http://www.straightdope.com/mailbag/mpipefreeze.html
I know this one goes back a ways, but I’ve had personal experience with sub-zero temps and plumbing nightmares. The staff report on running water was right on. Nothing to change, just something to add. While a dripping faucet will keep the supply line open during freezing weather, it can cause the drainpipe to freeze solid. The pipe that goes to the sewer is vented through the roof (usually), to allow poisonous sewer gasses to escape. In really cold weather (the kind we get here in North Dakota) the vent also allows cold air to come in direct contact with drain water. No big deal if a lot of water goes down the drain at once, but if the sink is just dripping the flow is slow enough to freeze and build up enough ice to clog the drainpipe. Which makes things interesting when the washing machine tries to drain and pumps 25 gallons of soapy water all over the floor.
Mathias (mop-boy) Murphy