DIY & Home Improvement

Moving water won't freeze until you get to extremely low temperatures (Waaaaay below zero). So water flowing through the pipes in your crawlspace should just keep going to the sewer. As long as you don't have a sag in your drain line, you should not get any freezing there.

So what you have to worry about is water in the trap freezing, since that's the only place water stands still. I'm guessing the trap is in the crawlspace, so it's cold and can freeze.

The best solution is to rip open the wall, and move the trap from the crawlspace into the wall. That way the trap will be 50 degrees like the rest of the utility room and it will never freeze. If you don't want to do that, all you can really do is insulate the trap and apply something like heat tape to it.

In the short run, pouring very hot water down the pipe should melt the ice in the trap. Start the washing machine on "hot", let it fill a little, then turn the knob to drain or spin. Be ready to stop the washer so that it does not flood the room.

You should not need the water to be near boiling - even if the initial blast of hot is not sufficient to melt the ice, you will end up with room-temperature water sitting on top of the ice. That should get enough heat down to the ice to melt it.