Is $6,000 for a stainless steel tankless vent line actually the normal rate?

Category 3 stainless steel venting is definitely pricey, but $6,000 for a straight run through a side wall is basially a "we don't want to do this" price or a total fleece. Most of those big-box plumbing outfits in the PNW have huge marketing budgets and high commissions for their "sales techs," so they quote projects at triple the actual cost hoping you'll just finance it and not look twice. The soot you're seeing at the seams is the real concern. That means your unit is backdrafting or leaking combustion gases, wich is a massive carbon monoxide risk. Category 3 is necessary because these units produce acidic condensate and operate under positive pressure—if you use regular B-vent or galvanized, it'll literally rot through in a season. The actual **AL29-4C stainless** pipe itself might cost $300 to $600 depending on the length and the wall termination kit, but the labor for a simple punch-through shouldn't be five grand.

As for the $15k Rinnai quote, that's just insane. In Washington, a standard swap usally runs between $5k and $8k including the unit, depending on if they have to upsize the gas line or move the venting. I'm glad you found someone for $700, but you relly need to make sure that guy is actually using the rated stainless steel and not just swapping one DIY mess for another. If he's using the right materials and sealing the wall penetration properly, you saved yourself a fortune.

You might also want to have him check the gas pressure and the burner while he's there. Soot usually indicates the gas-to-air ratio is off or the heat exchanger is already partially clogged with carbon. If the unit was improperly vented for a long time, it might need a deep cleaning to stay efficient and safe, but that's still a far cry from needing a $15,000 replacement.

$15k for a Rinnai is nuts. I'm looking into getting a tankless myself, but all this talk about AL29-4C is making my head spin. Since you're dealing with soot, does that mean the heat exchanger is toast?

I’m curious if there’s a way to tell the difference between the high-end stainless pipe and the cheap stuff just by looking at it. I'd hate for you to get a "pro" who just buys a $20 dryer vent and calls it a day. Is there a specific brand name or marking I shoud look for on the metal?

To spot the real deal, look for the UL label stamped directly on the metal—it should explicitly state 'Category III' or 'AL29-4C.' High-end stainless has a mirror-like finish and uses internal silicone gaskets with locking bands. If the guy starts reaching for foil tape, he’s doing it wrong.

Also, that soot is a major red flag for the burner. It means the unit is 'running rich' becuase it’s starving for air. Since you’re saving five grand, have the tech actually pull the burner and clean the heat exchanger fins. If those are choked with carbon, a new pipe won't fix the underlying combustion issue.