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This article discusses Required Venting Changes For A Gas Water Heater When Installing Any New Gas Furnace (standard or high efficiency) IF the home has a shared exhaust vent (serves both furnace & water heater).  Most homes built before 1987 have a shared vent.  Home Pro Plumber, Heating & A/C in Plano, Texas provides; maintenance & repairs for all brands of Central A/C, Gas & Electric Furnace, and Heat Pumps. Additionally, we sell and install new HVAC Systems from American Standard (same company as Trane), Ameristar (owned by American Standard), and Coleman HVAC (same company as York).

Click On This Link To Read About The Six Companies That Make 24 Central A/C Brands: HomeProPlumber.com Six Companies Make 24 Central A/C Brands

Home Pro Plumber also provides Full-Service Plumbing; maintenance, repairs, and replacements for every plumbing component in your home.   We repair, sell & install gas & electric water heaters.  We’re near your home in Plano, TX, Garland, TX, and Richardson, Texas.  We service all homes in southern Collin County with no additional travel charges.

Call Home Pro Plumber to discuss concerns about your Plumbing or HVAC System.  We’ll arrange an appointment at your convenience.

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When Installing Any New Gas Furnace — Changes To Gas Water Heater Venting

Are Required By Building Code.  Read below to understand why. 

(IF The Home Has A Shared Exhaust Vent For The Water Heater & Furnace.)

Copy The Link Below Into Your Browser To See A Photo Of A Shared Exhaust Vent (serving both the furnace & water heater). 

https://www.google.com/search?q=furnace+and+water+heater+side+by+side&sxsrf=AOaemvIW6aVrt8IRcwSMyjzhsipz-2qtPw:1630519165081&source=lnms&tbm=isch&sa=X&ved=2ahUKEwj73Iidrd7yAhWMmGoFHT8mDF0Q_AUoAnoECAEQBA&biw=1097&bih=554#imgrc=1L8teikJw5kSVM

In 1988: 80% Efficient Gas Furnaces became required.   Today 90% are required in northern states.  80% allowed in southern states.

• Nearly all gas water heaters are naturally vented.  This means the heat the create causes exahust to rise up the flue and out of the house.
• All new gas furnaces are Mechanically Vented.
• This means a fan forces the furnace’s exhaust out of the home.

• Many homes (built before 1987) — have a combined exhaust vent — shared by both the furnace & gas water heater.

• Furnaces built before 1987 sent 30%–50% of the heat they generated up the flue.  That kept the flue warm enough for the water heater’s exhaust to rise too.

• When any new gas furnace (standard or high efficiency) is installed in homes with a shared exhaust vent — the water heater must then have it separate venting.

• With a combined vent — a mechanically vented furnace can force the water heater’s exhaust downward into the house.

NOTE: Today — An 80% efficient gas furnace is required (in southern states).  A 90% is required in northern states.

Is Your Existing Furnace <80% Efficient?

Shown: Older Gas Furnace <78% Efficient.

If a gas furnace is less than 80% efficient:

• Its exhaust vent pipe is larger than 3 inches around.
• The furnace is silent before the burner’s light.   In newer furnaces — there’s a fan motor that pushes exhaust out of the house.  You hear this motor start before burners light.
• The gas water heater & furnace often share the same exhaust vent.

Homes Built Before 1987 Likely Have A Combined Gas Water Heater + Gas Furnace Venting. 

Click Here To See One Version Of A Combined Exhaust Vent: Combined Vent For Furnace & Water Heater

• This venting arrangement was allowed with furnaces <78% efficient — because they aren’t mechanically-vented.
• The furnace’s wasted heat (going up the flue) — was enough to get both the furnace & gas water heaters’ exhaust to rise up the flue.

In 2003, The International Residential (Building) Code Changed

Nearly All States’ Building Codes Match (Or Are Bused Upon) The International Residential Building Code (IRC)

IRC Building Code No Longer Allows Combined Vents Serving Both A Gas Furnace & Water Heater — Because All New Furnaces Are Mechanically-Vented.

The New IRC Code (2003 and later) States:

4. Vent connectors serving equipment vented by Natural draft (typical water heater) Shall Not Be Connected Into Any Portion of Mechanical-Draft Systems (all new gas furnaces).

Source: https://telluride-co.civicweb.net/document/32311  Page: 360.  Section: G2427.3.3

The Water Heater Must Be Vented Through Its Own “B-Vent” Flue

Image Source: Amazon.com Embedded Link

Click On Photo To; View Product, Read Details, or Purchase from Amazon.com

A B-Vent Pipe — Has 2 Pipes:

• The inner pipe heats easily.  This assist flue gases to easily rise.
• There’s an air-gap between the inner & outer pipes.
• The air-gap provides some insulation between the inner & outer exhaust pipes — keeping the outer pipe cool.
• Exhaust vents must run (as vertically as possible) — from the water heater through the roof.
• A single-wall pipe creates a fire hazard.  It gets too hot to be near wood.  Home roofs are wood on the attic side.

• 80%  gas furnaces — also require a (B-Vent) metal exhaust pipe.
• 80% furnaces draw their combustion-air (for the burners) — from the area near the furnace.  (There are a few models that bring in outdoor-air for combustion).
• If the furnace is inside living space — it’s using heated indoor air for combustion.
• This causes the home to leak outdoor air inside at air leaks.

• 90%+ gas furnaces — have 2 plastic pipes.
• 1 pipe brings in outdoor air for comustion (used by the burners).

• The 2nd pipe vents the furnace’s exhaust outdoors.
• Plastic venting is used because the furnace’s exhaust is quite cool — (it won’t melt the plastic).

Copy The Link Below To See The Metal Exhaust Venting (B-Vent) For An 80% Gas Furnace:

Click Here To See Metal Exahust Venting For An 80% Efficient Gas Furance: Metal Exhaust Vent For 80% Gas Furnace

90%+ High-E Gas Furnace Venting

Image Source; CanStockPhoto

Shown: HI-E Gas Furnaces Intake Combustion-Air (for the burners) From Outdoors — AND Vent Exhaust Outdoors.

SHOWN: The 2 white plastic pipes in the photo (on left of the furnace at top) and (right of the furnace, near the top) — Are the Exhaust-Vent & Combustion-Air Intake.

The Combined Current Exhaust Venting — Is Too Large For A Gas Water Heater. The flue gasses can’t rise in a pipe that size.

 

On A Side Note:

A Water Heater’s Exhaust Vent Pipe Must Match The Water Heater’s Draft Hood’s Size (from water heater through the roof).

Image Source: Amazon.com Embedded Link

Shown: Draft Hood For Gas Water Heater

Click On Photo To: See, View Details, or Purchase the item from Amazon.com

What Happens If An Existing Vent Pipe Is Not Upgraded To Meet Building Code?

With a vent pipe that’s too large — the water heater (alone) can’t produce enough heat — to get exhaust gases to rise out of the home.

• In this case, the exhaust gases backdraft to inside the home (because cool air sinks — down the exhaust pipe).

• Backdraft means the exhaust gasses move downward instead of upward.

• Gas Equipment’s Exhaust Gases Contain Carbon Monoxide (CO).

• In small concentrations — CO will make the home occupants sick.  In larger enough concentrations — CO can kill.

How To Test If Your Gas Water Heater Is Venting Safely

To Test:

• Be sure all windows & doors are closed & locked.
• Close fireplace dampers.
• Turn all kitchen & bathroom exhaust vents on
• Turn clothes dryer on. 

• If the water heater & gas furnace share the same vent —  lower the furnace’s thermostat setting.
• Run enough hot water for the water heater to start heating.
• Allow the water heater to run continuously for 10 minutes.

With this test, with all venting -devices (described just above) pulling air from the house — after 10 minutes a properly vented gas water heater will not backdraft.

To See A Water Heater Backdrafting — Click On Arrow (in center of image)

Image Source: YouTube Embedded Video

Click On Arrow (in center of image) — To See A Water Heater Backdrafting.

HomeProPlumber 

This article discussed Gas Water Heater Required Venting Changes — When Installing Any New Gas Furnace (standard or high efficiency) In Any Home Built Before 1987 — IF the home has a shared exhaust vent (serving both the gas furnace & gas water heater).  Most of these homes have a shared vent.   Home Pro Plumber, Heating & A/C in Plano, Texas provides; maintenance & repairs for all brands of Central A/C, Gas & Electric Furnace, and Heat Pumps. Additionally, we sell and install new HVAC Systems from American Standard (same company as Trane), Ameristar (owned by American Standard), and Coleman HVAC (same company as York).

Click On This Link To Read About The Different Central A/C Outdoor Units Speeds Available: HomeProPlumber.com Different A/C Speeds Available

Home Pro Plumber also provides Full-Service Plumbing; maintenance, repairs, and replacements for every plumbing component in your home.   We repair, sell & install gas & electric water heaters.  We’re near your home in Plano, TX, Garland, TX, and Richardson, Texas.  We service all homes in southern Collin County with no additional travel charges.

Call Home Pro Plumber to discuss concerns about your Plumbing or HVAC System.  We’ll arrange an appointment at your convenience