Heat Pump Not Heating — Causes and Fixes
Heat pumps work differently from furnaces, so "not heating" can mean something normal (defrost mode) or a real problem. Here's how to tell, and what you can check yourself.
Understand "Cool" Air Is Normal-ish
Heat pumps deliver warm air that's cooler than a furnace's (often 85-95°F, which can feel lukewarm). And in very cold weather they rely on backup/auxiliary heat. So mildly cool air isn't always a fault. But cold air or no heat is.
1. Check Thermostat and Filter
Set the thermostat to HEAT (not "emergency heat" unless needed) above room temp. Replace a clogged air filter — restricted airflow is a top cause of poor heating and can cause the indoor or outdoor coil to ice up.
2. Outdoor Unit Iced Over
Heat pumps periodically run a defrost cycle to melt frost off the outdoor coil — during which the indoor air briefly goes cool and you may see steam outside. That's normal. But if the outdoor unit is encased in a thick block of ice that won't clear, the defrost system has a problem, or airflow is blocked. Clear snow/leaves from the outdoor unit and keep it clear.
3. Reversing Valve or Refrigerant
If the heat pump blows cold in heat mode and the outdoor unit runs, a stuck reversing valve (which switches between heating and cooling) or low refrigerant could be the cause — both need an HVAC technician. Low refrigerant also shows as an iced coil and weak heat.
4. Auxiliary Heat Not Engaging
In very cold weather, the heat pump should bring on electric/gas backup heat. If "aux heat" never comes on and the home stays cold, a control or wiring issue may be preventing it — worth a technician check, especially in a cold snap.
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FAQ
It may be in a normal defrost cycle (brief), or it could be a clogged filter, an iced-over outdoor unit, a stuck reversing valve, or low refrigerant. Check the filter and clear the outdoor unit first; valve/refrigerant issues need a technician.
Light frost is normal and the defrost cycle clears it periodically. A thick, persistent block of ice is not normal and means a defrost, airflow, or refrigerant problem.
Always unplug an appliance and shut off its water supply before servicing. This guide is informational and not a substitute for a qualified technician.