I have central air. When the furnace comes on, we can feel air coming out of the registers for a few minutes, then the airflow stops. If we turn it off for a few hours, we can turn it on again and get airflow for a few minutes, then it stops. There is a copper pipe coming out of the furnace that gets up to a half inch of ice when the furnace is running. What might be wrong? What should I check?

  • is this a heat pump?
    – BMitch
    Commented Apr 28, 2011 at 19:23
  • it is a heat pump. it has permanent filters. There is also ice building up on giant metal heat sinks, but they might be the air filters. They're between the intake and the fan. Commented May 1, 2011 at 17:01
  • In that case, as msemack says, something is likely wrong with the coolant. I could also see an issue with the valves switching from AC to heating mode. Either way, you almost certainly need someone that's licensed to work on them to have a look.
    – BMitch
    Commented May 1, 2011 at 23:53
  • I'm note sure what you mean by "permanent filters". In your indoor air handler (what you call the "furnace"), there should be a square air filter. If this filter is plugged (you should be changing it regularly), it can choke the system and cause problems. Commented May 3, 2011 at 14:20
  • The "giant metal heat sinks" are called coils. There is a coil in the outdoor unit and the indoor one. In air conditioning mode, heat is pumped from the indoor coil to the outdoor one. Commented May 3, 2011 at 14:23

1 Answer 1


If the copper lines are frosting over, it sounds like you are low on coolant. If the coolant levels are low, the unit is unable to cool the house, which means it keeps running, and the copper pipe keeps getting colder and colder, until the humidity condenses on it and eventually freezes.

If the lines are frosted, it means the unit is overworked. Stop running it ASAP so you don't burn out the compressor. Check the coolant levels before you turn it back on (you may want to call an HVAC company).

If you are low on coolant, you probably have a leak somewhere.

Other things to check:

  • Make sure the indoor air filter(s) are clean.
  • Make sure the outdoor unit fan is running properly.
  • Make sure the vents/coils on the outdoor unit are clean, and airflow is unobstructed.
  • the coils in the indoor unit were dirty, causing them to frost over and totally block the airflow. Commented Oct 25, 2011 at 16:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.