Why do I have such terrible allergy and sinus problems?
Yes, this pipe should be replaced with ABS pipe. The cost to replace this section will depend on how accessible the existing pipe is. If it is simply suspended from the floor above and easily accessible with a 6 foot ladder, you can count on an easy fix. LIcensed plumbers charge about $110/hour in most parts of the country these days, so if everything is out in the open and nothing is buried or covered with drywall you can expect to pay for at least 2 hours plus $100 or less for the pipe and couplings to connect to the floor drains.
Check for drafts of warm air, anything that might simulate a human body moving. If your detector has a sensitivity control you can de-tune the sensor. If not and the problem is with a heat supply, try masking off the lens section looking at the heat. Try a piece of masking tape first just to see if you have a solution. If that works, cut the shape neatly and stick it in place. You may also be able to redirect the sensor. Beware cats and dogs and false signals. GL
