather and that defrost should clear it every 30 to 90 minutes. Carrier draws the line at about 1/4 inch; thicker ice, or ice that outlasts defrost, needs a technician.

Can I pour hot water on a frozen heat pump?

No. Lennox says the sudden temperature change can crack components. Turn the heat pump off or to EM heat, brush off loose snow with the power off, and let the ice melt.

How long does it take a frozen heat pump to thaw?

About 2 to 4 hours above freezing, longer when it's colder, Lennox says, with emergency heat keeping the house warm. American Standard and Trane say to call if the ice isn't melting after a few hours; Lennox's outer limit is 6 to 8.

Should I cover my heat pump in winter?

No. Goodman says a cover keeps a heat pump from drawing air in through its sides and out the top and may damage it, and Lennox says heat pumps that run year-round should never be covered. Lennox allows only a top snow guard; Trane, only a breathable cover, never a solid one.

The Bottom Line

Thin frost that melts in a short defrost, with steam, a stopped fan, and a whoosh, is normal. Thick ice, or ice on the top, fan, or lines, is not. Switch to OFF or EM heat, kill the power, brush off snow, change the filter, stop dripping water, and let it thaw, with no chipping and no hot water. If it won't clear in 6 to 8 hours, call a licensed HVAC technician.

Sources