First: two of the most common "not heating" reports aren't faults at all. A mini split in defrost (DF on many displays, steam rising off the outdoor unit) pauses heating for several minutes by design. And below its rated minimum temperature, a unit protects itself rather than heats - that's a spec limit, not a breakdown. Rule those out, then work the list.
Safe checks first
- Mode. HEAT (sun icon), not AUTO or FAN; setpoint several degrees above room temperature. Some remotes have a heating-lock or eco mode that caps output.
- Is it defrosting? Steam or dripping at the outdoor unit and a pause in warm air = normal defrost. Frequent, back-to-back defrosts with poor heat between them = worth a service call.
- Know your unit's floor. Every model has a published minimum heating temperature - some stop in the teens, cold-climate models run to -13F or -22F. Check your model's page in our database or its manual. Below that floor, reduced or paused output is the design working.
- Filters and indoor airflow - same as cooling; a blocked filter guts heating too.
- Outdoor unit clearance. Snow drifted against the coil, ice sheeting off a roof drip line, leaves - the outdoor unit must breathe to extract heat. Clear it (power off first). If it sits in a snow zone without a stand or roof, that's an installation improvement worth making.
- Power cycle at the breaker, five minutes.
- Error code? Look it up in the library.
Beyond the safe checks (technician scope)
- Reversing-valve problems - unit cools fine but won't heat (or blows cold in heat mode long after startup).
- Low refrigerant - weak heat AND weak cooling, ice where it shouldn't be. A2L refrigerants are certified-handling only.
- Defrost sensor/board faults - constant defrost cycles or an iced outdoor coil that never clears.
Cold-climate reality check
If your unit routinely can't keep up on the coldest nights and it's a standard (not low-ambient) model, the fix may be capacity/spec, not repair - see the cold-weather guide before paying for service on a unit doing exactly what its spec sheet says.
Sources
No model-specific systems are linked from this guide yet.