Heat Pump Not Heating? Here’s the Checklist (Including Defrost Mode)

Heat pump blowing cold air in Heat mode has six common causes: defrost cycle, wrong O/B wire setting, low refrigerant, outdoor unit iced over, balance point exceeded, or AUX heat not wired. Most are DIY-diagnosable in under 10 minutes. Work through the steps below in order.

TL;DR: If it’s below 40°F outside and the heat just quit, wait 15 minutes first. The most common cause is the defrost cycle (the system is temporarily melting ice off the outdoor coil) and it will return to heating on its own.

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Step 1: Confirm it is not defrost mode (check this first)

Most likely cause in cold weather. When outdoor temps drop below 40°F, frost forms on the outdoor coil. The heat pump periodically reverses refrigerant flow to melt that frost. This cycle typically runs every 30–90 minutes and lasts 5–15 minutes.

During defrost you will notice:

• The outdoor fan stops spinning.
• The unit may hiss or click (the reversing valve switching direction).
• Steam or vapor rises from the outdoor unit as frost melts.
• Air from your supply vents turns cool or room-temperature for several minutes.

This is normal behavior, not a fault.

What your thermostat shows during defrost:

• ecobee: The status bar reads “Heat On” but supply air feels cool. Defrost is handled by the outdoor unit’s own control board, so ecobee doesn’t always flag it explicitly.
• Nest: Screen stays orange (calling for heat). Some Nest installs show “Auxiliary Heat” activating to bridge the gap while the compressor is in defrost.
• Honeywell Home T6/T9/T10: The flame icon stays lit; there’s no separate defrost indicator on these models.

How to confirm defrost was the cause: Wait 15 minutes without changing settings. If heat returns, defrost was the issue and the system is working correctly. If no heat after 15 minutes, continue to Step 2.

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Step 2: Check the thermostat settings

Mode set to Heat. Not Auto, not Cool. Auto mode can switch to cooling on a mild afternoon and not flip back until room temp drops significantly.

Setpoint at least 2–3°F above current room temp. If the room reads 68°F and the setpoint is 69°F, most systems won’t generate a strong call for heat. Set it to 71–72°F and confirm the thermostat shows an active heat call (flame icon or “Heating” in the status line).

O/B wire setting. If you recently replaced the thermostat, this is a common miss. The O/B terminal controls the reversing valve that switches the refrigerant between heating and cooling direction. Set to the wrong position, the system literally pumps heat out of your house while in Heat mode.

• Most systems use O (the valve energizes in Cool mode). Set to B on an O-system: cold air in Heat mode.
• Rheem and Ruud systems use B (the valve energizes in Heat mode). Set to O on a B-system: same result, opposite direction.

On Nest, check: Settings > Equipment > Heat Pump > O/B. On ecobee, check the installer configuration for O/B direction. See our full walkthrough at /what-is-ob-wire-thermostat/.

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Step 3: Check the outdoor unit for ice

Go outside and look at the outdoor unit.

Normal: Light frost or a thin white coating on the coil between defrost cycles. The fan is spinning and you can hear refrigerant moving.

Problem: Unit is completely or heavily encased in ice. A thick ice shell blocks airflow, kills heating capacity, and can damage the compressor if left running.

Common causes of heavy icing:

• Debris or vegetation blocking airflow around the unit (snow drifts, leaves, a fence too close)
• Low refrigerant charge (a refrigerant problem, not a thermostat problem)
• Stuck reversing valve (unit won’t enter defrost even when the control board commands it)
• Failed defrost control board

What to do: Set the system to Fan Only at the thermostat and let the unit thaw for 30–60 minutes. Do not chip ice off the coil; you will damage the fins. Once thawed, resume Heat mode and monitor.

If the unit ices over again within a few hours, stop running it. Repeated icing points to refrigerant or defrost board failure. Call a licensed HVAC technician.

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Step 4: Is it cold enough to need AUX heat?

Every heat pump has a balance point: the outdoor temperature below which the heat pump can no longer meet your home’s heating load on its own. This is typically 25–40°F depending on your equipment and house. Below that point, auxiliary (backup) heat should activate automatically to keep up.

If the heat pump is struggling in cold weather and no AUX heat is activating, check:

Is AUX heat wired? On the air handler’s terminal board, look for wiring at W2, Aux, or E. If those terminals are empty and there are no backup electric strips, your system has no backup heat source. It will underperform at low outdoor temps regardless of thermostat settings.

ecobee Compressor Min Outdoor Temperature: ecobee has a setting that locks out the compressor below a configurable threshold (Main Menu > Settings > Installation Settings > Thresholds > Compressor Min Outdoor Temp). The default is often 35°F. If set too high, the compressor will not run at all when it’s cold, and if AUX strips are not wired correctly, you get no heat. Adjust or confirm this setting.

Nest Heat Pump Balance: Nest’s “Heat Pump Balance” setting (in the app under Settings > Equipment > Balance) has a Max Savings option that delays AUX activation as long as possible to cut electricity costs. This can leave your home underheated at low temps. Set it to Comfort or a balanced midpoint during a cold snap.

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Step 5: Is the outdoor unit running at all?

Go outside. Is the fan spinning? Is there any hum from the compressor? A unit that is completely silent while the thermostat is calling for heat is not a thermostat problem. It is a power or mechanical fault in the outdoor unit.

Check the outdoor disconnect. The disconnect is a pull-out fuse block or lockable switch mounted within a few feet of the outdoor unit. Pull the fuse block and inspect visually. A blown fuse usually has a visible gap or darkened interior. Replace blown fuses with the same amperage rating.

Check the 240V breaker. Heat pump outdoor units run on 240V AC and typically have a dedicated 30–50A double-pole breaker in the main panel. If it’s tripped, reset it once. If it trips again, stop. Do not reset a repeatedly tripping breaker. That indicates a short or a compressor fault.

If the outdoor unit runs but indoor air is still cold: you are likely looking at a refrigerant charge issue or a stuck reversing valve. Both require an EPA 608-certified HVAC technician to diagnose and repair.

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Step 6: When to stop and call a licensed HVAC tech

Safety callout. Line-voltage work (120V/240V), refrigerant handling, and heat exchanger inspection all require a licensed technician. Do not open the electrical compartment of the air handler or outdoor unit.

Call a tech if any of these apply:

• The outdoor unit ices over again within a few hours of thawing
• Cold air continues after a 15-minute wait (defrost ruled out)
• The outdoor unit is completely silent with power confirmed and thermostat calling for heat
• The breaker trips a second time after reset
• Neither heat pump nor AUX/emergency heat produces warm air
• You smell refrigerant (sweet, faintly chemical odor) near the outdoor unit or air handler

The 24V AC control wiring at the thermostat is safe to inspect. Everything else on the line-voltage side stays closed until a tech arrives.

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A note on installation quality and defrost

When heat pump troubleshooting leads to a replacement conversation, bid completeness matters. A quote that doesn’t mention commissioning or defrost/staging verification as part of the scope is a warning sign — those are exactly the steps whose absence tends to surface as defrost or AUX-staging problems in the first winter of operation. Use BidSmart to compare your own quotes and check whether commissioning is actually itemized.

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Frequently asked questions

Why is my heat pump blowing cold air in heat mode?

The most common reason is the defrost cycle: the outdoor unit temporarily reverses refrigerant flow to melt ice off the coil, and air from your vents goes cool for 5–15 minutes. Other causes include a wrong O/B wire setting (reversing valve configured backward), low refrigerant, or outdoor temps below the system’s balance point with no AUX heat wired.

How long does heat pump defrost mode last?

Typically 5–15 minutes per cycle. The outdoor unit initiates defrost automatically when ice accumulates on the coil, usually every 30–90 minutes in cold weather. If your unit seems to be in defrost for 30 minutes or more without returning to normal operation, that is a defrost control fault, not a normal cycle.

At what temperature does a heat pump stop working?

Standard heat pumps lose significant capacity below 20–25°F and may not keep up with heating demand without AUX heat. Cold-climate heat pumps (Mitsubishi Hyper-Heat, Bosch IDS, Carrier Infinity series) maintain usable capacity down to -13°F. The specific outdoor temperature at which your system needs AUX backup is called the balance point, and it depends on your equipment model and your home’s heating load.

Should I turn on emergency heat if my heat pump isn’t heating?

Only if the heat pump has a confirmed fault (outdoor unit not running, refrigerant issue, physical damage). Emergency heat bypasses the heat pump entirely and runs only the electric resistance backup strips, which cost roughly two to three times more per hour of operation. Use it as a temporary bridge while waiting for a tech, not as a routine workaround for cold-weather underperformance. If the problem is just defrost mode, AUX heat will activate on its own when needed. You do not need to switch to Emergency.

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Related

• Why Won’t My Heat Turn On?
• Aux Heat vs Emergency Heat
• What Is the O/B Wire on a Thermostat?
• Switch Thermostat from Cooling to Heating