Any burning smell, smoke, or sparks from the indoor or outdoor unit?

Is the outdoor unit currently covered in frost or ice, with the fan off and steam rising? This is a normal defrost cycle (typically 5–15 minutes).

Defrost cycles run periodically in cold weather. The reversing valve sends hot refrigerant to the outdoor coil to melt frost — air handler may blow cool air temporarily.

What is the current outdoor temperature?

Is the thermostat showing 'AUX' or 'Emergency Heat' on?

Below the heat-pump balance point (typically 25–35 °F depending on unit and load), auxiliary electric strips or a backup furnace should kick in.

Check the indoor air filter and supply registers. Is the filter heavily clogged or are registers blocked?

hvac troubleshooting

Heat pump not heating in cold weather

Common symptoms: heat pump cold air; heat pump not heating; heat pump weak heat; cold weather no heat

Stop and call a pro if:

burning odor
smoke
sparks
frozen outdoor unit

Step-by-step diagnostic flow

Step 1
Any burning smell, smoke, or sparks from the indoor or outdoor unit?
Step 2
Is the outdoor unit currently covered in frost or ice, with the fan off and steam rising? This is a normal defrost cycle (typically 5–15 minutes).
Defrost cycles run periodically in cold weather. The reversing valve sends hot refrigerant to the outdoor coil to melt frost — air handler may blow cool air temporarily.
Step 3
What is the current outdoor temperature?
Step 4
Is the thermostat showing 'AUX' or 'Emergency Heat' on?
Below the heat-pump balance point (typically 25–35 °F depending on unit and load), auxiliary electric strips or a backup furnace should kick in.
Step 5
Check the indoor air filter and supply registers. Is the filter heavily clogged or are registers blocked?

Possible outcomes

Stop and seek professional help immediately

high confidence

Burning smell, smoke, or sparks indicate active hazard.

Electrical or fire hazard present

Normal defrost cycle — wait and observe

high confidence

Defrost cycles are a designed part of heat-pump operation in cold weather. Cool air from the indoor unit during defrost is expected.

Safe next steps

Wait 5–15 minutes for the cycle to complete
Verify heat returns when the outdoor fan restarts
If defrost recurs more often than every 60 minutes or runs >20 minutes, escalate to a pro

What to document for a pro

Defrost frequency and duration
Outdoor temp and humidity
Equipment make/model

Below balance point — auxiliary heat is running as designed

high confidence

Heat-pump capacity falls with outdoor temperature. Below the balance point, auxiliary heat is supposed to engage.

Safe next steps

Run the balance-point calculator with your AHRI 47/17 °F capacities to confirm expected behavior
Expect higher utility bills during cold snaps when aux runs
If aux stays on for days even after weather warms, escalate

What to document for a pro

Make/model
AHRI rated capacities at 47 °F and 17 °F
Design heating load

Auxiliary heat should be on but isn't — call a pro

medium confidence

Below 20 °F with no aux indicator suggests a wiring, control-board, or staging issue.

Safe next steps

Document indoor and outdoor temps at multiple times
Photograph thermostat display
Do not attempt to wire the aux relay yourself

What to document for a pro

Thermostat make/model
Equipment make/model
Time-stamped temp readings

Aux heat issues involve line-voltage circuits

Restricted airflow — replace filter and clear registers

high confidence

Heavy filter restriction starves the indoor coil and reduces capacity.

Safe next steps

Replace filter with same MERV rating (do not jump to MERV 13+ without checking blower static pressure)
Open all supply registers; do not close more than 10% of the system
Re-test heating after 30 minutes

Heat pump underperforming — professional diagnosis needed

low confidence

Outdoor temp is above the typical balance point, no obvious airflow restriction, and heat is still weak.

Safe next steps

Note supply-air temperature with a probe thermometer (should be 90–105 °F in heat mode for a working HP at 35–47 °F outdoor)
Capture system make/model and any error codes
Do not open the outdoor unit

What to document for a pro

Supply air temp
Outdoor temp
Equipment make/model
Any error codes

Refrigerant work and capacitor service require certification

Diagnostic guidance only. If unsure, stop and call a licensed professional — gas, electrical, and refrigerant work is hazardous to untrained users.

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