This one needs a licensed pro
Likely cause: refrigerant, a reversing valve or an electrical fault
Thick ice that won't clear, a breaker that keeps tripping, hissing or oily residue, or a unit that won't run with power on all need an HVAC technician.
Why this happens
Trane lists the reversing valve and a refrigerant leak among the causes of a heat pump that isn't heating, and Carrier adds a frozen outdoor unit and low charge. Hissing or liquid pooling points to a refrigerant leak. The EPA says "only Section 608 certified technicians can purchase refrigerants intended for use with stationary refrigeration and air-conditioning equipment," and its rules prohibit venting refrigerant while servicing a system.
What to do right now
- If a breaker keeps tripping, leave it off.
- Set the thermostat to Emergency Heat if you have it and the house is getting cold. Carrier says to use it sparingly. Otherwise set the system to Off.
- Don't chip ice off the outdoor unit or pour water on it.
- Don't open the outdoor unit or try to add refrigerant.
- Write down the brand, model and serial from the outdoor unit's data plate.
What to tell the HVAC technician
- What you saw: thick ice, hissing, oily residue, pooling, or a unit that won't start.
- Whether a breaker tripped and whether it held after one reset.
- That the thermostat setting and filter checked out.
- The outdoor temperature and how long it has been happening.
Build the full list: what to tell the HVAC technician.
Start this troubleshooter over
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