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What this covers
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The first cold morning of the season produces the same phone call in a lot of houses. Someone looks out the window and sees a cloud of steam rising off the outdoor unit. The fan has stopped. There is a whoosh, then quiet, then the fan starts again a few minutes later.
Most of the time nothing is wrong. The heat pump is defrosting, which is a normal part of how the equipment runs in cold, damp weather. The trouble is that a few genuine faults look very similar from the kitchen window, and telling the two apart is easier once you know what the cycle is supposed to look like.
What a Heat Pump Does in Winter
A heat pump is a refrigeration system that moves heat rather than making it. In summer it pulls heat out of the house and dumps it outdoors. In winter it runs the same loop in the other direction, pulling heat out of the outdoor air and delivering it inside.
That works because outdoor air still holds usable heat well below freezing. The outdoor coil runs colder than the air around it, so heat flows from the air into the refrigerant. A component called the reversing valve decides which direction the refrigerant travels, and it is the part that makes one machine serve both seasons.
The cost of running the outdoor coil colder than the air is that moisture in that air condenses on the coil and, once the coil is below freezing, turns to frost.
Why Frost Forms on the Outdoor Coil
Frost forms on a heat pump coil when two things are true at once: the coil surface is below 32 degrees and the outdoor air carries enough moisture to deposit on it.
The worst conditions are not the coldest days. They are the raw, damp ones, roughly in the 20s and 30s with high humidity or light precipitation. Very cold air is usually dry and frosts the coil more slowly.
That matters locally. Springfield and the rest of Greene County sit in IECC climate zone 4A, a mixed-humid zone. Winters here are full of exactly those damp days near freezing, rather than long stretches of dry deep cold. A heat pump in this climate defrosts often, and that is expected.
A thin, even layer of frost is normal. A coil that is completely encased in ice is not, and the difference is the most useful thing a homeowner can learn to see.
How the Defrost Cycle Clears It
The defrost cycle is a short, deliberate reversal. The control board decides frost has built up, using either a timer, a temperature sensor on the coil, or a combination of sensors on newer equipment. It then:
- Shifts the reversing valve so the system briefly runs in cooling mode.
- Stops the outdoor fan, so the hot refrigerant warms the coil instead of the air.
- Brings on backup heat indoors, so the house is not fed cool air while this happens.
- Ends the cycle once the coil warms past a set point, then returns to heating.
The whole event usually lasts somewhere between a few minutes and around ten. On a damp day it may happen every half hour to hour and a half, depending on the equipment and its settings.
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What you notice |
What is causing it |
Normal during defrost? |
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Steam or fog rising off the outdoor unit |
Frost melting on a warm coil |
Yes |
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Outdoor fan stops for several minutes |
Fan held off so the coil heats up |
Yes |
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A whoosh or a short clunk |
Reversing valve shifting position |
Yes |
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Water running out under the unit |
Melted frost draining away |
Yes |
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Indoor air feels slightly cooler for a few minutes |
Backup heat covering the gap |
Yes, if brief |
The Signs That Point to a Fault Instead
The defrost cycle has a shape: frost builds, the unit clears it, and the coil goes back to bare metal. A fault breaks that shape somewhere. These are the patterns worth taking seriously.
- Ice that never clears. The coil stays white or turns into a solid block across several hours. A defrost cycle that never runs, or runs and fails, leaves ice that keeps building.
- Ice on the top of the unit or the fan blades. That often points to water dripping from a gutter or roof edge onto the unit and refreezing.
- Ice on the refrigerant lines but a clear coil. That points away from defrost and toward a refrigerant or airflow problem.
- The fan never stops. A unit that frosts heavily but keeps its fan running may not be entering defrost at all.
- Constant backup heat. If the auxiliary heat light stays on all day, something is preventing the heat pump from carrying the load.
A Decision Table for the Window Check
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What you see |
Most likely cause |
Safe for a homeowner to act on? |
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Light frost that clears on its own |
Normal defrost |
Nothing to do |
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Heavy ice, fan still running |
Defrost sensor, control board or timer fault |
No, call for service |
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Ice on top from dripping water |
Gutter or roof runoff landing on the unit |
Yes, redirect the water |
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Unit buried in drifted snow |
Airflow blocked around the coil |
Yes, clear it gently by hand |
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Coil clear, ice on the thick suction line |
Low refrigerant or a blower or filter problem indoors |
Check the filter, then call |
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Loud repeated banging during the shift |
Reversing valve or its solenoid |
No, call for service |
Two rules sit under that table. Never chip ice off a coil with a tool, because the fins are thin aluminum and the tubing behind them carries refrigerant under pressure. And never pour hot water over a running unit; switching the system to emergency heat while you wait is the safer choice.
Why the House Feels Cooler During Defrost
A common complaint is that the vents blow cool air for a few minutes. During defrost the heat pump is technically cooling, so the indoor coil is cold. The backup heat is meant to cover that, which is why most systems bring on electric strips or a gas furnace during the cycle.
If the house gets noticeably cold every time the unit defrosts, the backup heat may not be engaging, or a strip may have failed. That is a fault worth having traced, even if the heat pump itself is fine.
The opposite complaint also happens. A system whose backup heat runs far longer than the defrost cycle needs will push the electric bill up quickly, because electric resistance heat costs more to run than the heat pump it is covering for.
What a Technician Checks on a Defrost Complaint
A defrost complaint is a diagnostic job, not a parts swap. A technician working through it will typically check:
- The coil sensor and its reading against the actual coil temperature
- The defrost control board and the timer setting it uses
- The reversing valve and the solenoid coil that shifts it
- Refrigerant charge, since a low charge makes the coil run colder and frost faster
- Outdoor fan motor and capacitor, which have to restart cleanly after each cycle
- Indoor airflow, because a choked filter or weak blower changes how the whole system behaves
The useful thing to tell the technician when you call is what you saw and when: steam or no steam, whether the fan stopped, whether the ice cleared overnight, and whether the backup heat light has been on.
When the Repair Becomes the Bigger Question
Frost problems on an older system often turn out to be one symptom of several. If a heat pump has already needed a reversing valve, a compressor part and a control board, the defrost fault may be the moment to weigh repair against replacement.
Federal efficiency ratings changed on 1 January 2023, when the Department of Energy moved to the SEER2 and HSPF2 test procedures. A quote on new equipment will use those numbers, and they are not directly comparable with the SEER and HSPF figures printed on older units.
For houses around Springfield, the detail on diagnosing heat pump faults in Springfield covers how a visit runs and how the cost is agreed before any work starts. The company behind that page, Redeemed HVAC & Plumbing, works on all makes and models rather than one brand. Where the heat pump is part of a bigger comfort problem, the wider heating and cooling work in Springfield is the place to start, and the Springfield office listing shows hours and service area.
The Short Version
Steam, a stopped fan and a whoosh on a damp winter day are normal. Frost that clears is normal. Ice that keeps building, ice on the top of the unit, ice on the lines with a clear coil, or backup heat that never switches off are the patterns that deserve a service call.
Clear snow and redirect dripping water yourself. Leave the ice on the coil alone. And when you call, describe what the cycle looked like, because that description narrows the fault faster than anything else.