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HVAC Diagnostic Guide HIGH URGENCY

Heat Pump Not Cooling

Heat pump running in cooling mode but no cold air, or much weaker than expected.

Quick Check Before You Call a Pro

  1. Set thermostat to COOL (not HEAT, not AUTO) — verify the call
  2. Check the outdoor unit is running
  3. Note temperature differential — should be 18-22°F drop across the indoor coil
  4. Inspect for ice on the suction line (large copper line outside)

01 · Most Likely Causes (Ranked)

25%

Stuck Reversing Valve (in Heating Position)

What's happening

Heat pump's reversing valve stuck in heating mode. You're calling for cooling but the system is pumping heat INTO the home instead.

How to check

Feel the smaller insulated copper line at the outdoor unit during a cooling call. Should be cold. If it's warm/hot, the valve is stuck in heating position.

How to fix

Tap the valve with a rubber mallet to try freeing it. If still stuck, replacement is required — significant brazing job, call a tech.

Shop Reversing Valves
20%

Low Refrigerant

What's happening

Refrigerant leak — same symptoms as in AC mode. Reduced cooling capacity, longer run times, possible ice on suction line.

How to check

Long run times, weak temperature drop (less than 15°F), ice on copper lines, hissing near connections.

How to fix

EPA-certified tech finds leak, repairs, recharges.

18%

Failed Capacitor

What's happening

Same as AC. Dual run capacitor failed completely or weakened, reducing compressor/fan performance.

How to check

Outdoor unit hums but fan or compressor doesn't run. Test MFD with multimeter, must be within 6% of label.

How to fix

Replace with matching MFD and voltage capacitor.

Shop Capacitors
15%

Dirty Outdoor Coil

What's happening

Same as AC. Outdoor coil can't reject heat. Especially common on heat pumps because they run year-round (more dust accumulation).

How to check

Visual: coil should show clean aluminum fins.

How to fix

Power off, gentle hose wash from inside out.

12%

Failed Defrost Board (causing reverse-mode issues)

What's happening

Sometimes a failing defrost board causes inappropriate reversing-valve activation, swapping you between heating and cooling unintentionally.

How to check

Symptoms vary — intermittent cold air, brief blasts of warm air during cooling, etc. Diagnostic blink codes from the outdoor board usually present.

How to fix

Replace defrost board. Match by manufacturer part number.

Shop Control Boards
10%

Compressor Issues

What's happening

Compressor failure or severely reduced output. Same as AC diagnosis.

How to check

Outdoor fan runs but you can't hear the compressor humming. Or full unit runs but ZERO cooling output. Amperage well below RLA.

How to fix

Compressor replacement rarely makes sense on aging equipment. Full system replacement usually the answer at 12-15 years.

Need the Replacement Part?

OEM-quality HVAC parts shipped same day on orders before 3 PM CST. Contractor pricing available.

Reversing ValvesCapacitorsControl Boards

02 · Frequently Asked Questions

Why does my heat pump only cool weakly?
Most common: low refrigerant from a leak, dirty outdoor coil, or weakening capacitor. Heat pumps tend to leak more than ACs because they run year-round (more wear on connections).
Should my heat pump cool the same as a regular AC?
Yes. In cooling mode, a heat pump is functionally identical to an air conditioner — same refrigerant cycle, same cooling capacity per ton. If yours cools poorly, troubleshoot like an AC.
How do I tell if my heat pump's reversing valve is stuck?
Set to COOL. After 5 minutes, feel the small insulated copper line outside — should be cold. If warm, valve is stuck in heating position. Conversely in heat mode, the large line should be hot.
Can a heat pump run in AC mode all summer?
Yes, that's its design. Most heat pumps spend more hours in cooling mode (long summer) than heating mode (shorter winter). Maintenance becomes critical because of the high run-hour count.

03 ·Related Symptoms

Heat Pump Not HeatingAC Not CoolingAC Blowing Warm Air
⚠ Safety notice: This guide is for informational purposes only. HVAC systems involve high-voltage electricity, refrigerants under pressure, and natural gas — all of which can cause serious injury, death, or property damage. Refrigerant handling requires EPA Section 608 certification by federal law. When in doubt, contact a licensed HVAC professional.
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