A heat pump that runs but won’t heat usually traces to one of six causes: wrong thermostat mode, a stuck reversing valve, low refrigerant, a stuck defrost cycle, an iced outdoor coil, or failed backup heat strips. Start by setting the thermostat to Heat at 75F, then check a supply register. The symptom-to-cause table below narrows it down in about 10 minutes.
One thing first. Heat pumps deliver warm air around 85-93F, cooler than a gas furnace. Skin sits at 98.6F, so that air can feel lukewarm at the register even when the system works fine. Hold your hand over a vent for a minute before assuming a failure. If the house is slowly warming, nothing’s broken.
Here are the six things that actually cause a real no-heat call, ranked by how often we see each in San Diego, with the test you can run for each.
The fast answer
| Cause | How often we see it | DIY-fixable? |
|---|---|---|
| Thermostat set to “Cool” or “Emergency Heat” | 15% | Yes |
| Reversing valve stuck in cooling mode | 25% | No |
| Low refrigerant from a leak | 20% | No |
| Defrost cycle stuck on | 10% | Sometimes |
| Outdoor coil iced over | 15% | Partial |
| Failed auxiliary heat strip (in dual-fuel and emergency-heat-equipped units) | 15% | No |
Symptom-to-cause decision tree
Match what you’re seeing to the likely cause. Read top to bottom and stop at the first row that fits.
| What you’re seeing | Most likely cause | DIY check | Section |
|---|---|---|---|
| Air feels lukewarm but house is warming | Normal. Heat pump air runs 85-93F | Hold your hand on the vent 60 seconds | Intro |
| Fan runs, air is cold, thermostat says Cool or Em Heat | Wrong thermostat mode | Switch to Heat, set 3-5F above room temp | 1 |
| Fan runs, air stays cold, switching to Cool gives cold air | Reversing valve stuck in cooling | Set to Cool and feel the register | 2 |
| Fan runs, air is room temperature, no cold in either mode | Low refrigerant from a leak | Listen for hissing, check for ice | 3 |
| System blows cool 5-15 min, then warms, repeats | Normal defrost cycle | Wait it out, this is by design | 4 |
| System blows cool over an hour and never warms | Defrost cycle stuck on | Tech needed, defrost board or sensor | 4 |
| Outdoor unit is a visible block of ice | Iced outdoor coil | Turn system Off, let ice melt 4-8 hrs | 5 |
| Worked last winter, no heat only on cold mornings | Failed aux heat strips | Tech needed, listen for strip contactor | 6 |
| Nothing runs at all, no fan, no outdoor unit | Power, breaker, or control board | Check breakers and disconnect switch | Diagnostic sequence |
| Air filter visibly clogged, weak airflow | Blocked airflow | Replace the filter, then retest | Diagnostic sequence |
1. Thermostat in the wrong mode
The first check, every time. Heat pump thermostats have more modes than standard AC/furnace thermostats, and it’s easy for the system to land in the wrong one.
- Cool. The compressor runs and produces cold air. If your thermostat is in Cool but the room is colder than the set point, you’ll feel air moving with no heat.
- Heat. Normal heating mode. The system reverses the refrigerant flow and pulls heat from the outdoor air.
- Emergency Heat. Bypasses the heat pump and runs only the electric resistance strips (if you have them). Expensive to run. Some homeowners set it during a cold snap and forget to switch back.
- Auto. Switches between heat and cool based on temperature. Generally fine for San Diego.
Fix. Set the thermostat to Heat. Set the temperature 3-5 degrees higher than the current room temperature. Listen for the outdoor unit to start, then walk to a supply register and check if air is warm within 5-10 minutes. If yes, you were just in the wrong mode.
2. Reversing valve stuck in cooling mode
The reversing valve is what makes a heat pump different from an air conditioner. It physically reverses the direction of refrigerant flow so the same equipment that cools your house in summer can heat it in winter. When the valve sticks in cooling position, you get cold air out of the supply registers in heat mode, because the system is still trying to cool.
Tell. Outdoor unit runs hard. Indoor blower runs. Supply air is the same temperature as outdoor air or colder. Set the thermostat to Cool and check what comes out. If it gets cold, the reversing valve is stuck in cool. If it stays room-temperature, the issue is elsewhere (probably refrigerant).
Fix. This is a tech job. The valve can sometimes be freed by cycling the system between modes several times, but a truly stuck valve needs the solenoid replaced or the whole valve swapped. Expect $400-$900 for the repair in San Diego, depending on whether the solenoid alone fixes it or the entire valve needs replacement.
3. Low refrigerant from a leak
Heat pumps need the right refrigerant charge to move heat efficiently. A slow leak that takes the system below charge means it can’t move enough heat to warm your house. Symptoms often look like the reversing valve issue: outdoor unit running, indoor blower running, no warm air.
Tell. Coastal San Diego homes are most likely to see this. Salt air corrodes the aluminum fins and copper line set over years, eventually opening a slow leak. Outdoor unit ices up partially or entirely. Hissing or bubbling sound from the line set. Refrigerant line is the same temperature as the surrounding air instead of warm during a heat call.
Fix. Tech-only. We use an electronic leak detector or UV dye to find the leak, repair it, and recharge the system. Costs run $400-$1,500 depending on leak location and refrigerant type. R-410A systems are most common; older R-22 units cost more to recharge because the refrigerant is no longer made.
Coastal San Diego homes (Coronado, La Jolla, Carlsbad, Oceanside) lose heat pumps to corrosion faster than inland ones. We cover the salt-air pattern in our coastal AC repair page.
4. Defrost cycle stuck on
In heat mode, the outdoor coil gets cold (often below freezing) as the system pulls heat out of the air. When the coil ices over, the system automatically reverses to cooling mode for a few minutes to melt the ice, then reverses back to heat. This is normal and called the defrost cycle. You’ll see steam coming off the outdoor unit and feel cool air out of the supply registers for 5-15 minutes.
When the defrost cycle gets stuck on, the system stays in cooling mode and never returns to heat. The defrost board or sensor has failed.
Tell. Outdoor unit runs continuously. Indoor air is cool, not warm. Steam may or may not be visible. The system never seems to “kick back into heat.”
Fix. Tech-only. Replace the defrost board or the outdoor sensor. Costs $250-$600 in San Diego depending on the brand and parts availability.
5. Outdoor coil iced over
Not the same as a stuck defrost cycle, but related. If the system has a defrost problem or if airflow over the outdoor coil is blocked, ice can build up and prevent any heat absorption. The whole outdoor unit becomes a block of ice. Heat transfer goes to zero.
Tell. Outdoor unit is visibly covered in ice or frost. Indoor air is at room temperature or cooler.
Fix. First, turn the system off at the thermostat and let the ice melt naturally (4-8 hours, faster if you can move warm water over the coil from a distance). Once melted, restart and see if the problem returns. If it ices over again within hours, you have a defrost problem (see #4) or a refrigerant problem (see #3). Either way, call a tech.
San Diego rarely gets cold enough to ice a heat pump from outdoor temperature alone (most coastal areas never see overnight lows below 38F). When we see iced coils here, it’s almost always a defrost or refrigerant issue rather than a climate problem.
6. Failed auxiliary heat strips
Many heat pumps have backup electric resistance strips that engage when outdoor temperatures drop below the heat pump’s efficient range, or when the thermostat detects the house can’t keep up. In San Diego we rarely need them, but they’re installed on most systems.
If the heat pump itself has failed but the strips are still working, you’ll get warm air. If the strips have failed and the heat pump is borderline, you may get no warm air at all on the coldest mornings.
Tell. System worked fine last winter. This winter, mornings below 45F outdoor temperature produce no heat. Or you can hear the contactor click for the strips but no warmth follows.
Fix. Tech job. Strip replacement runs $300-$800 in San Diego depending on the size of the heating elements and the air handler model.
The diagnostic sequence (do this in order)
- Set the thermostat to Heat at 75F. Listen at the outdoor unit. Does it start?
- If nothing runs at all, check the basics first. Look at your electrical panel for a tripped breaker (heat pumps use a 240V outdoor breaker and a separate indoor air handler breaker). Check the outdoor disconnect box on the wall near the unit. Check the power switch on the air handler indoors. Reset anything that’s off, then retest.
- Check the air filter. A clogged filter chokes airflow and can stop heat at the register even when the system runs. Replace it if it’s dirty, then retest.
- Walk to a supply register. Is air moving? Is it warm, cool, or room temperature? Remember, warm air at 85-93F can feel lukewarm but is normal.
- If air is cold: flip the thermostat to Cool, same temperature. If you get cold air, reversing valve is stuck in cool. If still room temperature, low refrigerant is likely.
- If air is room temperature: check the outdoor unit. Iced? Stuck defrost? Refrigerant lines warm or cold?
- If the outdoor unit is silent and breakers are fine: thermostat wiring, control board, or contactor. Tech needed.
San Diego-specific patterns
Heat pumps in San Diego have an unusual advantage: they’re rated efficient down to about 5F, and we never see temperatures even close to that. Most coastal homes never see overnight lows below 38F. This means the heat pump itself almost never struggles for climate reasons. When you get a no-heat call here, it’s a failed part, not a weather problem.
The most common heat pump failures we see in San Diego, in order:
- Salt-air corrosion in coastal homes (Coronado, La Jolla, Carlsbad, Oceanside, Encinitas). Slow refrigerant leaks from corroded coils show up at 8-12 years.
- Stuck reversing valves, often after a long cooling season with no heat use. The valve sits in cooling position from April through October and seizes when winter calls.
- Failed defrost boards in inland homes that see actual cold mornings (Escondido, San Marcos, Vista, El Cajon, Santee).
- Aging electric strips on heat pumps installed before 2010. The strips were undersized for the air handler and have shorter lifespans.
If you’re considering heat pump replacement instead of repair, the SDG&E and TECH Clean California rebates make this an unusually good time to upgrade. We break it down in our 2026 heat pump rebate guide.
What it costs to fix in San Diego
| Issue | Typical 2026 cost |
|---|---|
| Thermostat reset (DIY) | $0 |
| Refrigerant leak detect + repair + recharge | $400-$1,500 |
| Reversing valve solenoid | $250-$450 |
| Reversing valve full replacement | $600-$1,200 |
| Defrost control board | $300-$600 |
| Defrost sensor | $200-$350 |
| Outdoor contactor | $200-$350 |
| Aux heat strip replacement | $300-$800 |
If your heat pump is over 12 years old and the repair quote is above $1,200, ask for a replacement number before committing. The repair-vs-replace math often favors replacement at that age, especially with current rebates.
FAQs
Why is my heat pump running but not heating?
The six common causes: thermostat in the wrong mode, reversing valve stuck in cooling, low refrigerant from a leak, defrost cycle stuck on, outdoor coil iced over, or failed backup heat strips. The diagnostic sequence above narrows it down in about 10 minutes.
How can I tell if my reversing valve is bad?
Set the thermostat to Heat. Wait until the system runs. Then set it to Cool with the same air-temperature target. If air comes out cold in Cool mode but room-temperature or cold in Heat mode, the valve is likely stuck in cooling.
Is it normal for my heat pump to blow cold air sometimes?
Yes, briefly. When the defrost cycle activates (usually every 30-90 minutes in cold weather), the system reverses to melt ice off the outdoor coil. You’ll get 5-15 minutes of cool air during defrost, then warm air resumes. If it stays cool for an hour or more, the defrost cycle is stuck.
Why is my heat pump iced over?
Defrost cycle failure, low refrigerant, or extended cold weather beyond what the system is designed for. In San Diego it’s almost always the first two; we rarely get cold enough for ice to form from temperature alone.
Should I turn my heat pump off if it’s iced over?
Yes. Set the thermostat to Off and let the ice melt naturally over 4-8 hours. Restart and see if the problem returns. If it ices again within hours, call a tech.
My heat pump won’t turn on at all. What should I check first?
Check the electrical basics before assuming a major failure. Heat pumps use a 240V outdoor breaker and a separate indoor air handler breaker, so look at your panel for a tripped breaker. Check the outdoor disconnect box near the unit and the power switch on the indoor air handler. Reset anything that’s off and retest. If breakers keep tripping, that points to a short or a failing component, so call a tech.
Does running emergency heat all winter damage my heat pump?
It doesn’t damage the heat pump itself, but it costs 2-3 times more to run than normal heat mode because it relies entirely on electric resistance strips. If your heat pump is fine and you’re just running emergency heat from habit, you’re spending money for no reason.
How long does a heat pump last in San Diego?
Coastal homes: 10-13 years before salt-air corrosion forces a coil or full system replacement. Inland homes: 14-18 years. Well-maintained units approach the top of those ranges; neglected ones fail at the bottom.
How much does it cost to fix a heat pump that won’t heat in San Diego?
A thermostat reset is free. Refrigerant leak detection, repair, and recharge runs $400-$1,500. A reversing valve solenoid is $250-$450, or $600-$1,200 for a full replacement. A defrost control board is $300-$600, and an aux heat strip replacement runs $300-$800. Most San Diego shops charge a flat diagnostic fee credited toward the repair if you move forward.
When to call us
If the diagnostic sequence points to refrigerant, the reversing valve, or the defrost system, those are tech jobs. Call (442) 777-6440 for a same-day diagnostic at $89 flat, credited toward the repair.