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.

Thanks! Water heater vent (right) vs crawl space level (left) Pressure sensor I think


2 Answers 2


The water heater is a condensing one. It should not be vented through galvanized metal pipe or through an existing chimney or through a common vent. It says these things in the installation manual. You should read the manual and re-install the venting correctly or hire a plumber with training in installing condensing boilers.

Yes the water heater's exhaust could be putting back pressure on your existing furnace but in addition it could be (?is?) dripping condensate into your chimney and possibly into your furnace, which will eventually destroy both.

It has been a while since you posted this question. If the original installation is still intact you probably have some new problems you can report.


they can not share one chimney vent because your old furnace vented thru metal sheet duct(high temp dry flue) , your hi-effiency water heater vented thru PVC 636 (wet flue)

  • 1
    Hello, and welcome to Home Improvement. Would you add some more info to your answer? And, you should probably take our tour so you'll know how best to contribute here. Dec 28, 2020 at 12:58
  • This doesn't answer the OP's question about the venting pressure of the new water heater.
    – JACK
    Dec 28, 2020 at 14:58
  • @DanielGriscom Does my "flag" stay disputed forever??
    – JACK
    Jan 3, 2021 at 1:59
  • @JACK Seems like a reasonable answer to me; the two flues shouldn't have been joined, causing the draft inducer switch to trip. (It's true that it needs more info, though.) Jan 3, 2021 at 13:21
  • Do you have some new neighbors that have built A home in the flu?
    – Gil
    May 22, 2022 at 20:15

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