If your heat pump isn't heating, check the thermostat, the filter, and the outdoor unit first, because most fixes are one of these three. Beyond them, the usual fault groups are airflow, defrost and ice, electrical trips, and undersizing or short cycling. Refrigerant leaks, reversing valve faults, repeated breaker trips, and a coil that re-freezes after clearing need a certified technician.
TL;DR:
- After changing thermostat settings, wait 10 to 15 minutes; check filters monthly in heavy use seasons, and reset a tripped breaker only once.
- Cold weather can bring steam and cooler indoor air during defrost for 5 to 15 minutes, every 30 to 90 minutes; lingering ice needs service.
- Cold air in Heat mode despite normal cooling suggests a valve fault; suspected refrigerant leaks need certified technicians and can cause weak heat or unusual icing.
- Short cycles, rooms below setpoint, or constant auxiliary heat can signal poor sizing; ask a contractor to compare a heat loss calculation with system capacity.
Table of Contents
- Which quick checks can you do yourself?
- What is normal at the outdoor unit, and what is a fault?
- What do sealed-system faults look like?
- Could your system be undersized or short cycling?
- When should you call a professional, and what should you tell them?
- Why maintenance and correct design prevent most no-heat calls
- We diagnose, clean, and maintain heat pumps across Maine
- FAQ
- Sources
Which quick checks can you do yourself?
Start at the thermostat, because it causes more "no heat" calls than most people expect.
- Set the mode to Heat, not Cool or Auto, and replace the batteries if the display looks dim.
- Raise the setpoint a few degrees above the room temperature, then wait 10 to 15 minutes.
- Flip the indoor power switch at the air handler, then check the circuit breaker. Reset a tripped breaker once. If it trips again, stop and note that for a technician.
- Pull out the air filter. If it looks gray, matted, or clogged, replace or clean it. ENERGY STAR recommends checking filters at least monthly during heavy-use seasons, since a clogged filter chokes airflow and stresses the system.
- Make sure return grilles and supply registers are open, and move furniture or rugs that sit in front of them.
Pro Tip: Photograph your thermostat screen and the filter before you change anything. Those two pictures tell a technician a lot.
If you want a full walkthrough of keeping filters and drains clean, see our guide to heat pump cleaning for homeowners.
What is normal at the outdoor unit, and what is a fault?
In cold, damp weather the outdoor coil collects frost, so the unit periodically switches into a defrost cycle. You may see steam rising from the cabinet and feel cooler air indoors for a few minutes. That is normal. Practitioner troubleshooting guidance puts a normal defrost at roughly 5 to 15 minutes, recurring about every 30 to 90 minutes in cold, damp conditions. We cover that pattern in more detail in our post on cold air from a heat pump.
Safe actions for you:
- Clear leaves and snow from around the unit.
- Gently brush off accessible ice, or rinse it with lukewarm water.
- Never chip at ice or pour boiling water on the coil.
Red flags that point to a failed defrost sensor, a control board problem, or low refrigerant:
- A coil that stays iced after a defrost cycle ends.
- Defrost cycles that repeat endlessly.
- A fan that does not run during defrost.
A coil that remains iced or never leaves defrost is a fault, not normal winter behavior, and it needs professional diagnosis. If the coil re-ices quickly after you clear it, schedule a service visit.
Also look at what sits above the unit. Dripping meltwater from a roof edge can refreeze on the cabinet, and roofing contractors explain how roof and curb leaks create that kind of drip.
What do sealed-system faults look like?
The refrigerant circuit is sealed, so problems there show up as patterns rather than obvious parts you can see.
- Low refrigerant or a leak: weak heat output, long runtimes, an outdoor coil that ices unusually, and rising electricity use.
- Reversing valve trouble: the unit blows cold air in Heat mode but cools normally in summer. That pattern points to the valve or its solenoid, and it is not a homeowner repair.
The legal line is clear. Under EPA Section 608, anyone who adds refrigerant, attaches gauges, or does work likely to breach the refrigerant circuit must hold technician certification. That rules out DIY recharges and gauge hookups. Our post on heat pump refrigerant recharges explains why.
When you call, report what you observed, whether cooling works normally, and whether the coil re-ices after thawing.
Could your system be undersized or short cycling?
Sometimes nothing is broken. The system simply cannot keep up. Watch for these signs:
- Many short on/off cycles instead of long, steady runs.
- Rooms that never reach the setpoint.
- Unusual noise or wear from constant starting and stopping.
Cold-climate output drops as the outdoor temperature falls, so a system that was not sized to the home's design heating load will struggle on the coldest days. Sizing guidance for cold climates recommends matching capacity at design temperature or planning supplemental heat sensibly. Leaning on auxiliary heat all the time is a warning sign, since it raises operating costs and suggests a performance or sizing gap. We go deeper on this in our piece on fixing heat pump short cycling.
Pro Tip: If comfort gaps persist, ask your contractor for a heat-loss calculation and a comparison against the installed capacity.
When should you call a professional, and what should you tell them?
Call now if you notice any of the following:
- A suspected refrigerant leak.
- Cold air in Heat mode with normal cooling (reversing valve symptoms).
- A coil that stays iced after defrost.
- Breakers that trip repeatedly, or a burning smell.
- Defrost cycles that never end.
Before you dial, gather the facts that speed up diagnosis:
- The thermostat mode and exact settings.
- Indoor and outdoor temperatures.
- How long the symptoms have lasted.
- Whether cooling works normally.
- Any recent service or changes to the home.
When you choose a contractor, ask about EPA 608 certification, experience with cold-climate heat pumps, and whether they perform load calculations or custom sizing. And do not attempt refrigerant recharging or sealed-system repairs yourself. EPA's rules exist for safety as well as the environment.
Why maintenance and correct design prevent most no-heat calls
In our view, most "no heat" calls trace back to two things people can control: neglected upkeep and a system that was never sized properly. ENERGY STAR advises a professional heating checkup in the fall, ahead of peak demand. Installation matters just as much. Cold-climate installation guidance calls for heat-loss calculations and following manufacturer instructions. If you keep losing heat or see a re-icing pattern, write down the symptoms and get a professional diagnosis.
— John
We diagnose, clean, and maintain heat pumps across Maine
When your checks do not solve it, we can take it from here. Our services include one-time deep cleaning, preventive maintenance and troubleshooting, new installations, and existing heat pump change-outs. Deep cleaning starts at 249 USD one-off, and the Annual Cleaning Plan is 219 USD per year. A visit typically covers an inspection, system checks, a heat-loss review where it applies, and a written estimate. Refrigerant work is handled by technicians only. If triage points to a sealed-system fault, repeated electrical trips, or ice that keeps coming back, book a diagnostic visit with us today.
FAQ
Why is my heat pump blowing cold air when the heat is on?
Brief cool air can be a normal defrost cycle. If it blows cold for long stretches while cooling works fine in summer, the reversing valve may be at fault and needs a technician.
How do you reset a heat pump?
Switch the thermostat off, turn off the indoor power switch, wait a few minutes, then restore power. Practitioner guidance says to reset a tripped breaker only once. If it trips again, call a professional.
Why is my heat pump fan running but no heat?
Check the thermostat mode, the filter, and the outdoor coil for ice first. If those look fine, the cause may be a defrost control issue, low refrigerant, or a sealed-system fault that needs a certified technician.
What is the most common problem with heat pumps?
Neglected basics lead the list. ENERGY STAR notes that dirt and neglect are top causes of HVAC failure, which is why monthly filter checks matter.
Sources
- Section 608 Technician Certification Requirements | US EPA
- Heat Pump Not Heating (or Defrosting Constantly): Troubleshooting