Heat Pump Troubleshooting

Your heat pump is running, the fan is blowing, but the house keeps getting colder—or the system won’t come on at all. Heat pumps fail in a handful of predictable ways, and most of them trace back to thermostat settings, airflow, a frozen outdoor coil, or the defrost and reversing controls that let the system switch between heating and cooling. This guide walks you through the checks you can safely do yourself, explains what each symptom usually means, and gives you real 2026 US repair costs so you know what a fair quote looks like.

How a Heat Pump Actually Heats (and Why That Matters)

Unlike a furnace, a heat pump doesn’t create heat—it moves it. In heating mode, the outdoor coil absorbs heat from outside air (yes, even cold air contains usable heat energy down to well below freezing) and the compressor raises its temperature before it’s released through the indoor coil. A reversing valve flips the refrigerant flow direction between heating and cooling, and a defrost cycle periodically melts frost off the outdoor coil.

That’s why heat pump troubleshooting looks different from furnace troubleshooting. A heat pump that “runs but doesn’t heat” is often a defrost problem, a reversing valve stuck in cooling, or a refrigerant charge issue—not a failed ignition or a cracked heat exchanger.

Key numbers to know:

• 1 ton of capacity = 12,000 BTU per hour. A typical 2,000 sq ft US home needs roughly a 3-ton (36,000 BTU) system, depending on climate zone.
• Supply-to-return temperature rise in heating mode should be about 15°F to 25°F after 10–15 minutes of steady runtime. Under 15°F suggests a refrigerant or airflow problem.
• Auxiliary electric strip heat is commonly sized at 5 kW to 20 kW (17,000 to 68,000 BTU) and is expensive to run—it’s backup, not primary heat.

Quick Checks Before You Call Anyone

Work through these in order. They cost nothing and resolve a surprising share of “heat pump not heating” calls.

1. Confirm the thermostat mode and fan setting

• Mode must be HEAT, not AUTO or COOL.
• Fan should be AUTO, not ON. A fan set to ON runs constantly and pushes room-temperature air that feels cold, which convinces people the system is broken.
• Check the setpoint is at least 2°F above current room temperature.
• Replace thermostat batteries if it’s battery-powered, and check the breaker at the air handler—many systems have a separate 240V disconnect and a 24V control fuse on the board.

2. Check or replace the air filter

A clogged filter is the single most common cause of weak heating. Restricted airflow drops the refrigerant pressure, can ice the indoor coil, and can trip the high-limit switch. Replace 1-inch filters every 30–90 days; use a MERV 8–11 filter, not a MERV 16, unless your system was designed for the pressure drop.

3. Look at the outdoor unit

• Is it running? If the outdoor fan and compressor are silent, you likely have a tripped breaker, blown capacitor, bad contactor, or failed defrost board.
• Is it iced over? Light frost that clears every 30–90 minutes is normal. A solid block of ice on a 45°F day means the defrost cycle has failed.
• Clear 2 feet of clearance around the unit and remove snow, leaves, and mulch. Never cover a heat pump while it’s operating.

4. Verify you’re not in a lockout or emergency heat mode

Many thermostats display “AUX,” “EM HEAT,” or an alert code when the system has locked out after repeated faults. Emergency heat will keep the house warm but at 2–3x the operating cost. Note any error codes—they shorten diagnostic time considerably.

Common Causes of a Heat Pump Not Heating

Frozen Outdoor Coil and Defrost Failure

When the outdoor coil drops below roughly 32°F, frost forms and the defrost control board initiates a reversal into cooling mode with the outdoor fan off to melt it. If the defrost sensor, board, or reversing valve fails, ice builds until airflow stops and the system can’t absorb heat.

Repair cost: defrost control board $300–$600 installed; defrost thermistor $150–$350; reversing valve $600–$1,200.

Reversing Valve Stuck in One Position

A reversing valve stuck in cooling mode means the system air-conditions in January. Symptoms: outdoor unit runs, indoor coil gets cold instead of warm, and the house never heats. This is a sealed-system repair requiring refrigerant recovery under EPA Section 608 rules.

Repair cost: $600–$1,200 for the valve plus $150–$500 in refrigerant, typically $800–$1,500 total.

Low Refrigerant Charge or a Leak

A system that’s undercharged can’t build enough pressure differential to deliver heat. Signs include a hissing or bubbling sound at the line set, ice at the outdoor service valve, and a temperature rise under 15°F. Topping off refrigerant without finding the leak is a temporary fix.

Repair cost: leak detection $150–$350; leak repair $200–$1,200; recharge $200–$900 depending on refrigerant type and quantity.

Failed Capacitor, Contactor, or Compressor

A weak run capacitor prevents the compressor or outdoor fan from starting—you’ll hear a humming or clicking from the outdoor unit. A pitted contactor prevents the 240V circuit from closing. A seized compressor is the worst case.

Repair cost: capacitor $150–$300; contactor $150–$300; compressor $1,200–$2,500.

Airflow and Ductwork Problems

Leaky or disconnected ducts in an attic or crawlspace can lose 20–30% of your conditioned air. Undersized return ducts starve the coil. Both show up as a heat pump that runs continuously without reaching setpoint.

Repair cost: duct sealing $400–$1,200; duct replacement $1,500–$4,000.

Auxiliary Heat and Balance Point Issues

Every heat pump has a balance point—the outdoor temperature where capacity equals the home’s heat loss, usually 25°F to 35°F for standard models. Below that, the thermostat should stage on auxiliary strip heat. If aux heat never engages, the house will drift cold on the coldest nights. If it engages constantly above 40°F, you likely have a charge, airflow, or control problem.

If you’ve worked through the checks above and the system still won’t heat, a systematic heat pump troubleshooting walkthrough can help you narrow down which component is at fault before you schedule service.

Defective Thermostat or Control Wiring

A failed thermostat, a broken wire at the outdoor unit, or a blown low-voltage fuse will leave the system unresponsive. Technicians typically confirm this in minutes with a multimeter at the control board.

Repair cost: thermostat replacement $150–$500 installed; low-voltage wiring repair $150–$400.

What Repairs Should Cost in the US (2026)

Rule of thumb: if the repair exceeds 50% of the cost of a new system and your unit is over 12–15 years old, replacement usually wins on lifecycle cost. Modern units must meet SEER2 14.3 / HSPF2 7.5 minimums for split systems; ENERGY STAR certified models go further (generally HSPF2 of 8.1 or higher), and in 2023+ federal tax credits under the Inflation Reduction Act can cover up to $2,000 for qualifying heat pumps.

DIY vs. Professional Repair

Safe to DIY: thermostat settings, filters, clearing snow and debris, breaker resets, coil cleaning with manufacturer-approved cleaner, and visual inspection.

Call a licensed technician: anything involving refrigerant, the compressor circuit, reversing valves, control boards, or gas/electric aux heat wiring. Refrigerant handling is federally regulated, and a botched repair can void your manufacturer warranty. Look for a NATE-certified technician and confirm the company holds state licensing and EPA 608 certification.

When you’re ready to book service, compare quotes for professional heat pump repair from at least two licensed contractors—pricing varies significantly by region and season.

Maintenance That Prevents Repeat Failures

• Twice a year: hose down the outdoor coil, check refrigerant line insulation, and test the defrost cycle by watching a full cycle in cold weather.
• Monthly: inspect the filter and the condensate drain line.
• Annually: schedule a professional tune-up ($100–$250) that includes a charge check with superheat/subcooling, amp draw on the compressor and blower, and a control board inspection.
• Keep the outdoor unit elevated above typical snow depth and off the landscaping fabric.

Frequently Asked Questions

Why is my heat pump blowing cold air when the heat is on?

The most common causes are a thermostat stuck in cooling mode or with the fan set to ON, a clogged air filter restricting airflow, a frozen outdoor coil caused by a defrost failure, or low refrigerant charge. Check the thermostat mode and filter first, then look at the outdoor unit—if it’s caked in ice on a mild day, the defrost cycle is failing and you need a technician.

How much does it cost to fix a heat pump that isn’t heating?

In the US, a diagnostic visit typically runs $75 to $150. Common fixes range from $15 for a filter to roughly $150 to $300 for a capacitor or contactor, $300 to $600 for a defrost control board, $600 to $1,200 for a reversing valve, and $1,200 to $2,500 for a compressor. Refrigerant leak repair and recharge usually falls between $200 and $900 depending on the size of the leak and the refrigerant type.

At what outdoor temperature should I expect my heat pump to stop keeping up?

Standard heat pumps have a balance point between roughly 25°F and 35°F, meaning below that they need auxiliary electric strip heat to maintain indoor temperature. Cold-climate heat pumps rated for ENERGY STAR Cold Climate designation can deliver rated capacity down to 5°F and continue operating well below 0°F, so if your standard unit struggles at 30°F something may be wrong.

Conclusion

A heat pump that isn’t heating almost always fails in one of five places: the thermostat setting, airflow, the defrost cycle, the refrigerant charge, or the reversing valve. Start with the free checks—mode set to HEAT, fan set to AUTO, a clean filter, and a clear, ice-free outdoor unit—then measure the supply-to-return temperature rise. If you’re seeing less than 15°F of rise, ice that never melts, or an outdoor unit that won’t start, it’s time for a licensed, NATE-certified technician to test the charge, the control board, and the reversing valve. Catching a defrost sensor or capacitor early keeps a $200–$400 repair from becoming a $1,500 reversing valve job or a full system replacement—and if you’re still chasing symptoms, the dedicated guide to a heat pump not heating covers the edge cases in more depth.