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I recently purchased and am installing a 96% efficiency two-stage gas furnace. The old unit was an 80% efficiency unit using a 4" metal flue. This flue merges the exhaust of both the hot water heater and the old furnace.

With the new furnace however, I need to switch to 2" PVC and I am unable to connect this to the metal exhaust as the condensate created by the higher efficiency furnace is corrosive and will cause the exhaust flue to rust and create a carbon monoxide hazard.

As it so happens, however, I have been wanting to upgrade to a tankless hot water heater. I am aware that there are condensing and non-condensing tankless models, and that the condensing models also use 2" PCV.

Would it be safe to join the exhausts similar to the old setup with this configuration? Or do I need to run two separate exhausts? If I can merge the exhausts of these two appliances, Do I need to go larger above the y joint? How large? Would 3" be sufficient?

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  • Note that PVC is generally considered marginal at best as a venting material for condensing water-heating appliances (or really any condensing gas appliance, for that matter); there have been some fairly disturbing field failures of the material when water heaters and boilers have scaled up, increasing flue gas temps beyond what PVC is rated to withstand. Listed stainless steel Category IV vent systems are available, and are what is supposed to be used for this application, as the manufacturer can't instruct you to outright violate the Codes in their manual (as they do right now). Commented May 10, 2020 at 14:06
  • Is this the kind of furnace that has an air intake running around the outside of the exhaust flue? Do you have a plan to provide for makeup air? Commented May 10, 2020 at 14:17
  • @ThreePhaseEel - yes, I am aware. That's why it is important that this would be a condensing tankless water heater - the manufacturers usually call for 2" PVC for this application as the gas temps are reduced by the condensing. Commented May 10, 2020 at 14:22
  • @JamesShewey -- what I'm saying is that the manufacturers only really tested that with a shiny new water heater, with no scale on the heat exchanger to reduce its efficiency and raise flue gas temps; there's also no shutoff that kills the water heater if the flue gas temp becomes excessive Commented May 10, 2020 at 14:29
  • @Harper-ReinstateMonica It's a Goodman *MEC96 series model two-stage gas furnace. The manual states that the furnace is dual certified and may be configured with direct or non-direct venting (eg, 1 pipe for intake, one for output, or just one for output if installed in a large enough space with proper ventilation) Commented May 10, 2020 at 14:45

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Even if material and size of 2 merged tubes weren't an issue, merging the 2 exhausts would still not work.

The exhaust from HE gas appliances is normally relatively cool and humid. They require a blower to actively push the exhaust out. If you tied 2 exhausts together, then when only one of them was running it would likely push exhaust down into the other appliance.

Even for 80% furnaces, exhaust temp ~350F, they recommend against merging pipes for this reason. (Though it is quite common to see with 80% appliances) For +96% furnaces the exhaust can be barely above 100F, its guaranteed to flow down into the other appliance.

Especially assuming the old flue route goes +10ft straight up thru a cold attic in the winter. If you give the exhaust gas a route to simply flow back down into the other appliance, then its going to take that easy option.

You could try to counteract this with check valves in both exhausts, but then you have you worry about the pressure to overcome those check valves being a problem and condensation building up on the valve. Even if that is not a problem you have to worry about the possibility of the valve being stuck open/closed and causing the same exhaust backfeed problem.

So unless you want to install some electrically actuated valves in each exhaust that open/close with the appliance on/off and have a positional failsafe to tied back into the appliance (even that would technically not be as safe).....you're left with needing 2 separate exhausts.

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  • Thanks for this - while I was able to find the "don't do this" in the manual, the logic of "why you shouldn't do this" is the ideal - so I've marked this as the accepted answer. The why is always far superior to "because I said so". Commented Aug 17, 2023 at 17:20
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In answering @Harper-ReinstateMonica's question in the comments, I stumbled across something on page 12 of the manual which states "never common vent this appliance with another appliance or use a vent which is used by a solid fuel appliance."

So it looks like common venting with a tankless condensing water heater would be generally unsafe, is not recommended by the manufacturer, and may be against code.

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  • In general, yes, any appliance that runs at positive vent pressure needs its own vent system. (Some commercial applications use banked venting, but this requires a specially engineered system, and generally will only be accepted by a Code official with a mechanical P.E. stamping the plans) Commented May 10, 2020 at 14:54

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