Why this happens

Trane and Carrier both list a dirty filter among the causes of a heat pump that isn't heating. Less air across the indoor coil means less heat reaches the rooms. Trane recommends replacing the filter every 1 to 3 months.

How to fix it

Set the thermostat to Off before you pull the filter.
Replace the filter with the same size, arrow pointing toward the air handler.
Set the thermostat back to Heat, fan on Auto, and check the air after several minutes.
Write the date on the new filter's edge so you know when it's due again.

When to stop. If the air is still cool with a clean filter, call an HVAC technician.

If this does not fix it, tell the HVAC technician

How dirty the filter was and when it was last changed.
Whether heating improved after the new filter.

Build the full list: what to tell the HVAC technician.

Start this troubleshooter over

References (every fact above comes from one of these pages)

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