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Orphaned water heater vent replacement

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Some background:  About 11 years ago, replaced my original (1964) gas furnace with heat pump; no problems.  About 10 years ago, I replaced my gas water heater myself, no problems.

Yesteday, gas utility replaced my meter; wouldn't re-light the water-heater as vent is corroded, not good!  I thanked them for the free inspection and heads up.

I've been reading on the "orphaned" vent from going from two to one exhaust sources, but since this has all worked OK for years, I'm confident the venting worked well, but may have caused condensation that took out the original galvanized pipes.  Still, they are 47+ years old, so not surprising.   

So reading your site, about single wall (it is 6 inch, water heater is 3 inch "y'd" into the 6 inch just above the water heater.  From there it goes about 3' out the basement, into crawl space (problem 1), is bent in a bit with no rise to clear a heat vent in about 8'(problem 2 and 3) then goes about 6' to the chimney with slight rise.  It is about 2" from the floor joists in places (problem 4).

Tactically, I don't want to tackle the chimney immediately.  Does it make the best sense to replace the 6" single wall with 3 or 4" double wall for this horizontal section, so I could fix all the problems above?  I'd have to cut the single wall a couple inches from the chimney and use an adapter to get from the new smaller double wall to the 6".   I know the whole chimney section should be evaluated by a pro, but to keep some hot water going until then, does this seem sound?

Very nice site you have here.   Regards,    ADS
asked in Gas Water Heater by ads (120 points)
edited by darekrudy
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1 Answer

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Thank you, I’m trying to make it helpful for visitors smiley.
Based on your description (unless I misunderstood) the lateral of the water heater vent pipe is about 17’ with only a slight rise somewhere along its run... that’s a very long run. For a 3” draft hood the biggest pipe you can use as a connector (between the water heater and the chimney) is 5” and the maximum recommended horizontal length of that 5” diameter connector is 7 ½’.
That’s one of the things responsible for corrosion and deterioration of the connector.
Since I don’t know BTU’s of your water heater, chimney type, chimney height, liner (if any) material and its diameter you need to use Gama Tables - http://icpindexing.toddsit.com/documents/003046/gama%20vent%20tab.pdf to determine if your orphaned water heater can be safely vented into the chimney without violating local codes. The table for your new installation would be most likely #5 but I highly recommend reading all the information in this manual. There are some examples of how to use tables starting on page #18.
Let me know if you need further assistance. Just click “comment” below.
answered by darekrudy (21,730 points)
Can't thank you enough, the tables were very helpful.  I way overestimated the horizontal run in the dark of the crawl space; couldn't get the tape measure from the angle I was at when I first wrote you, but was able to measure about 60% of the run today, which let me estimate the rest much closer.  Table 5 is dead on my situation, and the lateral run is really 11' max, and I've got a good 20' elevation, 40k btu water heater, so I'm fine at 4" b type.  Bought most the parts today and did some pre-assembly in front of having a guy help me wrangle things in place.

Thanks again!

You are welcome ADS. I hope the chimney won't become a problem after the inspection. Good luck!