Humming from Furnace. Ice on pipes.

Started by Levi-Leroy, January 15, 2023, 03:00:07 PM

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Hello folks,

Having a small problem with my furnace which started just recently.
When the furnace is off, I can hear a hum from within the furnace. This is accompanied by a hissing sound coming from the pipes which lead to the outdoor AC unit.

Frost collects on those pipes fairly quickly after the furnace shuts off.
I'm wondering if its drawing freezing cold fluid from the AC unit outside.

I'm not sure if this is related to the -20°C weather we have been having recently. Otherwise, the furnace has been functioning fine. Is this something to worry about?


Are you certain the AC outside has not been running?  Sometimes a wiring issue can cause the AC to run during a heat cycle so you could turn off the 240V AC breakers for now.

There's also a chance the AC is a heat pump and designed to run during the winter.


Disconnecting the black wire which was connected to the (Y) cooling link in the thermostat has stopped the humming.

This is interesting. I will investigate the AC unit. I understand that heat pumps function poorly in very cold climate. Though I doubt the this unit is a heat pump.

My current thermostat wiring has green, red, white, and black. I understand that usually instead of black, one would have a blue or yellow wire to connect to the cooling circuit (Y).

Thank you for the insight.


If it was a heat pump it would seem it has an issue with the reversing valve if the lines are frosting up. It's clearly trying to run the outside unit. Take pictures of your wiring setup at the stat and inside the unit.


Here are the requested pictures. The green wire is obscuring the letter G on the furnace end.

I can confirm that the AC unit is not a heat pump.

The loose black wire you see in the thermostat picture is currently disconnected. This was connected to the Y location, since disconnecting it the humming and frost have stopped.

Everything looks like its matching to me, other than the black line. What do you think?


The second black wire on Y and the white wire on C go to the outdoor unit.

It sounds like you either have a bad thermostat or damaged thermostat wire.  You need a multimeter to troubleshoot.

Hopefully you caught it in time and the AC compressor is not damaged.  It's best to turn the AC breakers or disconnect box outside off during the winter.