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I replaced my hot water heater. I used a single flexible 3" metal duct tube to go from the new unit to a an old wye connector metal duct that is near the ceiling in the utility closet.

I noticed that Home Depot doesn't sell high temp foil duct tape. Is it required or forbidden to wrap the joints/seams of the hot exhaust stack with foil tape?

The utility closet is inside the house, so I'm worried about CO leakage.

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If the vent is not going into a masonry chimney or equal, Type B vent should be used ; it is double wall to lower the temperature of the outer surface. I recently put in a hot water heater and had to replace the original Type B because it could not be changed after 20 years to meet the new unit. I don't believe Home depot has Type B; try plumbing supply or Mcoys.

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Duct tape and flexible ducts are OK for a fume hood exhausts or for ventilation ducts, but not for combustion exhausts.

Although regulations depend on where you live, for your own safety I would not use a flexible metal duct for combustion exhausts as they are not designed for this and can be prone to leaking. Same goes for duct tape. The glue or tape could melt, or the tape could perhaps even catch fire from the hot fumes.

So, use fixed tubing and connect it as per the instructions, probably with screws. There are also systems that are self sealing with built in rubber rings.

Furthermore, if possible for your hot water heater, I would recommend that you also connect an intake duct for fresh air. This will lessen the amount of CO gas is produced, and make it less likely that the combustion exhaust product escape inside the house in stead of through the exhaust pipe. If you have a heater with condensation, it is possible you need to use specific concentric tubing to do this. It might be necessary to build a new connection to the outside for the intake, or, if the existing exhaust is wide enough, you could install an air intake tube inside the existing exhaust tube.

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    CO gas is always produced. More is produced when Oxygen is limited, less when abundant. What you want to reduce the chance of is "back drafting" where all of the combustion exhaust remains in the the building rather than being vented out the exhaust pipe to the outside.
    – Dan D.
    Commented Jul 27, 2018 at 14:25
  • thanks Gents! I did use a very heavy duty (and expensive) flexible 3" pipe from homedepot. I chose this rather than having mutiple smaller sections for the 2.5' run from waterheater hood to ceiling connection. I did attach with screws. just to clarify... are you saying the 'metal foil tape' designed for ductwork could actually be a fire hazard? thanks!
    – Cool Pontiac
    Commented Jul 31, 2018 at 19:03
  • It depends on which tape we are talking about. Duct tape is a fire hazard because it's made from flammable plastic and cloth. HVAC tape is made from metal and might be safe to use, depending on its specifications. Do check them!
    – user89225
    Commented Aug 1, 2018 at 5:11

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