Can an 80% efficiency, draft hood, 40k BTU, water heater be vented through a 2x10 wood rim joist with painted cedar siding? Assume proper pipe sloping.

The termination would be roughly 2 feet above ground (knee height). The house is colonial and the eaves/soffit are about 25 feet up.

Does a Type B double wall pipe make a difference? How far from the siding does the pipe have to extend before terminating? If it can go out the rim joist then do I have to terminate above the roof line?

Why am I asking this?

It currently vents through a chimney but it would be nice to demolish the chimney. The water heater is fairly new and works really well; even during power outages!

If it makes a difference, I live in Central NY.

  • What do the directions say about that? I will take a SWAG and if the stack is single wall metal probably not, Type B double wall pipe should work, check your local codes. It may be less expensive in the long run to buy a new ultra high heater, depending on the age of this one that uses plastic as the vent.
    – Gil
    Sep 9, 2021 at 17:43
  • @Gil The directions don't say much. Yes, I'm aware of high-efficiency water heaters. I installed mine 2 years ago and in the summer my gas bill is like $12 when the stove/oven gets used the least. I cannot imagine that number getting significantly lower with a high-efficiency heater. If it does then the savings might get eaten by the blower motor.
    – MonkeyZeus
    Sep 9, 2021 at 18:23
  • There are two other questions today that are similar. One comment/answer said that Cat I needed to go up a chimney, but Cat III could go though a wall. Cat I could be made into Cat III with the use of an added fan/blower.
    – crip659
    Sep 9, 2021 at 20:18
  • Thanks for the directions, they state: "Horizontal vent connectors must be pitched upward to the chimney at least 1/4” per foot of length. Single wall vent connectors must be at least 6” from adjacent unprotected combustible surface. Vent joints must be securely fastened by sheet metal screws or other approved method." The 6" applies to all sides so you cannot use single wall pipe. You must use the double wall pipe if your code permits it. To be safe check with your local building department to check this.
    – Gil
    Sep 9, 2021 at 21:14
  • @Gil Hmm I read that as well but since you pointed it out too I wonder if "to the chimney" is my answer. They give no alternative...
    – MonkeyZeus
    Sep 10, 2021 at 14:16

2 Answers 2


4" B vent requires 1" to combustibles and has an OD of 4.5".

So, can you put a 6.5" hole in a 10" rim joist? IDK, the rule for joists is one third their depth.

I wouldn't butcher a hole that big for an 'eighty'. When you upgrade to a power vent it will be a 4" concentric, or two 2" holes.

  • If the rim joist is sitting on a stone or concrete foundation the vent doesn't need an inch from that. You'd be cutting an arch from the bottom of the joist, 6.5" wide by 5.5" high, supported on both sides by the foundation, and with 4.5" of joist remaining above you probably wouldn't even need to worry too much about what's directly above ... although if there is say a hefty steel support column directly above, it would be wise to put the hole somewhere else. You'd need to fill the gap with an appropriate material, not flammable, not heat conducting.
    – jay613
    Feb 7, 2022 at 23:32
  • And then the pipe would need to be waterproofed and stood off from the wall all the way up to code-compliant height, probably above roof level. If there's an eave you'd need to go through or around that. I guess if you're demolishing a chimney stack none of this is too scary.
    – jay613
    Feb 7, 2022 at 23:35
  • Any size notch is worse IMO.
    – Mazura
    Feb 7, 2022 at 23:38

That may be why it was done that way in the first place. Just be cognisant of the minimum rise per foot then exceed it when you replace it and it should go great. Keep the 90s to a minimum, there is a maximum you can use.

This was published in my water heater instructions. It is probably correct but it must be checked with your local building department to be sure. Sorry I do not know put it in a table. These are maximum values.

Allowable Elbows & Lengths for Direct Vent Gas Water Heater Exhaust

With 1-90  3" = 35'  4" = 80' 4" 
     2-90  3" = 20'  4" = 75' 
     3-90  3" = 15'  4" = 70' 
     4-90  3" = 10'  4" = 65'

Many localities require the vent to be through the roof if there is no chimney. They also state it must have an approved cap on the pipe and it must be above specified roof line heidghts. Your best friend in this would be your local building department.

  • It's unclear what your opening sentence references. You should revise to include that, possibly as a quote.
    – isherwood
    Feb 7, 2022 at 19:13

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