For most homeowners, a heat pump not heating usually traces back to a wrong thermostat mode, a clogged filter, or a normal defrost cycle, not a broken unit. Before you assume the worst, run three checks: confirm the thermostat is set to HEAT (not just fan), pull the filter and hold it up to a light, and watch the outdoor unit for five to ten minutes. If air feels cool but the house is slowly climbing in temperature, that’s often just how heat pumps work. Red flags that mean it’s time to call a licensed technician include:

  • Air that stays cold for over 20 minutes with no warming trend
  • Ice on the outdoor coil that doesn’t clear after a defrost cycle
  • Hissing, bubbling, or a burning smell near the indoor or outdoor unit

If any of those apply, Empire Plumbing and Air Conditioning handles same-day diagnostics across the Phoenix area.

Key Takeaways

A heat pump not heating is usually a thermostat, airflow, or defrost issue you can check yourself, while ice, hissing, or repeated breaker trips signal a fault that needs a licensed technician.

Point Details
Check the basics first Confirm HEAT mode, replace the filter, and clear the outdoor unit before assuming a real fault.
Defrost cycles are normal Brief cold air lasting 5 to 15 minutes with a warm-air finish is expected system behavior.
Measure the temperature differential A healthy supply-return gap runs 25 to 40 degrees; anything lower suggests refrigerant or airflow trouble.
Know the pro-only repairs Refrigerant leaks, reversing valves, and compressor faults require a licensed technician every time.
Call Empire Plumbing and Air Conditioning for persistent faults Their licensed technicians handle refrigerant, defrost control, and AUX/EM heat diagnostics across the Phoenix area.

Table of Contents

Heat Pump Not Heating? Try This Checklist First

Run through these in order. Most homeowner calls get resolved somewhere in the first four steps.

  1. Check the thermostat mode. Confirm it reads HEAT, not COOL or OFF, and set the fan to AUTO rather than ON. Swap the batteries even if the display looks fine.
  2. Pull the air filter. A filter clogged with dust starves the system of airflow and can trigger it to run weak or short cycle. Hold it to a window; if you can’t see light through it, replace it.
  3. Open blocked vents. Furniture, rugs, or closed registers restrict airflow and make rooms feel underheated even when the system runs correctly.
  4. Check breakers and disconnects. Look at the indoor air handler breaker, the outdoor condenser breaker, and any disconnect switch near the outdoor unit.
  5. Inspect the outdoor unit. Clear leaves, dirt, or snow, and watch the fan. If you see light frost and the unit briefly blows cool air, it’s likely a normal defrost cycle.

Pro Tip: Time the cold-air blast with your phone. A defrost cycle typically runs 5 to 15 minutes and ends with a burst of warm air. If it drags past 20 minutes or repeats every hour, stop troubleshooting and call a technician.

Diagnosing the Real Cause of a Cold Heat Pump

Once the quick checklist is done and the problem persists, the cause usually falls into one of seven categories. Here’s how to tell them apart, and which ones you can safely handle yourself.

Thermostat problems go beyond mode selection. Smart thermostats sometimes hold onto an old schedule or a “hold” setting that overrides your manual changes. Try a full reset (power cycle it for 30 seconds) and check the schedule tab for conflicting programs. If the thermostat display temperature seems off by several degrees compared to a separate room thermometer, it may need recalibration, a job your thermostat troubleshooting guide can help walk through before calling anyone.

Airflow restrictions beyond the filter include blocked return grilles, collapsed flex ducts, or closed dampers left over from a summer zoning adjustment. A system straining against restricted airflow often runs longer cycles while barely warming the house.

Blocked HVAC return air grille in home

Outdoor unit and defrost issues are the most misunderstood cause. Trane’s own troubleshooting guidance lists thermostat settings, dirty filters, reversing-valve faults, and refrigerant leaks as the four most common reasons a heat pump stops heating properly. Light frost that clears on its own is normal. A solid sheet of ice that doesn’t melt after a defrost cycle points to a failed defrost sensor or control board, and that’s a call for a pro.

Refrigerant leaks show up as hissing sounds, persistent ice buildup, and a system that runs constantly without ever reaching setpoint. Refrigerant work requires EPA certification and specialized gauges, so this is never a DIY repair.

Reversing valve failure has a distinct signature: the system cools your home fine in summer but blows cold or lukewarm air in winter. The valve directs refrigerant flow between heating and cooling modes, and when it sticks, only a licensed technician can replace it.

Electrical issues include a breaker that trips once (check it, reset it) versus one that trips repeatedly (stop resetting it and call someone, since repeated trips usually mean a short or a failing component).

Auxiliary and emergency heat confusion causes more “not heating” complaints than most homeowners realize. AUX heat kicks in automatically to supplement the heat pump in cold weather; EM HEAT bypasses the heat pump entirely and relies solely on electric strips, which costs significantly more to run. If your thermostat shows EM HEAT lit up without you selecting it, that’s the system telling you it detected a fault.

Sizing and insulation problems make a perfectly healthy heat pump seem broken. If your system runs nonstop without reaching setpoint on cold days, but AUX is engaging and the outdoor unit looks fine, the issue may be an undersized system or a leaky building envelope rather than mechanical failure. A Manual J load calculation from an installer can confirm whether your equipment was ever sized correctly for your home.

Keeping Your Heat Pump From Failing Again

A few consistent habits prevent most of the causes above from happening in the first place.

  • Replace or clean the air filter every 1 to 3 months, more often if you have pets or run the system heavily.
  • Keep a clearance of at least 24 inches around the outdoor unit, clear of plants, debris, and stored items.
  • Schedule an annual professional tune-up that includes a refrigerant charge check, electrical inspection, defrost control test, and airflow measurement.
  • Ask for a sizing or envelope review if the system has never kept up with your home’s heating load, even when running well.

Routine care from a seasonal maintenance program catches small issues, like a slightly low refrigerant charge or a weakening capacitor, before they turn into a no-heat call in January. Homeowners in colder climates sometimes follow more aggressive seasonal maintenance schedules, but the core tasks stay the same regardless of region.

When to Stop Troubleshooting and Call a Technician

Some symptoms simply aren’t safe or practical for a homeowner to chase down. Call a licensed technician if you notice:

  1. Cold air blowing continuously for more than 20 minutes with no warming trend
  2. Ice that builds back up immediately after a defrost cycle finishes
  3. Hissing, bubbling, or oily residue near refrigerant lines
  4. A breaker that trips more than once after resetting

Before you call, jot down your thermostat’s brand and model, a photo of the outdoor unit, and roughly how long the defrost cycles are lasting. If you own an infrared thermometer, note the supply and return air temperatures. That handful of details lets a technician triage the problem before they even arrive, and Empire Plumbing and Air Conditioning offers 24/7 emergency HVAC repair for exactly these situations.

Who’s Actually Behind This Advice

Empire Plumbing and Air Conditioning has served Phoenix, Scottsdale, Glendale, Peoria, Mesa, Tempe, and Gilbert for more than 25 years under ROC licenses 342717 and 342718. Our technicians handle the repairs homeowners can’t: refrigerant leak detection, reversing-valve replacement, defrost control diagnostics, and AUX/EM heat lockout troubleshooting.

  • Licensed, background-checked HVAC technicians
  • Same-day and emergency service across the greater Phoenix area
  • Transparent, honest pricing before any work begins

Compressor Malfunction or Failure

The compressor is the heart of your heat pump. It pressurizes refrigerant and pushes it through the system, and when it fails, heating output drops sharply or stops altogether. A failing compressor often hums or clicks without starting, trips its breaker repeatedly, or runs but produces only lukewarm air even with clean filters and a correct thermostat setting.

Common causes include electrical faults (a bad capacitor or contactor prevents the compressor from starting cleanly), refrigerant problems (running low on charge forces the compressor to work harder and overheat), and simple age. Most compressors are rated for 10 to 15 years of service, and by the time one fails outright, other components have often been under stress for months.

You can’t test or repair a compressor yourself, and attempting to do so risks electric shock or refrigerant exposure. What you can do is watch for early warning signs: a loud clunk when the system starts, breakers that trip specifically when the outdoor unit kicks on, or a noticeable drop in heating performance over a few weeks rather than a sudden stop.

If a technician confirms compressor failure, they’ll also check why it failed. Replacing a compressor without addressing an underlying refrigerant leak or electrical fault just sets up the new one to fail the same way, often within a year or two.

Faulty or Dirty Indoor Coil

The indoor coil, sometimes called the evaporator coil in cooling mode, is where the refrigerant absorbs or releases heat that eventually reaches your living space. A layer of dust and grime across the coil fins acts like a blanket, blocking heat transfer even when every other component works fine.

Dust-covered indoor HVAC evaporator coil

You’ll often notice weak heating performance, longer run times, and sometimes ice forming on the coil itself if the restriction is severe enough to affect refrigerant flow. A dirty coil frequently shows up alongside a dirty filter, since the same lack of filtration that lets dust reach the coil also clogs the filter first.

Homeowners can visually inspect the coil if it’s accessible behind an access panel, but cleaning it properly requires a coil cleaner solution and careful rinsing that won’t damage the fins or nearby electrical components. A bent or corroded coil, especially one showing pinhole leaks, needs replacement rather than cleaning, and that’s a job for a licensed technician with the right tools to recover refrigerant safely.

Annual maintenance visits typically include a coil inspection specifically because this component gets overlooked until performance has already dropped noticeably.

Improper Refrigerant Charge Diagnosis and Solutions

Refrigerant charge, the exact amount of refrigerant in the system, is one of the most precise measurements in HVAC work, and even a slight deviation hurts heating performance. Too little refrigerant (undercharge) usually results from a leak somewhere in the line set or coils. Too much (overcharge) sometimes happens after a poorly executed repair or installation.

Undercharge symptoms include weak heat output, longer run cycles, ice buildup on the outdoor coil, and a supply-return temperature differential that falls short of the healthy 25 to 40 degree range. A properly charged heat pump running in steady-state heating should hit that range consistently; anything noticeably lower suggests refrigerant, airflow, or compressor trouble.

Diagnosing the exact charge requires gauges connected to the refrigerant lines, along with readings for suction pressure, subcooling, and superheat, none of which a homeowner can safely measure without training and EPA certification for refrigerant handling. If a leak caused the undercharge, simply adding refrigerant is a temporary patch. The leak has to be found and sealed first, or the system will lose charge again within months.

This is one of the clearest lines between a DIY check and a professional repair: you can suspect low refrigerant based on symptoms, but confirming and correcting it requires a licensed technician every time.

Sensor Problems Affecting Operation

Modern heat pumps rely on several temperature sensors to make decisions: when to start a defrost cycle, when to engage auxiliary heat, and when to shut down to protect the compressor from overheating or freezing conditions. A failed or drifting sensor can cause the system to behave in ways that look like bigger problems.

A defrost sensor that reads warmer than actual outdoor coil temperature might prevent defrost cycles from starting, leading to the persistent ice buildup Kodiak HVAC identifies as a common non-normal cause of heating failure. A sensor that reads too cold might trigger defrost too often, wasting energy and reducing heating time.

Thermostat sensors matter too. If the thermostat’s internal sensor is miscalibrated, it might tell the system the house has reached setpoint when it hasn’t, or vice versa, leading to short cycling or a home that never quite feels warm enough despite the system running.

You generally can’t test these sensors yourself since they require a multimeter and knowledge of expected resistance values at given temperatures. What you can do is note the pattern: does the problem happen at specific outdoor temperatures, at specific times of day, or randomly? That pattern often points a technician straight to the faulty sensor without extensive diagnostic time.

How Cold Weather Affects Heat Pump Performance

Heat pumps extract heat from outdoor air, and as that air gets colder, there’s simply less heat available to extract. This isn’t a malfunction. It’s physics, and it’s why heating capacity drops as outdoor temperatures fall, even on a perfectly healthy system.

This is exactly where auxiliary heat is supposed to step in, adding electric resistance heat to make up the difference on the coldest days. If AUX never engages during a cold snap, either the lockout setting is too aggressive or something else is preventing it from activating, both of which are configuration issues rather than equipment failure.

Extreme cold also increases how often the outdoor unit needs to run defrost cycles, since frost forms faster on the coil. More frequent defrost cycles mean more moments where you’ll feel that cool air blast, which is normal, just more noticeable in January than in November.

Heat pump coil melting frost during defrost

If your system genuinely can’t keep the house warm during your coldest stretch of weather, and AUX heat is confirmed to be running, the underlying issue is very often sizing or insulation rather than the heat pump itself. A system sized correctly for your climate zone and home’s heat loss should manage all but the most extreme outliers without falling far behind.

Homeowners in mild climates sometimes discover their heat pump was never sized for the occasional hard freeze, and that gap only shows up once every few years, making it easy to mistake for a random mechanical issue rather than a persistent sizing shortfall.

What Homeowners Get Wrong About a Cold Heat Pump

Most advice online treats every cold blast of air as an emergency, and that’s the biggest disservice to homeowners trying to figure out what’s actually happening. A heat pump that blows air within a typical warm range during a defrost cycle isn’t broken; it’s doing exactly what it’s designed to do. The conventional wisdom skips right past that nuance and sends people straight into panic mode, or worse, straight into a repair call for something that resolves itself in ten minutes.

What actually matters is pattern recognition. Time the cold air. Check whether AUX heat engages when it should. If you have an infrared thermometer, measure the supply and return temperatures, since a gap under 20 degrees tells you more than any amount of guessing about “is it broken.” Most homeowners skip this step entirely and jump straight to worst-case assumptions.

Where the real judgment call comes in is sizing. A system that’s never kept up with a home’s coldest days isn’t failing. It was likely undersized from the start, and no filter change fixes that. When the basics check out and the problem persists, that’s exactly when Empire Plumbing and Air Conditioning’s licensed technicians earn their keep, running the diagnostics a homeowner physically can’t.

Get Your Heat Pump Fixed by Local Licensed Technicians

Empire Plumbing and Air Conditioning is the direct route to a fixed heat pump when the checklist above doesn’t solve it, no guesswork, no upsell pressure, just a licensed technician who diagnoses the actual fault the first visit.

Empire Plumbing and Air Conditioning

Homeowners across Phoenix, Scottsdale, Mesa, Tempe, Gilbert, Peoria, and Anthem call us for exactly the problems this article covers: a reversing valve stuck in cooling mode, a refrigerant leak that keeps draining charge, a defrost sensor that’s stopped triggering cycles correctly. Our technicians carry the gauges and training to confirm what’s wrong in one visit rather than sending you through a string of guesses. Every job comes with upfront pricing before work starts, and our 24/7 emergency HVAC repair line means a cold house in January doesn’t have to wait until Monday.

If your system passed the homeowner checklist but still isn’t heating right, schedule a diagnostic visit and get a technician out to your home this week.

Frequently Asked Questions

Why is my heat pump not heating even though the fan is running?
A running fan with no heat usually points to a thermostat set to the wrong mode, a tripped breaker on the outdoor unit, or a reversing valve stuck in cooling position. Check the thermostat first, since that resolves the issue more often than any mechanical cause.

Why is my heat pump blowing cold air instead of warm?
Brief cold air lasting under 15 minutes is typically a normal defrost cycle. If it continues longer or repeats frequently, suspect a defrost control or refrigerant issue and call a technician.

How do I know if my heat pump is not defrosting properly?
Watch the outdoor coil during cold weather. Light frost that clears within a defrost cycle is normal; a thick, solid layer of ice that doesn’t melt points to a failed defrost sensor or control board.

What’s the difference between AUX heat and emergency heat?
AUX heat automatically supplements your heat pump during cold weather and is normal, efficient operation. Emergency heat (EM HEAT) bypasses the heat pump entirely and relies only on electric resistance heat, which costs significantly more to run and usually indicates the thermostat detected a system fault.

Why does my heat pump run constantly but never keep the house warm?
This pattern often means the system is undersized for the home, refrigerant charge is low, or insulation gaps are letting heat escape faster than the system can replace it. A supply-return temperature check or a load calculation from a technician can confirm which cause applies.

Sources

Do not wait for a small AC issue to become a major breakdown. If your air conditioner is blowing warm air, making strange noises, leaking water, or failing to cool your home, Empire Plumbing & Air Conditioning can help.

Call 602-275-8400 today to schedule professional A/C repair in Phoenix.

Serving Phoenix, North Phoenix, Scottsdale, Peoria, Glendale, Anthem, Cave Creek, Mesa, Tempe, Gilbert and surrounding Valley communities.

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