Common symptoms

• Thermostat displays “Heat On” or flame icon but vents blow cool air.
• System cycles on/off rapidly (short cycling).
• Outdoor heat pump runs but indoor air feels cold or lukewarm.
• Auxiliary heat never engages even below freezing.
• Breaker trips when switching to heat mode.

Quick diagnostic flowchart

Use this simplified text chart to isolate your issue:

  ┌─> 1️⃣ Is fan blowing but air cold?
  │       ↓
  │   Yes → Go to Step 2.  
  │   No  → Check breaker / system power.
  │
  ├─> 2️⃣ Verify thermostat mode = HEAT.
  │       ↓
  │   Wrong? Switch to HEAT.
  │   Correct? Go to Step 3.
  │
  ├─> 3️⃣ Check outdoor temp below 35°F?
  │       ↓
  │   Yes → Aux heat may be locked out (check settings).
  │   No  → Go to Step 4.
  │
  ├─> 4️⃣ Heat pump running but air cold?
  │       ↓
  │   Yes → Likely reversing valve or wiring issue.
  │   No  → Furnace may not ignite—check flame sensor.
  │
  └─> 5️⃣ Still cold?  
         → Power down system. Verify wiring per model and call a technician if uncertain.

Model-specific checks

Ecobee Smart Thermostat Premium

• Menu → Equipment → Verify heat pump or furnace detected correctly.
• Check “Heat Pump Reversing Valve” setting: should match your system (O or B). If reversed, cooling and heating are swapped.
• Inspect Aux Heat Lockout: set at 25–30°F for cold climates. Too high = no backup heat.

Need a replacement sensor or unit? Shop Ecobee on Amazon →

Nest Learning Thermostat (3rd-gen)

• Settings → Equipment → Heat Pump → “O/B” wire orientation. Swap if blowing cold in heat mode.
• Check “Heat Pump Balance” setting—set to “Max Comfort” temporarily to force more aux heat.
• Test each stage: Settings → Equipment → Continue → Test → Heat.

Check Nest price on Amazon →

Honeywell Home T10 Pro

• Menu → Installer Options → Equipment Type → Confirm Heat Pump vs Gas/Electric setup.
• Verify reversing valve (O/B) orientation; swap if air temp opposite of mode.
• Check sensor priority: T10 may average in unoccupied cold rooms; temporarily disable averaging.

Check T10 Pro availability →

Wiring & power

Loose or miswired connections cause 30% of “cold air” complaints. Use this checklist:

• Confirm power: thermostat screen lit, 24V present between R and C (use multimeter).
• Wires tight on terminals (especially O/B and W/Aux).
• C-wire firmly connected to HVAC control board (C-Wire explained here).
• No corrosion or fraying on thermostat or furnace terminals.

Tools you’ll need: Digital multimeter, screwdriver, flashlight.

Heat pump reversing valve issues

The reversing valve changes refrigerant flow for heating vs cooling. If miswired or stuck, the system cools when it should heat. Signs:

• Outdoor unit runs, but line feels cold during “Heat.”
• Switching thermostat to “Cool” makes warm air (reversed mode).

Fix: Toggle “O/B Reversing Valve” in your thermostat’s heat-pump menu. If no change, valve coil or control board may need service.

Aux heat lockout & outdoor temps

Modern thermostats disable electric or gas backup heat above a threshold (25–40°F) to save energy. In very cold climates, that can backfire.

1. Ecobee: Settings → Equipment → Aux Heat Lockout. Lower to 25°F for cold regions.
2. Nest: Settings → Heat Pump Balance → “Max Comfort” temporarily forces more aux.
3. Honeywell T10: Installer Options → Aux Lockout → adjust to local climate.

For deeper guidance, see Best Thermostats for Heat Pumps.

When to call a professional

If you’ve confirmed correct settings but still get cold air:

• Outdoor coil frozen solid (defrost cycle stuck).
• Blower running but no heat from furnace—possible igniter or flame sensor failure.
• Breaker tripping or fuse blowing repeatedly.

At that point, shut off power and contact a licensed HVAC technician. DIY fixes beyond wiring or settings can risk system damage.

FAQ

Why does my heat pump blow cold air sometimes?

During defrost mode, it temporarily switches to cooling to melt outdoor ice. The thermostat usually signals “Aux Heat” to balance it—if that’s disabled, you’ll feel a chill.

Can I just raise the temperature setting?

Raising setpoint works short-term but wastes energy. Instead, check lockout temps and sensor placement (guide here).

Does a dead C-wire battery cause this?

It can. A thermostat losing power intermittently might mis-trigger modes. Ensure solid 24V supply from control board.