House Water Heater Vent Pipe, How To Do It Right

Post 145 of 157

Natural Draft Hot Water Heater Venting / Water Heater Vent Pipe

There’s more about the water heater vent pipe in my article below the video. Follow the links for more important information.

Watch Home Maintenance videos on Youtube.com – House Water Heater Venting

To fully understand house natural gas water heater venting I highly recommend reading House Brick Chimney Problems & Gas Water Heater and Furnace Chimney Flue Sizing posts – several of the water heater vent pipe conditions described below depend on properly operating and installed chimney!

Check my new post explaining tank gas water heater maintenance and water heater inspection It’s important!

There are two common natural draft gas water heater vent pipe systems:

Natural draft gas water heater vent pipe system is still dominating and as long as the property has favorable structural conditions and a chimney dedicated for use with gas burning appliances, you can use this type of venting.

Typical, natural draft water heater vent pipe assembly - missing screws at draft hood connectionTypical connection of the natural draft gas water heater vent pipe would look more or less like one on the picture (one of the problems with this connection – missing screws at draft hood). There might be several variations… some good and some bad of course.

Assuming, that you don’t know anything about water heater vent pipe subject, this is how the natural draft gas water heater venting should be installed:

Water heater vent pipe draft hood

On top of natural draft gas water heater, in its center section, right above the water heater vent, you should have a draft hood installed. It sits on 3-4 short legs, sometimes secured to the water heater top plate with screws, or (depending on design) has its legs shaped like pins or hooks at the end, inserted into the holes in water heater top cover.

Water heater vent pipe - collapsed natural draft gas water heater draft hood poses safety hazardFor the gas water heater vent pipe system to operate properly its Draft Hood Has to be Centered over the vent hole, and its legs must be straight – any displaced or deformed draft hoods should be serviced / replaced if necessary. Water heater draft hoods serve a very important purpose and if not installed correctly, carbon monoxide gases may be expelled into the living space. I’m pretty sure you’re aware how dangerous Carbon Monoxide can be…

  1. Gas water heater draft hoods provide additional air for the combustion process gases, to be properly pulled out from the burner chamber (base of the water heater), into the water heater vent pipe and the chimney
  2. Gas water heater draft hoods act as a device, which in case of down-draft (a condition which forces air / wind back into the vent pipe / chimney), prevents air from extinguishing the gas burner.

Loose water heater vent pipe, oversized - requires reducing connectorWater heater venting - installed vent pipe reducing connectorGas water heater draft hoods come in different sizes, and if you are replacing water heater, make sure, that the draft hood and the water heater vent pipe match each other. If the draft hood that came with your water heater has a top opening diameter designed for 3″ vent pipe, and you have 4″ vent pipe installed, use an adapter / increaser to join both of them together.

Downsizing of water heater vent pipe is not permitted and poses safety hazardWater heater vent pipe and draft hood properly secured with screwsHowever, if the gas water heater draft hood is larger, designed for a 4″ pipe – do not downsize it! / DON’T install reducing connector from 4″ draft hood to 3″ water heater vent pipe just to accommodate an old venting system.

Replace the smaller size vent pipe with a proper / required size:

  • most 30 gallons, 40 gallons, and 50 gallons gas water heaters use 3″ diameter vent pipe
  • some 50 gallons might require 4″
  • 75 gallons and more will call for 4″ and more

Always follow the appliance manufacturer’s recommendations and comply with your local code requirements – again – never downsize the water heater vent pipe!

Natural draft gas water heater vent pipe material

Flexible, aluminum connector used as a water heater vent pipe is not permittedAluminum water heater vent pipe section is not permittedFood can used as a water heater vent pipe is not permitted

For natural draft type just use galvanized steel pipe and don’t experiment with aluminum pipes, food cans with removed both ends, stainless steel sections, blue stove pipes, high temperature plastic tubing, flexible pipes…, etc.

Water heater vent pipe example - developed heightMake the vent pipe section between the water heater and the chimney as short and as straight as possible. The common rule is that the horizontal part of the single wall vent pipe must be equal or shorter than 75% of it’s total developed height.


Draft hood and gas water heater vent pipe connections

The connection between the draft hood and the vent pipe should be secured with sheet metal screws – three per connection on a single wall pipe are recommended.

DO NOT use regular duct tape to secure or seal connections. Some building inspectors don’t even allow aluminum tape on joints. Simply because it hides problems like corrosion or holes developing on surface.

If you are using a B-vent which is a double wall vent pipe, you supposed to use screws on the first connection only – to the draft hood or to the single wall water heater vent pipe. Smaller diameter double wall to double wall vent pipe sections utilize twist lock fittings with no screws / larger diameter B-vent pipes might require screws on its joints.

Natural draft gas water heater vent pipe pitch

Water heater venting - WH vent pipe horizontal run / pipe has a negative pitch

Gas water heater vent pipe connector must continuously run upward towards the chimney entrance, rising not less than 1/4″ per linear foot, to provide proper draft. Some installation might be a significant challenge, or even become impossible, due to a water heater height and chimney flue connection level, in those cases, induced draft motor equipped water heater should be considered.

Natural draft gas water heater vent pipes corrosion

Heavily corroded water heater vent pipe, holes on surface might cause Carbon Monoxide spillageSeverely corroded water heater vent pipe poses safety hazard, possibility of Carbon Monoxide poisoningWater heater venting - corroded vent pipe connections might cause CO leakage and joints separation

WH vent pipes should be monitored periodically for corrosion and deterioration. Problems with proper drafting, chimney conditions and combustion air issues usually cause galvanized pipes to corrode.

Small holes start appearing on their surface – most common areas are along the bottom portion and on connectors. At some point, corroded gas water heater vent pipe wall becomes very soft, and might fall apart when under even slight pressure.

Corroding gas water heater vent pipes should be replaced as soon as possible, to prevent possibility of exhaust fumes / Carbon Monoxide contamination.

Water heater vent pipe clearances and restrictions

Gas water heater vent pipe located to close to combustible materialsSingle wall gas water heater vent pipe / connector cannot be installed closer than 6″ from any combustible materials such as floor / wall framing, paper, etc.).  Such installation could create pyrophoric conditions and a fire-hazard.

Pyrophoric condition is when a material  ignites spontaneously at significantly lower temperature if it is constantly exposed to heat. So, if something would normally ignite at 500F, by applying constant heat to it, that ignition point might be lowered to let’s say 250F – it’s just an example…

  • Single wall natural draft gas water heater vent pipe cannot be used in unheated areas like attic or garage (even if only partially penetrating that space) , because such vent pipe installation will cause excessive condensation on vent pipe walls and compromise proper drafting – double wall pipe / B-vent type is required.
  • Single wall natural draft gas water heater vent pipe can not be used inside the walls, ceilings and any inaccessible areas – double wall type vent pipe / B-vent must be installed.

Thinking about relocating your natural draft gas water heater to the garage area? Check this important information – garage water heater requirements.

Check gas water heater vent pipe clearances post for more about natural draft water heater venting.

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This article was written by Dariusz Rudnicki

I'm a retired Illinois home inspector, founder and editor of checkthishouse.com, a blog which attracts around 2 thousand readers daily and is dedicated to answering the many questions of home owners and home buyers. Connect with me on Google+ Find me on Google+ Local

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