Improperly-vented gas fired water heater in a Minneapolis home (C) Daniel FriedmanGas Water Heater Vent Codes & Standards
15 water heater venting checkpoints

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Venting requirements for gas-fueled water heaters:

This article describes proper ventilation details for handling the exhaust from gas fueled water heaters, both LPG or propane and natural gas.

We include building code citations for gas water heater venting and we quote excerpts from manufacturers' water heater installation manuals, U.S. and Canadian model building codes, and other sources and water heater venting standards.

We also illustrate improper and unsafe gas fired water heater venting installations.

The natural gas fueled water heater shown at page top is improperly vented, lacking the recommended minimum 12" of vertical rise before the elbow and vented directly into a large masonry flue where it cannot develop adequate draft. This heater installation may be unsafe.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Proper & Safe Exhaust Vent Codes & Standards for Gas Fired Water Heaters

Gas water heater venting up through a roof  at excerpted from Whirlpool gas water heater manual cited in this articleSummary of Good Venting Practices for Gas Fired Water Heaters

Water heaters must be vented using a listed and approved venting system that provides adequate size, height, and draft.

Otherwise such vents may be unsafe, venting combustion products, including dangerous carbon monoxide, into the building.

Illustrations of proper gas fired water heater venting using Type B- double wall vent pipe through a roof (above) or through a chimney (below) include details for the flue vent connector such as the requirement of a minimum vent connector upwards slope of 1/4" per foot of horizontal run.

These illustrations are adapted from WHIRLPOOL RESIDENTIAL GAS WATER HEATER MANUAL [PDF] (2015) retrieved 2018/03/05, original source:

[Click to enlarge any image]

  1. Watch out: Carbon Monoxide Hazard. Gas fired water heaters like those illustrated here must be supplied with adequate combustion air and the heater's exhaust gases must be vented to outdoors.

    Failure to properly vent a gas-fueled water heater can result in severe injury or death from carbon monoxide poisoning.

    The vent system must be installed by a qualified person. Examples of a qualified person include gas technicians, authorized gas company personnel, and authorized service technicians.
  2. Carbon Monoxide Detector(s): To reduce the risk of carbon monoxide poisoning, install a fuel gas and carbon monoxide detector. Install and maintain the detector in accordance with the manufacturer’s instructions and local codes.

    Review the location of the CO detector to be sure that it will protect building occupants. More information:

  3. Clearance to combustibles: for gas fired water heater vents: 6" minimum clearance from single-wall vent pipe or flue-vent connector to any combustible material.

    Single wall vent pipes cannot pass through a ceiling, floor, firewall, or wall. [An exception is made for non-combustible walls such as solid masonry or concrete.]

    Clearances for double-walled B-vent piping are given by the manufacturer and can be found also

    at TYPE B-VENT CEILING FLOOR WALL CLEARANCES. Typically a double-walled metal B-vent chimney section is stamped as requiring 1-inch clearance to combustibles.

    If you cannot meet the 1-inch B-vent clearance you may be able to use a zero-clearance chimney material. More information:

    CLASS A CHIMNEYS, MetalBestos™
  4. Code compliance: The vent system for a gas fired water heater must be installed according to local and state codes, or in the absence of local or state codes, the current edition of

    the NATIONAL FUEL GAS CODE, ANSI Z223.1 (NFPA 54) [link to 2012 edition].
  5. Draft hood: use the manufacturer-provided draft hood. Don't omit it as shown in the photo below.

Gas fired water heater missing its draft hood (C) Daniel Friedman San Miguel de Allende

  1. Draft hood connection to flue vent connector: provide at least 12" vertical inches of flue vent between the draft hood outlet and the first elbow or sloping flue vent connector.

    This is a manufacturer-recommended detail to improve the flow of exhaust gases [we have not found this in the model codes].
  2. Horizontal flue vent connection length & slope:

    Horizontal distance: no more than 75% of total vertical height of the entire chimney assembly

    Horizontal slope: no less than 1/4" per foot upwards slope per foot of horizontal run.

    A portion of the vent pipe (up to 75% of the total vertical height) can be horizontal, but the termination must be vertical. For the horizontal section, install without dips or sags with an upward slope of at least ¼ inch per foot. Install pipe avoiding unnecessary bends.

    Pipe joints must be fastened by sheet metal screws or other approved means.

    Support the pipe to maintain clearances and to avoid separation of joints or other damage.

    Vent pipe must be accessible for cleaning, inspection, and replacement. Running vent piping through a concealed space or inaccessible space is improper and unsafe.
  3. Existing Vents: If using an existing venting system: Inspect the existing vent system for obstructions, corrosion, and proper installation. Repair or replace the existing flue vent connectors and chimney components if necessary.

    The existing vent system must be UL listed Type B double wall or single wall metal vent pipe of either 3 inch or 4 inch diameter and installed according to the vent manufacturer’s instructions and the terms of its listing.

    More information: B-VENT CHIMNEYS.

    Do not use other materials such as dryer vent hose.

    Local codes may be more restrictive and may not allow single wall vent pipe.

    Single-wall vent pipe cannot be used for water heaters located in attics and may not pass through attic spaces, crawl spaces, or any confined or inaccessible location.

    Single-wall vent pipe cannot pass through any interior wall. [Exceptions may be made for non-combustible walls such as solid masonry].
  4. Power Vent Sharing - Common Venting: Do not share a power-vented-appliance flue: Do not common vent a natural-draft gas fueled water heater with any power vented appliance.

    The sketches below, adapted from the Whirlpool installation guide we have cited, are examples of vent pipe system installations and may or may not be suitable for your specific application.

    Consult the NATIONAL FUEL GAS CODE, ANSI Z223.1 (NFPA 54) [link to 2012 edition] -current edition and your local building & fire codes.

Gas water heater venting up through a roof  at excerpted from Whirlpool gas water heater manual cited in this article

[Click to enlarge any image]

  1. Venting through existing chimney: [Illustration above] An unused chimney flue or masonry enclosure may be used as a passageway for the installation of the water heater vent pipe.

    Watch out: Before connecting a vent to a chimney, make sure the chimney passageway is clear and free of obstructions. The chimney must be cleaned if previously used for venting solid fuel appliances or fireplaces.

    Also consult local and state codes for proper chimney sizing and application or, in the absence of local and state codes, the “National Fuel Gas Code”, ANSI Z223.1(NFPA 54)-current edition. 

Gas water heater venting up through a roof  at excerpted from Whirlpool gas water heater manual cited in this article

  1. Termination of flue vent connector at an existing chimney: [Illustration above]. This item refers to terminating the flue vent connector for the water heater right in the chimney, without extending the vent upwards through the chimney interior, relying instead on the existing chimney flue.

    Improperly-vented gas fired water heater in a Minneapolis home (C) Daniel FriedmanIf local codes allow, some [manufacturer specified models] gas-fueled water heater may be terminated into an existing chimney using the instructions below.

    Otherwise, this water heater’s vent must terminate vertically (sidewall or other horizontal terminations are not allowed).

    Watch out: Do not terminate the vent pipe in a chimney that has not been certified for this purpose.

    Some local codes may prohibit the termination of vent connectors in a masonry chimney. [We warn about this just below].

    Some local codes may prohibit the termination of vent connectors in a masonry chimney.

    Common (combined) venting is allowable with vertical Type B vent systems and lined masonry chimneys as long as proper draft for the water heater is established under all conditions of operation.

    As you can see the photo this gas-fueled water heater vent was installed without the manufacturer-recommended 12" vertical rise in the flue vent connector before the first elbow.

    Below we show a close-up of heat damage around the draft hood of a gas-fueled water heater installed in a Minneapolis home.

    As the homeowner pointed out, that's because this water heater is essentially TOO TALL for this LOCATION [photo of the entire Rheem gas-fired water heater below a low ceiling].

    Improperly-vented gas fired water heater in a Minneapolis home (C) Daniel FriedmanBut in our OPINION even with a 12" vertical rise this heater would not vent adequately because it is directly-vented into a large, otherwise unused masonry flue.

    Watch out: in our experience and opinion the direct termination of a gas fired water heater into a large masonry flue, as illustrated just above, is likely to give serious venting problems, particularly in cold weather when the heater is the only appliance venting into the chimney.

    The lower-BTUH of a gas water heater (compared with a heating furnace or boiler) will very often be unable to heat the chimney interior sufficiently to develop a draft.

    The result is a back-draft of combustion products out of the water heater's draft hood and into the building.

    Such installations are potentially unsafe and risk carbon monoxide poisoning.


    Do not install the flue vent connector from the water heater into the very bottom of a chimney. The vent pipe must be installed above the extreme bottom of the chimney to prevent potentially blocking the flue gases.

    The vent pipe must be firmly attached and sealed where it enters the chimney
    in order to prevent it from falling out, to prevent air leaks into the chimney - reducing the effective draft, and to prevent flue gas leaks out of the chimney. To aid in removing the vent pipe, a thimble or slip joint may be used.

    The vent pipe must not extend beyond the inner edge of the chimney as it may restrict the space between it and the opposite wall of the chimney.

  2. OK to vent into a shared B-vent chimney? Common (combined) venting is allowable with vertical Type B vent systems and lined masonry chimneys as long as proper draft for the water heater is established under all conditions of operation. Do not common vent this water heater with any power vented appliance.
  3. Vertical Termination with Listed Chimney Cap: The gas vent must be terminated in a vertical position to facilitate the removal of exhaust gases.

    Vertical exhaust vents must terminate with a listed cap or other roof assembly and be installed according to their manufacturer’s instructions.
  4. Vent dampers: Do not install a vent damper anywhere in the venting system of natural draft gas fueled water heaters.

Improper & Unsafe Gas-Fueled Water Heater Chimneys & Vents

Gas fired water heater vented horizontally through a dryer vent with a painted screen (C) InspectAPedia.comQuestion: gas-fueled water heater horizontal vents terminate in a dryer vent flapper: is this acceptable?

I have recently inspected a large apartment complex with more than 100 units, each served by an individual gas-fired water heater.

The water heaters that I saw (I haven't seen all of them yet) vent horizontally out through a building side wall.

[Click to enlarge any image]

There the heaters terminate in either a painted-over (and so blocked) screen or a clothes dryer flapper-type vent.

In my opinion these water heaters are not safely vented. A building manager asserted that because the apartments have carbon monoxide detectors the installations are "Safe".

I think that proper venting is a necessary safety correction for the property and, knowing the property owner I'm confident that the proper repairs will be made.

Below is what I wrote.

Is there more I could or should say? - A.P. ASHI Home inspector by private email 2018/02/09

Many gas fueled water heater exterior vent covers at xxx are dangerously blocked. Many (maybe 30) vents are obstructed by tight meshes (mosquito netting?) which are painted, many are covered with louvered dryer vent covers (which are designed for the forced air pressure of a dryer vent, not for the gentle flow of a water heater flue), and some are crushed and some have animal nesting.

The argument that the units have carbon monoxide detectors should not satisfy you because UL listed carbon monoxide detectors are not designed to detect low levels of carbon monoxide and because low levels of carbon monoxide can make people sick. Please see the photo below and see:

Also, attached are two pages from a random water heater installation manual

Please contact me with questions or comments, - AP

Reply: No AP, the natural draft gas fired water heater vent you show through a sidewall is improper and unsafe.

Venting a Natural Draft Gas Fired Water Heater Out Through a Sidewall & Into a Dryer Vent Termination

P. In my OPINION you've met the requirement of pointing out an unsafe condition. What I provide below in amplification will provide more authoritative citations and excerpts on the topic of proper venting of gas fired water heaters.

In your note you could consider expanding this phrase:

... should not satisfy you because ...

because I want to make clear that the "you" is not a legal, competent, nor acceptable authority and because I'd want to have made clear that there are grave liabilities and life-safety risks at stake.

A longer explanation of the hazards around a blocked or improperly-designed gas fueled water heater might be excerpted from the following:

... cannot satisfy your assertion that the building and its mechanical systems are safe and acceptable.

The water heater vents and vent terminations are incorrectly designed, improperly installed, and blocked. CO detectors are intended as a last line of defense against CO poisoning.

CO detectors are never accepted as a substitute for proper heater installation and venting. The building code for your area (citation needed), amplified by the manufacturer's instructions (citation needed) do not permit a water heater to be improperly nor ultimately un-vented.

That condition forces combustion gases into the building's interior and risks forcing them into occupied spaces.

The attached installation instructions from the manufacturer of these water heaters require a minimum vertical rise in the water heater exhaust flue as well as a proper UL listed chimney cap or termination for safe venting of combustion products.

There are grave liabilities and risks involved in violating basic mechanical and safety standards.

A water heater whose exhaust vent is inadequate due to insufficient chimney height or worse, blocked by an improper termination is unsafe regardless of whether or not CO is detected. Even a small change in a building, opening or closing a door or window can lead to illness or to a fatality.



Attachment: sketch from this company showing acceptable water heater venting details.You will see that in no case is short horizontal sidewall venting acceptable for a conventional gas fired water heater. That is because the system will not develop adequate draft to vent flue gases outdoors.

OPINION: When a water heater vented in that manner also has the vent exit blocked by painted screen material or by use of an improper vent termination such as one intended for a clothes dryer (not listed nor approved for water heater use) the venting can be nearly 100% blocked.

Such installations are improper and unsafe. The details for acceptable water heater venting are in the manufacturer's instructions as well as in the model codes cited below.

The gas-fueled water heater shown below is discussed at GAS FIRED WATER HEATERS where we provide a checklist and table of gas fired water heater safety points.

Burn marks on top of a gas fired water heater (C) Daniel FriedmanIn the photo the scorch marks are a tell-tale indicator that there has been back-drafting at this gas water heater.

The following might best be handled as an attachment demonstrating that you are not simply stating an opinion you are citing applicable building codes and thus law that pertains. (You don't give the country, state, city, or province or we could add local code citations).


Watch out: WARNING: Obstructed or deteriorated vent systems may present a serious health risk or asphyxiation." - source: "Instruction Manual for Residential Gas Water Heaters", State Water Heaters, retrieved 2018/02/09, original source:

Authority: This gas-fired water heater is design certified by CSA INTERNATIONAL under American National Standard/CSA Standard for Gas Water Heaters ANSI Z21.10.1 • CSA 4.1 (current edition).

The installation must conform with this manual, Local Codes and with the current edition of the National Fuel Gas Code, ANSI Z223.1.


Watch out: Carefully inspect the venting system of a replacement water heater installation before connecting to the venting system. All joints in the vent connector must be securely fastened with screws and fit tightly together.

Inspect the venting system for signs of deterioration (rust and perforation) and replace any sections that are not in good condition.

The chimney must be lined and in good condition. Check to make sure the venting system is properly sized for the water heater. If the venting system was previously sized for another gas appliance that has been removed, the venting system may now be too large.

Refer to the latest edition of the National Fuel Gas Code (ANSI Z223.1-latest edition), or in Canada, the Natural Gas and Propane Installation Code (B149.1-00 latest edition) for the correct sizing of venting systems and common venting with another gas appliance.

Do not vent this water heater into the venting system of another gas appliance designed to vent under positive pressure. The water heater should be installed as close as practical to the venting system to minimize the vent connector length required.

Refer to local codes for the distance limitations on vent connector lengths. At the completion of the water heater installation, the burner and venting system must be checked for proper operation with all other commonly vented appliances in operation.

Check for spillage of flue products around the outside relief opening of the draft hood after several minutes of operation.

The flame from a match should be drawn into the draft hood. Do not use the water heater or connected equipment if spillage is detected until the problem is corrected.

Refer to the latest edition of the National Fuel Gas Code, or in Canada, the Natural Gas and Propane Installation Code for complete details on the “Procedure to Be Followed to Place Equipment in Operation”. - source: Bradford White Water Heaters, retrieved 2018/02/09, original source:

Gas Fired Water Heater Chimney Codes in the U.S.

Example of gas fired water heater B-vent properly terminated above the rooftop at InspectApedia.comSee the National Fuel Gas Code, Chapter 24, Fuel Gas, that includes discussion of venting requirements in Section G2427 p. 360, Venting of Equipment (cited at the end of this page).

G2427.3 (503.3) Design and construction. A venting system shall be designed and constructed so as to develop a positive flow adequate to remove flue or vent gases to the outdoor atmosphere.

G2427.6.5 (503.6.6) Gas vent termination. A gas vent shall terminate in accordance with one of the following:

1. Above the roof surface with a listed cap or listed roof assembly. Gas vents 12 inches (305 mm) in size or smaller with listed caps shall be permitted to be terminated in accordance with Figure G2427.6.5, provided that such vents are at least 8 feet (2438 mm) from a vertical wall or similar obstruction.

All other gas vents shall terminate not less than 2 feet (610 mm) above the highest point where they pass through the roof and at least 2 feet (610 mm) higher than any portion of a building within 10 feet (3048 mm).


G2427.7.3 (503.7.3) Termination. Single-wall metal pipe shall terminate at least 5 feet (1524 mm) in vertical height above the highest connected equipment draft hood outlet or flue collar.

Single-wall metal pipe shall extend at least 2 feet (610 mm) above the highest point where it passes through a roof of a building and at least 2 feet (610 mm) higher than any portion of a building within a horizontal distance of 10 feet (3048 mm).

(See Figure G2427.5.) An approved cap or roof assembly shall be attached to the terminus of a single-wall metal pipe. [Also see Section G2427.7.8, Item 3.]

G2427.7.6 (503.7.6) Installation. Single-wall metal pipe shall not originate in any unoccupied attic or concealed space and shall not pass through any attic, inside wall, concealed space or floor.

The installation of a single-wall metal pipe through an exterior combustible all shall comply with Section G2427.10.15.

G2427.7.10 (503.7.10) Termination capacity. The vent cap or a roof assembly shall have a venting capacity not less than that of the pipe to which it is attached.

Gas Fired Water Heating Venting Standards for Canada

See Chapter 7 beginning at page 152 and also see "Combustion Spillage" beginning on p. 160 in this example code: B149.1HB-00 Natural Gas and Propane Installation Code Handbook


Gas appliances may be vented to either a chimney flue or a certified vent or factory-built chimney.

When vents are used, they must be certified to the applicable standard and bear a label that indicates that they have been so tested and approved.

It is important not to remove these labels in order to enable field inspection. In the case of Type B or Type L vents, each length extending above the roof line must be certified for exterior use and so identified.


7.14.4 A chimney shall extend not less than 3 ft (900 mm) above the highest point where it passes through the roof of a building and not less than 2 ft (600 mm) higher than any portion of a building within a horizontal distance of 10 ft (3 m).

Nowhere in these standards is there an acceptance of a short-rise, horizontally-vented, dryer-vent flapper capped or painted-screen-capped water heater vent termination.

Side-wall vented appliances, without exception, make use of some form of power exhaust, never simply atmospheric exhaust. Such vent exits and terminations, if you water heaters were power-vented, cannot be obstructed and must use a listed (approved) termination cap or cover.

Excerpting from GAS FIRED WATER HEATERS - home

Check for proper exhaust venting of combustion products from your gas fired water heater.

Gas-Fired Water Heater Chimney & Vent Codes, Guides, Standards

For more details see the citations below as well as these gas water heater venting guides:

Question: what's the purpose of these fins on the gas water heater flue?

Fins on gas water heater vent extract heat to surrounding area, possible subtle safety hazard (C) MHI have never seen this on a gas fired water heater, can you tell me what the heck it is? - Anonymous by private email 2019/01/23

Reply: "energy savings" or heat extraction fins on a gas water heater: Safe?

Those fins on the exhaust vent were popular starting in North America during the Arab oil embargo in the 1970s and are still sold and installed on occasion.

The function of these fins on the gas water heater flue vent connector is to try to increase the transfer of heat from the water heater exhaust into the surrounding area - maybe to make it warmer, in a fantasy that we're cutting heating cost by exracting heat, perhaps in hope of preventing frozen pipes nearby.

Watch out: There is in my OPINION (there's no authoritative data I've found on this ) a subtle potential safety hazard of the heat transfer fin add-on.

**IF** the gas fired water heater is already venting into a chimney flue that is big, and perhaps sometimes cold (as in old buildings) and perhaps tall, THEN the heater may have trouble venting under all weather conditions, risking back-drafting and carbon monoxide poisoning hazards in the building.

That problem could be present with or without the heat transfer fins but would be exacerbated by the additional heat that the fins remove.

If there were any doubt about the chimney it would merit inspection by a certified chimney sweep, a check for signs of back-drafting. You probably already know those signs.

Thanks for the photo and question.

See also these examples of heat recovery methods using fins or devices attached to flue vents or other heater tuves:


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