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Relocating water heater - impact on vent size?

1303 Views 10 Replies 2 Participants Last post by  yuri
I currently have a furnace and water heater located next to each other, each with a 3" vent pipe connected with a 6" wye to a 6" horizontal which goes about 6 feet to the chimney. I am remodeling and reducing the size of my mechanical room so the water heater needs to move to the basement, where a new run of 3" vent pipe will connect to the chimney. I am assuming that this means I need to reduce the 6' horizontal run for the furnace down to 3" as well, but being this will be a bit of a pain I figured I'd ask before going through the trouble of replacing it. Is it necessary to get rid of the 6" pipe and install 3" or can I just cap one side of the wye and leave it as is?
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leave it the way it is and cap it is OK. try not to have more than 6 feet of horz pipe to the chimney from the water heater or you may not get enough draft and fumes will spill out. 95% of our water heaters where I am use 4" now for better draft.
Yeah I know most localities like to see 4" but around here it seems 3" is the standard - 4" horizontals with a 3" breech on a natural vent water heater is considered "less than desirable." I thought there might be a chance I could leave the 6" on the furnace because it has an inducer motor but I wanted to be 100% sure. Thanks for verifying.

As for the new location of the water heater, it's practically right under the chimney so there's almost no horizontal at all. If only I could afford a new power vent water heater this project would be much easier, but a few pieces of 26-ga c-vent is a whole lot cheaper...
actually I may be wrong. I believe that the water heater must enter the chimney above the point where the furnace enters so you cannot go below the furnace entry point. very rare that your case happens but you should check your gas code or local requirments. where do you live and maybe one of our other guys can verify that point.
Yeah, my local code needs the water heater flue to connect above the furnace for sure. Quick details on the construction of my home: It's a 1950s split level with the chimney in the middle of the house. The "chimney" is actually a round b-vent pipe that was installed inside the original brick chimney about 5 years ago. The bottom 4 feet or so of this pipe is exposed in my mechanical room (I kept the brick chimney from the second floor through the roof because I like the classic look of the chimney and it's in keeping with the other homes in my neighborhood). The b-vent changes to 26-ga. c-vent at this point as the walls on this level are concrete and besides that there is well over 9" of clearance on all sides from anything considered combustible.

So my plan was this:

Furnace: The furnace will remain on the first floor, but because it's a split-level it will only be about 5 feet higher than the water heater. The 3" furnace flue connector enters a 5' to 6' horizontal of 6" pipe via one side of a 6" wye that is capped on the other side. The 6" pipe connects to the chimney via a tee on the very bottom of the chimney pipe, right above the cleanout.

Water heater: The water heater, although physically located below the furnace, will have a 6-foot vertical run of 3" connector that takes its entry point to the chimney about 12" above that of the furnace, and it will connect to the chimney at a 45-degree angle via a wye with a reducer.

I'm relatively sure this meets local code, NFPA 211, and will be safe. If I've missed anything please let me know.
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Thanks, yuri - I will post pics of the current situation and my drawings of the proposed work as soon as I can.
This is the best photo I could get on short notice. No flash so you can barely see back to the chimney. A quickly-thrown-together diagram shows what I propose to do (not to scale, obviously). I'll try to get better photos but hopefully this gives you what you need


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1) where exactly do you live, for the other techs who may know your area

2) you know it is NOT easy to get back into the B vent chimney above the existing tee? cannot just drill a hole in the side of it but must get the proper B vent tee and then shorten one length and probably have to yank it upwards etc etc etc to do it. NOT an easy proposition IMO. B vent chimney tees and fittings are NOT always going to fit each other as one brand/manufacturer may be slightly larger/smaller than the next one so that may be an issue also. I would lean towards an electric water heater or thru the wall type. You are also going to have to move/extend the gas line and you cannot go thru floors with that yellow flex pipe so unless you are a licensed gas fitter or get one to do it your house insurance will likely be void and you can get yourself in one helluva lot of trouble.
I live in PA. Local inspectors in my area have a reputation for having unreasonably strict interpretations of the codes but the ones in my township are great to work with and just want to see good safe installations.

That said, im not doing any mods to the b vent. I would never drill or try to cobble together makeshift ductwork for any application, especially gas venting. The portion of the vertical pipe that is exposed is 26-ga single walled. My HVAC contractor adapted it to b-vent about 2 feet or so before it goes through the floor and into a chase leading to the chimney. i didnt climb into the crawlspace to take photos but if i go around the back I can actually climb in and there's room to stand and walk around the 26-ga pipe. All I need to do is unscrew the existing tee, insert wye, and screw everything back together.

As far as gas, I'm trained as as plumber. I don't hold a master license as I chose not to do plumbing for a living but I plumbed my entire house for gas and water and did a ton of my own HVAC work (although HVAC is its own art and I know where to draw the line and bring in a contractor). This is simple and I've got no problems doing it myself I just am a suspenders and a belt kind of guy so I figured I'd get some opinions before pulling the trigger.
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