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Ahh, the joys of an old house.

The former owners of my house did some funky stuff with venting the water heater. Let me explain:

  1. the water heater is in the basement
  2. there is a 3 in single walled vent pipe that goes up towards the first floor at about 45 degrees
  3. next there is a joint that points up into the first floor
  4. a short single walled 3 inch vent goes up into the floor
  5. now in the first floor in a closet there is another joint that connects horizontally into a vertical 12 inch(?) mega pipe.
  6. this "stand pipe" appears to have been used as a exhaust vent for an oven in the kitchen, but is no longer used for that.
  7. at the top of the pipe you can see that this big pipe is double walled and has a 10 inch (?) clay pipe going through the attic and out into a roof vent.
  8. the roof vent is new, it is a standard vent with the cap on it.

So, my question is: can I just connect a double walled vent pipe from the basement joint (#3) up through the attic (#8) (after removing all the funkiness (#4-7)

Here are a few issues:

  1. ideally this double walled vent pipe would be in the wall and not in the middle of the closet as it is now. I believe that is safe, and it can touch drywall/studs as long as it is double walled.
    1. Edit: No it is not safe. d-wall must be 1 inch from combustibles
  2. I'd like to plug the hole in the attic. I think I can safely just cut a piece of drywall to fit around the smaller double walled pipe.
    1. Edit: No it is not safe. d-wall must be 1 inch from combustibles so you can't let sheetrock touch the pipe. You must use a "firestop" flashing
  3. we have a whole house fan. This means the attic gets pressurized. Is there any issue with vent pipe being so loose in the old riser on the roof? Ie can the pressure go up the riser and then down the vent and backdraft? I suspect this is not an issue. I could also seal the riser from inside the attic to prevent issues.
    1. Edit: I should use a firestop again directly under the roof plywood so the pressurized attic air can't escape up the old vent riser.

Comments?

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3 Answers 3

up vote 1 down vote accepted

I did a lot more research and I have concluded that what I proposed is mostly safe. There are a few important points

  1. Single wall vent pipes must be 6 inches or more from combustibles (this includes sheetrock!). That means you basically can't use it in walls or near joists/rafters.
  2. Double wall vent pipes must be 1 inch or more from combustibles. You can't put it against sheetrock or studs, etc. That means for a 3 inch pipe (4-5 inches double walled) you need a 6-7 inch wall.
  3. You should use a firestop (a sheet of flashing with a pipe sized hole) at walls and floors/ceilings to keep the pipe a safe distance from studs/sheetrock.
  4. to prevent backdrafting from a Whole House Fan another fire stop should be installed directly below the roof plywood to limit the airflow from the attic to the top of the vent pipe.
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Backdrafting is a serious life and death issue. Do not connect two applicance exhaust pipes to the same stack. The pressure from one fan can overcome the other and one or the other will backdraft. Carbon Monoxide if the GWH looses.

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Sounds like the OP is talking about a new, single appliance stack. –  bib Aug 29 '13 at 15:26
    
@bib: That's right. It appears the previous owner's setup allowed the exhaust fan over the oven to push into the same vent pipe as the water heater. I'm trying to resolve that issue and reclaim a lot of wasted space in that closet. –  mlathe Aug 29 '13 at 23:42

I'm guessing that your water heater has a draft hood, and is not sealed combustion. When your whole-house fan is operating, it drops the pressure in the house, and probably in the basement also. If so, it can easily reverse the draft on your water heater even with a perfect venting system. This is not an immediate problem if the combustion is clean, which it normally is. But if it's a newer water heater, it is flammable vapor ignition resistant, which usually means the combustion air passes through a flame arrestor which can get clogged. When this happens, the flue gas gets very high in carbon monoxide and when that happens you really don't want a reversed draft. In a modern house, it's best not to have appliances with draft hoods indoors.

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