I have an older furnace and a brand new natural gas instant water heater that I installed. I only start this way because the furnace had one thing break last year, so I'm not sure if things on it are just starting to break or if the new water heater is causing issues.
This last winter the draft inducer pressure switch started tripping randomly. It's hard to tell exactly when it's tripping, but going into the crawl space and pushing in the sensor's reset button fixes it every time. This furnace vents into a chimney vent which also is the vent for the water heater (comes in about 4-5 ft higher than the furnace vent). I'm wondering if this water heater might vent at higher pressure and be tripping the furnace sensor.