A friend had an addition built that is 2 feet or so off the ground. There is a tub in it on the outside wall. So the drain runs back to the house in the "floor". When it gets really cold, the (metal) drain freezes at the P-trap. This can be avoided by pouring a little windshield washer fluid in the drain at night, but the tenants who rent the place often forget.
Of course the drain pipe has lots of pink stuff around it but that just minizes heat loss. It doesn't generate any heat though. We cut a hole in the base of the tub enclosure to permit warm air to circulate around the drain pipe, but it didn't help when the temp went down to -15 F last January.
Suggestions? Thanks.