House Water Heater Vent Pipe Tips & How To Do It Right

Improper house gas water heater venting could be hazardous for you. Have a look at the pictures, and get it right, get it now - it's easy and it can save your life.


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

House Water Heater Venting Video:

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 – explained below
  • induced draft / water heater PVC pipe venting / power venting

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 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.

Water heater venting - installed vent pipe reducing connectorIf 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 hazardHowever, 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.

Water heater vent pipe and draft hood properly secured with screwsReplace 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 height

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

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  1. jody says

    Hi. I am planning on replacing my old gas water heater with a 40 gal tank. The water heater is in my bathroom (12.5′x7.5′) on an outside wall and sits on a concrete slab foundation. Can I vent it out the side without a power vent?

    1. CTH Experts says

      Hello Jody,
      If it is a natural vent type water heater you can not vent it through the side wall, it has to vent above the roof. Other issue is the bathroom installation which isn’t permitted as well, unless the water heater closet is sealed from the bathroom area (air tight doors) and it has a separate, fresh air supply from exterior or other areas of the house that can be used for this purpose. Let me know if you have any other questions.

  2. Matt says

    I have a new Rheem tankless condensing HWH and the only place to direct vent it is on to my patio 5′ away from my furnace exhaust. To avoid another PVC exhaust stack in a commonly used area I would much prefer to run the intake and exhaust (separate 3″ PVC vents) up my chimney (30′ or so rise) in the existing, unlined 12″ chimney (previously used for an older furnace + HWH and now only used for soon-to-be-gone leaking HWH). How should I terminate the 2 PVC vents atop the chimney?

    1. CTH Experts says

      Hello Matt,

      Go to this page: and open the “Use and Care Manual” pdf file (last one on the list). Pages 44, 45, 46 show the details of the direct vent, condensing type through the roof vent installation. The easiest way would be to put a metal cap on the top of the chimney opening with one or two holes in it (depending on which installation you’re going to pick). Run double wall pipe or two single wall pipes through the holes, install proper flashing, and termination connectors just like on the Rheem manual pages. Make sure that you follow all of the directions in their manual, it’s critical for the appliance to operate safely and efficiently.

      1. Matt says

        Thank you for the quick response Dariuz,

        Any tips on supporting the pipes vertically inside the chimney (the manual states vertical supports every 4′). Also, does the intake still have to be 12″ above the chimney level? There are two other chimney ports for fireplaces, how can I direct the intake away from exhaust from the other ports (1 unused but functional wood fireplace in the middle and 1 seldom used natural gas fireplace on the other side)? The top of the chimney is at least 36″ above the peak of the roof line.

        1. CTH Experts says

          You’re welcome Matt,
          For the supports you can use plumbing pipe support clamps with horizontal rods attached to it and anchored to the chimney side walls. Unfortunately you’d have to make a few holes in the chimney along its length in order to install those rods. However, the biggest problem that I can see is the proximity of the other two chimney flues. If you run the fireplace and the water heater at the same time, your appliance might start recirculating exhaust fumes from the fireplace. To avoid any frustration in the future I’d call Rheem and explain this particular situation. According to the table on page 44 of the manual you can go 24″ above the roof (top of the chimney crown level) without any additional support. You could extend pipes further away from the fireplace flue and install some kind of a support, but the safest way would be to get that distance information from Rheem first.

  3. Cindy Libolt says

    We are planning on re-roofing our house this summer and the chimney which our water heater is vented is rusting out and needs replaced. The water heater is vented through the old chimney for a fuel oil furnace which has been replaced with a high efficiency furnace and no longer uses that chimney. The chimney looks to be 12″ double wall. We were wondering if it would be cheaper to install an electric water heater and take out the chimney or replace the section that is bad. The 4′ section that is rusting out is from the roof line up. Our water heater is in the basement and we didn’t know if there is any other way to vent it. The water heater is 14 yrs. old.

    1. CTH Experts says

      Hi Cindy
      Is your chimney 12″ in diameter with 4″ liner inside?

      Since you have an older water heater I would suggest to consider replacing it. It might run for a few more years or it might start leaking tomorrow. 14 year water heaters are unpredictable. While replacing it you have 4 choices.

      1. new electric water heater / remove old chimney / you have to run 220V power line to the water heater’s location.
      2. new power vented water heater / eliminate the existing chimney / WH vents through the side wall of the house using PVC pipe / you’ll need one electrical outlet to power the vent
      3. new natural draft water heater (this is probably something you have now) / correct any problems with the chimney / replace corroded, deteriorated sections.
      4. new tankless water heater / eliminate the chimney / vent through the side wall of the house / you might need larger size gas line for it / the most expensive WH, probably around $2000 installed

      Get some estimates for each option. Number #4 is the most efficient type but it usually takes about 10 years to get your invested money back.

      Let me know if there’s anything else I can help you with.

  4. Steven says

    Hi. Last year my old water heater went out, and I bought a brand new one from Home Depot. Rheem. Gas Residential – Low NOx Emissions. 40 Gallons. I installed it myself. The WH is inside a laundry room (17’x4′) on the back of a detached garage, and it sits on a concrete slab foundation. The single-wall vent (about 4′) sits on top of the draft hood, and it’s vented straight up thru the roof with a cap. Recently I noticed a problem which never happened before, and I’ve had this home for 7 years. When I turn on the hot water for about 45 seconds, I saw a small explosion (size of draft hood) from the WH by the draft hood. This is about the same time when I heard the burner came on. Then everything seemed ok. Once the burner stopped burning and someone turned on the hot water, the small explosion by the draft hood would happen again. What’s wrong? There is more than 12″ clearance in front and on the right side of the WH. Immediately to the left of the WH is the washer then the dryer to the far left. Please help.

    1. Steven says

      CORRECTIONS: The vent pipe is a 3″ Type-B double wall, and it’s 5′ straight up thru the roof with a cap. This is a dedicated vent pipe for the WH only. The dryer has its own dedicated vent pipe to the side of the bldg.

      CLARIFICATION: The laundry room and the garage is only connected by an exterior wall, and there is no doorway between the two separated spaces.

      1. CTH Experts says

        Hi Steven,
        From your description it appears that your water heater might be experiencing something similar to a puff-back. It’s a condition caused by delayed ignition of gas in a combustion chamber. The gas should ignite immediately after it’s released from the burner. If / when this is delayed, the WH’s igniter timing is off, there’s too much gas accumulated inside the burner chamber and when it finally ignites it creates that small explosion. It’s a newer water heater with a sealed combustion chamber and for that reason you can see or hear it at the draft hood area.
        One thing I can’t explain is the explosion at the end of the cycle. It might be the gas valve which is malfunctioning resulting in an erratic opening and closing.
        Another possible complication could be the amount of combustion air in this utility room. You didn’t mention how tall the room is and if it has any constantly open ventilation ports, but assuming 17’x4’x8’ ceiling you’d need 3 times as much of air to support complete (safe) combustion process of the water heater (based on 30000 BTU’s which should be stated on the label attached to the water heater / more BTU’s requires more air).
        Because of a sealed combustion I wouldn’t try any repairs by yourself. The appliance is still under warranty and you should call for the service ASAP. Let me know if you have any other questions.

        1. Steven says

          Thanks for the quick reply!!! Just to clarify my comments earlier regarding the “explosion at the end of the cycle.” That doesn’t happen. What I meant to say was after the the first “explosion” and when the user has finished using the hot water, the WH will bring temp back to its normal temp AND the burner will stop burning. UNTIL the next user turn on the hot water AGAIN, the cycle repeats like clockwork. There IS NO second explosion at the end of a cycle.

          The laundry room is 17′ x 4′. The height on the front 17′ wide (where the entry door is) is 6′, and the height on the back (where it shares the same wall as the garage) is 7′. So as you enter the laundry room, the roof slopes upward toward the back wall. On top of both (front and back) walls are openings between 2x4s at about 24″ on center. T’here are plenty of air flow betweeen the outside air and inside the front wall, and there are plenty of air flow between inside the back wall and inside the garage. Along the front wall, there is 8″ x 16″ vent. Along one of the 4′ sides, there is also another 8″ x 16″ vent. I’ve even let the 36″ entry door wide open, and the “puff-back” still happened. Do you think I still need more “vent?”

          I will call manufacturer while waiting for your response : )

          1. CTH Experts says

            It looks like there’s enough of fresh air in that room and since you’ve clarified that second “explosion” it seams to be ignition delay issue. Let me know “verdict”. Thank you.

  5. Dave says

    My newish (last 2 years) water heater as recently has been giving me some challenges. The pilot routinely goes out, if the A/C is running and the heater kicks on the exhaust does not go up the vent pipe and instead is pulled into the house. We recently replaced a very drafty sliding glass door with one that is energy star, tightening the house (so much so I often can’t hear my dog barking to be let back in). If I smell the fumes I turn off the AC fan the fume begin to vent properly than I am able to turn the AC back on, not the best solution but an effective one.

    My question is ….. Is this a water heater issue or HVAC or chimney issue? The furnace/ac unit is located off the kitchen sharing the closet with the water heater. The new door is on the opposite wall. Really who should I call to fix it or is there something DIY that can eliminate the problem. In addition, could the Water heater be defective, (still under warranty)?

    Thanks for your insight.
    Dave M.

    1. CTH Experts says

      Hi Dave,
      It appears that your house became excessively “air tight” and whenever an HVAC system starts operating, it takes away combustion air necessary for the water heater’s operation / burning process. Your newer water heater is equipped with a safety device (read my answer to Christa’s question below) which shuts off the burner / pilot whenever it senses inadequate amount of combustion air or a downdraft (created by your HVAC system).
      1. Is the utility closet equipped with a sufficient amount of combustion air providing vents – louver doors, ventilation ports between its interior and the rest of the house, exterior of the house (check combustion air article)
      2. Are there any HVAC system air return ports inside the utility closet or next to the combustion air supplying ports (if such exist)? Check this – air return and combustion air
      3. If your closet is “air tight” and its interior provides the only combustion air source for the appliances it contains, open its doors while running HVAC and monitor your water heater’s operation.

      Let me know if this helps and NO, I don’t think that your water heater is defective, it’s just its safety feature response to the insufficient air supply. An experienced HVAC professional would be the right person to call. Don’t forget to maintain a functional and up-to date CO and smoke alarms!

      1. Dave says

        1. Is the utility closet equipped with a sufficient amount of combustion air providing vents – louver doors, ventilation ports between its interior and the rest of the house, exterior of the house (check combustion air article)

        Yes, louvered bifold doors – four sections probably 6 ft. wide.

        2. Are there any HVAC system air return ports inside the utility closet or next to the combustion air supplying ports (if such exist)? Check this – air return and combustion air

        There is a return port around the corner none in the closet.

        I guess I need a HVAC guy.


        1. CTH Experts says

          If your HVAC system creates a back-draft there might be a “leak” in the air supplying ducts. Maybe an open air duct in the attic, crawlspace (don’t have enough info on your entire system). Somebody will have to look at it and find a reason of that negative pressure and “vacuum” like effect HVAC system creates. Test if opening the window or an exterior door would prevent the pilot from going out.

  6. Mike says

    Hi, I am adding a second hw heater to my house to serve the first floor( 2 family house). The existing water heater is 40gal natural gas, 3 inch vent to chimney about 6 feet away. The 2nd water heater will be the same, but i want to use the same vent pipe for both. Will i have to increase the vent pipe size to accomadate both? And what size do i need to increase to? Thanks!

    1. CTH Experts says

      Hi Mike,
      The general rule for venting two water heaters into a common connector is that its diameter must be equal or more than the larger diameter vent pipe + 50% of the smaller diameter vent pipe. In your case it would be 4.5″ or more (the closest you can get is 5″). You have a few options on the drawing. Let me know if this answered your question.

      1. Mike says

        That answers my question! Thank you sir.

        1. CTH Experts says

          You’re welcome Mike 🙂

  7. Tony Mann says

    I’ve been living in my 100 yearly old home for 5 years with the following natural gas equipment:

    Natural draft gas water heater, 14 years old,, vent sizes (from hood) 3″ into 4″ into 6″, ~ 10′, into chimney.

    Natural draft gas water heater, 14 years old, vent sizes (from hood) 4″ into 6″, ~8′ into chimney.

    Natural draft gas furnace, 24 years old, vent sizes (from furnace hood) 4″ into 5″ into 6″, L – shaped, 20′ into chimney.

    There are some rust & water stains near the water heater vent hoods. Noticed slow water drips from vent joints, lots of “calcium” on vent joints, and slight rust near screws.

    The equipment and vents are located in a crawlspace that is humid in summer and is very dry in the winter, but the temperature never falls below 50 F.

    Noticed water dripping and peak CO readings from two (Kidde Nighthawk) detectors increasing slightly since Feb 2011, CO/Gas detector went off May 14, 2011

    There is no chimney cap, but never noticed water until Feb 2011.

    Gas company visited today, confirmed CO detection, said vents/chimney were not exhausting properly, deemed installation unsafe, shut off gas.

    Gas service company rep said (on phone) “water leaks from vents” indicate chimney problem. Recommended calling a chimney company.

    Can you advise on the best steps to take so I solve the right problems?

    Here’s my guess:

    1) Get chimney inspection. Do I need to install a liner?
    2) Inspect vents and/or replace due to age? Do I replace with double walled
    3) Inspect gas equipment and/or replace due to age?

    Great website! Thanks.


    1. CTH Experts says

      Hi and thank you Tony,
      I’m assuming that you have a masonry chimney and since it’s an older home, the chimney may not have a liner at all. Missing chimney’s rain cap worsens the situation by allowing rain water penetration and causing interior wall surface and joints deterioration. Pieces of mortar, stones / brick often fall down the chimney, start accumulating, blocking the flue, restricting exhaust gases, resulting in condensation and elevated levels of CO. Your to-do list is perfect:
      1. Chimney inspection ASAP, they will tell you what needs to be done. If it’s interior is heavily deteriorated it might require partial rebuilding (if it runs through the attic, have this section evaluated too). Chimney liner is currently required by code so if you’re performing major repairs on your chimney or replacing any of the appliances – it must be installed and appliances connected directly to the liner (rain cap is required too).
      2. I would replace furnace and water heater vent pipes with double wall type. Your crawlspace is not a conditioned space (you do not control temperature and humidity in there?)
      3. Water heaters are unpredictable, they can last for a few more years or crack tomorrow. The heat exchanger of a forced air furnace installed in a crawlspace has a tendency to deteriorate much faster due to excessive humidity and temperature variations. 20 years lifespan is typical for a regular environment installation, I’ve seen severely corroded (with holes) heat exchangers in crawlspace furnaces at 10-15 years of age – have the heat exchanger checked!

      Let me know if there’s anything else I could help you with…

      1. Tony Mann says

        Thanks Dariusz for the quick reply and advice. With the gas shutoff, it’s a good thing the heating season is over so we’ll only have to do without hot water until I can get the chimney/gas appliance companies in.

        I don’t control the temperature in the crawlspace, it’s just well insulated due to the house being built on rock. I keep the “vent window” closed year-round and run a dehumidfier in the summer, but it’s a lost cause. That’s another issue, I’m hoping your website will answer in the future.

        By the way, what is the “white stuff” I found on the vent pipes?

        I’m sure you’ve heard this a few times, but your website/information is very useful! I’ve finally found what I was looking for about chimney caps!

        Keep up the great work!


        1. CTH Experts says

          You’re very kind Tony :-), thank you…
          The “white stuff” (most likely) on the vent pipes is a result of ventilation problems / possibly clogged chimney. It is caused by moisture condensation on the interior walls of the chimney and metal vent pipe. The exhaust gasses from your appliances supposed to be carried out to the exterior through the chimney. They contain slightly acidic water vapor which (with obstructed chimney) condensates on the chimney’s interior walls and flows back down through the metal pipes reacting with galvanized metal (zinc).

          That white residue could be also a product (or partial product) of efflorescence. Rain water inside the chimney and condensation soaks into the brick / mortar / stone surface dissolving the alkaline salts. It carries the solution down the chimney, furnace/water heater vent pipe, and evaporates leaving the salt deposits on surface / joints.

          As far as sealing off the ventilation in your crawlspace – very bad. You have 3 gas burning appliances that must have a constant fresh air supply to support clean and complete combustion process. The bi-product of incomplete combustion is carbon monoxide… Ask your HVAC guy about providing calculating sufficient combustion air supply for them.

          If you decide to go ahead and replace your appliances, consider stepping up to a high efficiency furnace and power vent type water heater(s). Calculate the cost of your chimney repair + natural vent type new appliances VS capping the old chimney and using new appliances venting through the side wall of the house with PVC vent pipes.
          Good luck with your project!

  8. Karl says

    We just got done replacing our old water heater, and need to replace the vent pipe. Really we needed to add about 2-3 inches before it 90’s into chimney. We noticed the old vent pipe didn’t stick into the chimney really far. About how far should it extend into the chimney, or do they make a plate to seal against the chimney, and then hook the vent pipe up to it?

    The old one was just sticking into chimney (barely), and then like a mortar material around it to seal then annular gap.

    1. CTH Experts says

      Hi Karl, ideally the water heater vent pipe connector should be even with the interior surface of your chimney’s flue. There are collars that can be installed against the chimney wall surface at the vent pipe penetration area. However, you have to make sure that it’s sealed properly, you can use high temperature caulking (if permitted for such applications in your area), or seal the penetration around the pipe with mortar.

      1. Karl says

        Okay, thanks. As far as sealing goes, someone gave me some firecaulk. Would that be a good sealer into the chimney?

        1. CTH Experts says

          Yes Karl, fire rated caulking would be good for this purpose.

  9. jay says

    Just wanted to say thanks for this excellent instruction on venting. I was absolutely clueless, I had ours all wrong. But now it’s perfect thanks to this video. Thank you.

    1. CTH Experts says

      You are welcome Jay, any home maintenance questions – just let me know.

  10. Steve says

    I have a gas water heater that vents out the chimney. The gas heater vents out the side of the house, so the water heater is the only item venting through the chimney. the chimney above the roof is in very poor condition. Here’s my question, can I disassemble the chimney to just below the roof boards and vent the water heater a few feet above the roof material using a liner of some sort. The chimney “liner” used to vent the water heater would be metal and I’d like to use some kind of flange similar to what’s used for for soil stack to keep it water tight. Is that within the realm of possibilities and code compliance

    1. CTH Experts says

      Hi Steve,
      1. You can use your old chimney as a chase for the new water heater vent pipe. However, you‘d have to run a B-Vent (double wall pipe) from the point where the water heater vent pipe penetrates the chimney wall, through the attic and above the roof. You’d have to install a vent pipe roof flashing at the surface penetration. If you have a corbelled chimney (angled) within the attic, assembling a b-vent pipe inside it would not be possible.
      2. You’d have to consult this second option with a local building department (HVAC division) or / and HVAC contractor. Run a flexible liner inside the chimney to the point of termination inside the attic, connect a B-Vent to the liner and continue this double wall pipe through the roof.
      In this configuration however, the chimney section inside the house and within the attic would have to be repaired, tuckpointed, sealed, closed-off properly around the B-Vent to flex liner connection (so the connection remains within the chimney walls). Due to some safety concerns this may not be approved by your local jurisdiction.

  11. Curtis says

    We had a new gas water heater installed a few months ago. I noticed today that the top of the tank was very hot to the touch and that the plastic ring around the outlet pipe had somewhat melted closest to the vent. I saw that the white plastic rings are also slightly melted even in the picture of a properly secured vent on this website . So, is this normal that the rings melt a little bit? The WH was installed by a plumbing team and told that the vent was good. I’ve also recently had the gas company here testing for a leak and he said the vent wasn’t leaking gases.

    When I touch the vent today, it wasn’t hot. Normally, the vent is hot up to a couple of feet from the tank.


    1. CTH Experts says

      Hello Curtis,
      Melting of the plastic ring (I’m assuming you’re asking about the water pipe union connector insulator) has not much to do with a properly secured vent / draft hood. It’s rather an indication that there might a problem with the water heater’s vent pipe or / and chimney draft. When the gas water heater burner ignites, a small amount of heat / exhaust gases is usually dispersed around the draft hood. However, this shouldn’t cause melting of that plastic, with a proper / strong draft, exhaust gases should be pulled through the vent pipe / chimney.
      The WH vent pipe / chimney drafting problems could be a result of the weather, combustion air problems inside your house, your house being too tight, some obstruction inside the chimney flue, oversized chimney, undersized chimney… So, without all those questions answered I can’t tell you what’s responsible for the melting, but it isn’t normal.

      The natural draft water heater vent pipe always becomes hot with operating burner, but there shouldn’t be heat buildup around / outside of the draft hood.

  12. scott says

    Hi, I replaced my 10 years old water heater one year ago because I smelled some kind of metal-stink after every re-boil cycle. However, only half year without the stink smell for the new one, it comes again. I asked the contractor to check any venting problem, like draft hood, pipe, cap, etc. They said nothing wrong. After I read through your web-side. I think the problem probably in condensation and/or ventilation (drafting) capacity not enough. Please give me your advice, thanks.

    1. CTH Experts says

      Hello Scott,
      Could you please describe the location of your water heater, ventilation of the water heater area (is it in a closet, basement, are there any vents that provide fresh air for the combustion process, what is your water source (city water or private well) and where is that bad smell coming from – is it only near the water heater, or is it from the hot water when you open the faucet after the re-boil.

      1. scott says

        Hi, my water tank is far from the water source, however is located closely at the wall corner right behind the furnace. Beside the water tank, there is a basement window, I could feel there is some fresh air getting in through the gaps. I am sure the stink smell coming out of the water heater after every re-boil cycle. It seems if water heater is just on pilot the smell becomes strong. I can tell you that, when the my old tank was removed, the metal piece inside the center of the tank (under the draft hood) was corroded half left. Once again, thanks for your attention!

        1. CTH Experts says

          Hi again,
          You didn’t answer my questions Scott, especially “what is your house’s water source -city water or private well?”, and that smell you’re referring to, is it only near by the water heater or / and when you open the hot water faucet.
          The metal baffle inside the water heater’s flue pipe has nothing to do with the smell, it’s there to deflect heat towards the tank wall and increase the efficiency of the burning process. Over the years it corrodes, deteriorates and sometimes falls in pieces onto the burner, which might cause problems, compromise combustion, and result in CO poisoning. The water heater itself doesn’t really smell unless there’s something that has fallen into or through the chimney flue and is burning inside it.
          However, if you’re using well water, city water from a local underground source (community well), or / and a water softener, you might need to replace sacrificial anode rod in your water heater tank. It’s a magnesium or aluminum that is wrapped around a steel core wire rod, screwed in into the WH top plate that prevents corrosion of your tank.
          Anode rods usually last 5 -6 years, which depends mostly on the quality of your water and how much of that water are you using.
          When using water softener sodium is added in the process and anode rods tend to corrode much quicker, sometimes in as little as six months if the water is over-softened. Also, well / municipal well water often contains anaerobic bacteria, which reacts with aluminum and magnesium anodes producing hydrogen sulfide gas, thus creating the rotten egg odor… (sodium from softening process makes it worse).

          If this is what you have, let me know. Otherwise, I’d look for some other source of that odor, a floor drain, open sewer line, sump / ejector pump well, etc.

  13. mike says

    I have a nautual draft water heater can i change it to a induced draft by just adding a motor or is there something I ned to do ?

    1. CTH Experts says

      Hi Mike,
      Natural draft type water heaters cannot be converted into the power vent operated appliances, unless the appliance in questions is listed / approved for such type of a ventilation. You can go over to the manufacturer’s website or simply give them a call and ask about your particular model.

      It’s all about the design of your appliance. Installing an induced draft motor on a natural draft type WH that has not been designed for it could disrupt proper combustion process, result in burner operation malfunctioning, and possible CO poisoning.

      Let me know if this helped and if you have any other questions.

  14. John says

    The vent coming off my gas water-heater and furnance join and run at an upward angle for 5ft or so, before turning straight up though the ceiling and into the attic. It stops about 6 inches from the roof. The house is almost 7yrs old. I was just rambliing around up there the other day and noticed what appears to be a soot/carmel like subtance, both on the underside of the roof and on the blown in insulation that is laying around the vent pipe. Is this normal, acceptable?

    1. CTH Experts says

      If I understand correctly, the builder forgot to put a chimney through the roof (the water heater and furnace vent pipes terminate in the attic?). If so, you are lucky to be alive and still have a house (or at least the roof). This furnace / water heater vent pipe must continue through the roof and terminate above it. Chimney’s minimum height above the roof will depend on its positioning / location at the penetration point, distance to any structure, roof slope, etc. (it’s all in my chimney posts). Get a licensed HVAC contractor, visit your local building department and find out who has approved such installation.

  15. brian says


    I have a vent pipe that handles both the hot water heater and the furnace. Both connections start above the bottom of the pipe, and the problem I have is that the bottom is not capped. Can you recommend a fix for me? Do I just buy a round piece of metal and caulk it in? Do you know why a builder would not provide a bottom cap? maybe just laziness? Also, I’m worried about possible gases in the house. It’s been like this for years. Better late than never, I guess.



    1. CTH Experts says

      Hi Brian,
      A picture would be helpful, but I believe it is something like on the one here. You should be able to purchase a cap designed for this purpose, they come in various diameters. It doesn’t have to be caulked, it’s even better to secure it with sheet metal screws only (3-4).
      Chimney vent stack pipe bottom cap
      Occasionally, there might be some condensation developing inside the vent pipe and if you seal the cap on the bottom, it will just result in corrosion. If you look inside the pipe and it appears to be clean / no rust, there is most like no draft problems and no worries about exhaust gases backing up into the house.

      1. brian says

        Thanks, Dariusz!

        My situation is similar to your pic, except that mine is all sheet metal. Thanks for the expert advice – you’ve really been helpful.


  16. Stacy says

    Caveat: I live in a condo, on the third floor, so all the draining, piping and infrastructure is what it is.

    I just had a ultra high efficiency water heater installed. To make a long story short, we’ve come to find out the drain (previously used only as the “emergency” drain from the drip-pan) cannot accommodate a continuous flow of water, mainly condensation from the Ultra High Efficiency unit, because it terminates outside and it freezes up every night. There are no other drainage options so they are going to swap out my new unit for one exactly like the one that was in there before, which was power-vented. Because of the angles in the utility closet, the water heater sits about 3-4 feet away from the exterior wall.

    The installer had an alternative water heater he recommended, but it did not have a power vent. I think to myself… there’s got to be some reason the builders originally put a power vent in, so we should continue to go with a power vent.

    What are the requirements for power venting? Can an non-power vented unit push 4 feet horizontally?

  17. Jim says

    I have just replaced a natural gas water heater and the burner flame is much too yellow. When it first fired the flame was “soft and blue” as described in the installation manual. After about ten minutes of operation the flame was much larger and nearly all yellow. According to the instructions this condition indicates a lack of combustion air and will produce excess soot. I performed the draft test using a flame at the base of the vent hood and there was a smooth steady draw toward the vent. The flame becomes smaller, more stable, and mostly blue if I pinch the gas supply valve back about 25%. The new water heater has a sealed combustion chamber with a screened combustion air intake around the bottom. The tank is 21″ in diameter with a 24″ diameter aluminum leak pan under it. The vent piping is unchanged from the old unit. Until last year the flue for the water heater was shared with the hot air furnace. Two PVC pipes connected through the wall provide combustion air and exhaust venting for the new furnace. From what I have read on your website I suspect the problem is too large of a flue for the water heater alone. I assume that operating with the gas valve pinched back will reduce the amount of heat the burner can provide and increase the recovery time. As long as the pilot is stable, the burner ignites ok, and the flame looks good do you think it is OK to operate this way until the chimney flue can be properly sized?

    1. CTH Experts says

      Hi Jim,

      Partially closing the gas valve is not a good idea. WH gas regulators are designed for dealing with the amount of gas provided from the gas meter / pressure regulator / gas piping. While running a few gas appliances at the same time, the one with a partially closed gas valve might be starving for fuel and shut down or start short-cycling.

      You didn’t mention the WH draft hood, vent pipe and chimney flue diameters, so I don’t know if this is the reason for the flame color issues. Usually it is not – oversized chimney might cause back-drafting, condensation inside the chimney flue, CO poisoning, etc.

      Flame discoloration is in most cases related (like you’ve mentioned) to inadequate amount of combustion air that must be supplied to sustain complete burning, without producing CO (yellow flame does that). Check those posts:
      Combustion Air
      Combustion Air Condominium (this one refers to Chicago but applies to “everywhere”) and make sure that your water heater has a sufficient amount of air supply from within the house or exterior.
      Let me know if you have any other questions,

  18. Lewis McDowell says

    I am currently extending out a bath that will require me to swing the gas vent pipe more toward the exterior wall to enable me to move the bath tub. It appears that I can spin or turn the pipe get it where I need to at the bottom but then my roof connection is crooked. Can I add a 45 to reconnect to the roof vent? this would mean a 45 or what ever the degree is coming out of the top of the water heater then straight pipe then 45 back to line up to current roof vent stack.

    1. CTH Experts says

      Hi Lewis,

      Without using vent pipe tables for your particular installation and just applying general rules, you can use 45° connectors and one 60° connector. However, to be on the safe side, you should properly calculate the entire run of your WH vent pipe. It might be OK, but it all depends on the BTU’s of the water heater, draft hood diameter, horizontal and vertical runs length + offsets.

      Without that information it’s difficult to say if what you’re doing is safe, because that additional length of the vent pipe might compromise proper drafting.

      You’d need Gama Vent Tables to perform those calculations ask ask your local building department / licensed plumbing contractor to evaluate your installation.

  19. Kathy says

    My vent pipes are 3 1/2 inch in diameter. During high winds the pilot light goes out. Can I put a 4 inch, high wind cap on a 3 1/2 inch pipe at the roof termination?

  20. Cheryl says

    I have a gas hot water heater, installed in 2004. Currently both the furnace and hot water heater are vented into an exterior brick chimney. I am in the process of purchasing a new hgh efficiency furnace. The furnace installer is telling me since the furnace will be vented out the wall, there will not be enough draw for the hot water tank to be safely vented up the chimney. He said carbon monoxide will back into the basement. He suggested I add a liner to the chimney, or purchase a new power venting hot water tank. Another installer has said that is not always true. How do I know if I need to replace the hot water tank, or if it is venting properly

    1. admin says

      Hi Cheryl,
      You can go to my post Water Heater Chimney Flue Sizing for a general answer. To calculate exactly the size of the required chimney diameter that will accommodate your water heater, you’d need to use Gama Tables and do some measurements of your installation. There is an easy to follow guide here – Hart & Cooley Type B Gas Vent & All-Fuel Chimney Sizing Guide – open the lower one of those 2 pdf files, go to page #8, it starts at “Single-Appliance Vent Systems”.
      There’s a 1” difference in the max. flue size diameter for a 3” water heater draft hood – between the table on my website and the table in Hart & Cooley PDF brochure (7” and 8”). I’d rather go with 7” maximum (if possible, keep the liner size closer to the minimum required diameter instead of a max.), especially with an exterior chimney.

      Let me know if you need some help with it.

  21. Clay says

    Should gas water heaters be raised off of the floor?

    1. admin says

      Hi Clay,
      I’ve put up together a little more information about raising the water heater – Gas Water Heater Raised Above the Floor

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