I went out and bought heat tape today, and was surprised that it wasn't sold out. But once we figured out the problem, there's no way to put heat tape on this pipe. It's got a vertical 20-foot run squeezed in with three plumbing pipes/vents and at least one electrical conduit. We decided that it's best to trim off the hole and put an antique furnace vent grate over it. When the temperature drops below zero, we can then blow warm air into it, or I may even consider putting some sort of vent fan in it that we can turn on in extreme weather.

We've lived in the house for 28 years, and it's been a problem the whole time, but only when we have two or more days of consistent subzero weather. It's always been temporary enough that we sweated it out, but it's stressful. We finally decided to bite the bullet and poke through the plaster, and it was enlightening. The pipe was coming from a completely different direction than we've always thought.

Better than a flooded house

It probably just needs some air flowing.
If it is a vertical pipe run (between 2 studs) cut the plaster and put a vent up high and one down low. A small return air type vent cover will hide the plaster you cut out.

Yep

Eliminates the need for trash bags too. Win/win.

Gat Danged sump pump not pushing water out. Submersible pump to the rescue!! Except now the hose I put through the basement window has frozen outside!! Lol
(Outside discharge line has frozen)

Next option is to run hose to an empty aquarium. Winter is so much fun!!

Happy Monday!!

Wouldn’t this work on a horizontal run as well?

the framing (studs) will prohibit the (natural) convection effect. My suggestion utilizes the space between the studs as a "box". A horizontal pipe runs thru too many "boxes" that would need vented.