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Discussion Starter #1
I am installing a gas water heater with a built-in ventilation fan. I'm venting it horizontally through the wall to outside. The installation instructions has two tables for the minimum and maximum vent pipe lengths. I understand why there's a maximum length, but is the minimum necessary? The table says 7 feet. Does that mean the exhaust of the vent pipe has to be 7 feet from the water heater? Why would that be when they are separated by an outside wall? Or do they mean 7 feet of vent pipe? I could achieve that easily, especially with elbows that have a pipe lenghth equivalency.

Any information would be helpful.

Thanks.
 

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That is so that the fan does not suck in the exhaust it just took the time to pump out. Contact the manufacture and ask them.
 

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Can you provide the manufacturer and model number, Jeff, so that we can look up the installation manual. Or give us a link to that.

Chris
 

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Discussion Starter #4
Skip at Stadium Hardware solved the mystery: the 7 foot minimum vent pipe lenghth is not a requirement, but a description. The water heater vent pipe starts with a 1 foot vertical section of pipe capped with a ninety degree elbow, which has the equivalent pipe lenghth of 5 feet, making for a sum of 6 feet, plus a foot to get from the water heater to the wall equals 7 feet. Thank you Skip, I shop at Stadium Hardware to purchase $13 worth of plumbing supplies and get $1,000 worth of knowledge for free.
 

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Hmmm…would need to see the manual for your unit to confirm that. Here’s a table from a GSW power vent water heater (as an example) showing a minimum equivalent vent length of seven feet. This refers to the vent that is added on downstream of the unit itself. It would be difficult to have an equivalent vent length less than seven feet, but depending on the vent pipe size and pressure switch setting that minimum equivalent vent length could be as long as 125 feet.

Chris

https://www.gsw-wh.com/assets/documents/current/manuals/GSW Power Vent Manual English 324089-001.pdf
 

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Discussion Starter #6
Chris, your understanding matches the interpretation that Skip from Stafium Hatdware had - that it is not physically possible to have less than 7 equivalent feet of vent pipe.

I asked the city inspector this morning and his interpretation was that the minimum length is needed for effective velocity and pressure, pointing out how the minimum length increases when the pipe diameter is 3 or 4 inches. As it is, the vent pipe is about 1 foot below and six inches to the side of the top air intake vent. The exhaust must be 4 feet below the top intake vent and 1 foot above the bottom intake. That means that the vent pipe will need an elbow and a vertical pipe over 3 feet and less than 5 feet long to keep the exhaust far enough from the intake vents. The pipe to the wall, two 90 degree elbows, and a 4 foot pipe adds up to about 16 feet, so the minimum pipe lenghth won't be an issue.

Thanks for the reply. Hopefully someone will be spared my confusion when they come across the column for the minimum exhaust length column in the tabe on page 14 of the pdf document you attached to your reply.
 

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I installed a gas water heater with a powered vent. Except for my 20+ year old appliances it is the loudest appliance in my home. It is not really that bad but I am easily annoyed by noises. Given a chance to do over, I would not install this type of water heater again, there are usually other options.
 
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