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what size vent do I need to connect two 35 gallon water heaters on same vent?flue water heater

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I have two water heater in an enclosed laundry room (20' X 6'). They are about 5 feet apart. They are vented by a common vent piper via a Y ducting. The vents from the water heater are 3" and they converge to a 4" Y before exiting through the roof. Is this venting system safe?
asked in Gas Water Heater by fmartinezd (120 points)
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1 Answer

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Hi FMartinezd,
I’m sorry but based on the information you’ve provided I cannot answer your question.
I need BTU’s from both appliances, length of both vent connectors (between the water heater and Y), vertical height of the chimney (from Y to the chimney cap), chimney type (metal, masonry), connector rise (how many feet between the top of the draft hood and Y). 
You can also look at the following question / answer and a picture at  http://checkthishouse.com/qa/544/can-vent-be-merged-with-the-furnace-vent-into-chimney-stack-6 and follow my calculations.
To calculate if your room 20x6 provides sufficient amount of combustion air click this link - combustion air
Let me know if you need help, just click “comment” below.
answered by darekrudy (21,730 points)