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Prelude:
First properly cold day of the season and I was expecting my furnace to kick in, but it didn't.
I went down into the basement to take a look at it and found it giving me an 2-blink error code which, for my furnace, apparently means something like "The pressure switch is stuck open or induced draft blower problem detected".
When I reboot the furnace I see it try to start up - the inducer motor appears to start & run, but I can see that the pressure diaphragm never activates its microswitch to tell the controller that it's working. If I pull the rubber pipe off the blower and suck on it, the diaphragm does move and activate the switch - so it seems that, although the the blower is running, it's not really blowing.
At the same time while I'm down there I'm noticing an unpleasant funky smell coming from the condensate pump ...
So I opened up the pump to find it has a little liquid in the bottom, but it smells really bad.
At this stage I'm almost certain that something has made its way into the furnace exhaust (a 3" PVC pipe which exits the furnace, travels about 4' up and then about 6' horizontally out through the side of the house), and died in there.

The Question:
Once I cut open the PVC exhaust and clean it out, is there any reason I couldn't re-connect it by some means (to be decided) to the same exhaust which my water heater uses (instead of having it run out the side of the hose as it is now and risk something getting back in again)?
The chimney from my water heater is about 2.5" and runs for about 3' before it joins into an enormous 8" wide metal pipe which runs up through the house and out of the roof.
Surely there's enough volume there for the furnace & water heater to share ... ?

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    If your water heater is an 80% unit then absolutely not. The water heater exhaust will melt the furnace PVC. You wouldn't want high-efficiency exhaust in a metal pipe anyways. It is very wet and will corrode the thin walls in no time. You usually cannot join two high efficiency exhausts either because one has the potential to backfeed the other.
    – MonkeyZeus
    Oct 18, 2022 at 19:53
  • @MonkeyZeus what is "an 80% unit" water heater? Good point about the wet exhaust from the furnace.
    – brhans
    Oct 18, 2022 at 19:56
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    Sorry, that simply refers to standard efficiency (<90%) units. See reliablewater247.com/high-efficiency-vs-standard-water-heaters
    – MonkeyZeus
    Oct 18, 2022 at 19:58
  • "First properly cold day of the season and I was expecting my furnace to kick in, but it didn't." - ALWAYS try to start your furnace at least a month before you need it.. If it were to have a problem which required an HVAC tech then guess what, you're up against thousands of other people in your area that tried turning on their furnace for the first time.
    – MonkeyZeus
    Oct 19, 2022 at 12:17

1 Answer 1

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If the appliance instructions say they can share an exhaust, then they can share. (Hint: The instructions won't allow it. The high-efficiency, also called "condensing" appliances, pretty much universally require their own independent exhaust and often their own combustion air intake pipe too.)

Before you go to the trouble of cutting the PVC exhaust or intake pipes for visual inspection, get an assistant to switch the furnace on while you stand outdoors listening to and feeling the flow of air at the intake and exhaust. You likely don't have prior experience to provide a baseline for what "normal" air flow there should feel like, but you may be able to make a judgement as to whether the air volume seems "reasonable" or "might be blocked by something."

The foul odor in the condensate pump may be normal especially if the water has been stagnant for 5+ months.

Keep in mind when testing the pressure switch that it is set to toggle at a particular pressure (or vacuum). The fact that the switch toggles when you suck on the tube doesn't tell whether it toggles at the correct pressure.

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    I do remember roughly what the exhaust looked like during operation previously - and I got virtually no output from it earlier today. I have since cut open the pipe and disassembled the inducer blower outlet pipework - extracted the remains of a bird, disinfected everything and reassembled - without the condensate pump as the old one was really nasty inside and I have a new one arriving tomorrow. Furnace is back up & running with a bucket to catch the water overnight.
    – brhans
    Oct 18, 2022 at 23:48

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