footer-logo

1970's built home

Image Source: Shutterstock

This article discusses the Required Venting Changes For A Gas Water Heater — when installing any new gas furnace (standard or high-efficiency) in a home built before 1987 — IF the home has a shared exhaust vent (serves both the gas furnace & water heater).  Most pre-1987 homes have a shared exhaust vent.   

Al’s Plumbing, 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 HVAC).

Click On This Link To See Who Manufactures Each Brand Of Central A/C: 6 Companies Make 24 A/C Brands

Al’s also provides full-service plumbing maintenance, repairs, and replacements for every plumbing component in your home.  Al’s sells and installs Rheem Professional Series gas & electric water heaters, and tankless water heaters.  Al’s is near your home in Plano, Garland, and Richardson.  We service all homes in southern Collin County with no additional travel charges.

Call Al’s today to discuss any concerns about your Plumbing or HVAC System.  We’ll arrange an appointment at your convenience.

Gas Water Heater Venting Changes When Installing A New Gas Furnace

In Any Home With A Shared Exhaust Vent (serves both furnace & water heater).

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 1987: 78-80% Efficient Gas Furnaces became required. 

  • Any new gas furnace installed today is 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.
  • Pre-1987 furnaces — 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 is installed in these homes — 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.

How To Tell If Your Current Furnace Is <80% Efficient

furnace blower motor at bottom of furnace

Shown: 80% Efficient (or higher) Ggas Furnace.  NOTE: The exhaust’s fan (black) is located in the lower left corner of the upper half of this furnace.

Your Current Furnace Is Less Than 80% Efficient If:

  • Its exhaust vent pipe is larger than 3 inches around.
  • You Don’t hear a fan motor start running before the burner’s light.
  • The gas water heater & furnace may currently share an exhaust vent.

Homes 1987 & Older  — Typically Have A Combined Vent For Gas Water Heater + Gas Furnace. 

  • This venting arrangement worked fine with <78% efficient gas furnaces — because they aren’t mechanically-vented.
  • The furnace’s wasted heat (going up the flue) — kept it warm enought that both the furnace & water heaters’ exhausts rose up the flue.

The 2003 (& later) International Residential (Building) Code States:

4. Vent connectors serving equipment vented by Natural draft (typical water heater) Shall Not Be Connected Into Any Portion of Mechanical-draft systems (80% and higher efficiency gas furnaces).

This Means: With Any New Furnace Installation — A Gas Water Heater Must Then Be Separately Vented. 

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

The Water Heater Must Then Be Vented Through Its Own, Separate “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 quickly — assisting flue gases to easily rise.
  • There’s an air-space between the inner & outer pipes.
  • That air space keeps the outer pipe cool.
  • A single-wall pipe would create a fire hazard — because it gets too hot to be near wood (like the underside of the roof).
  • 80%  gas furnaces — also require a metal (B-Vent) exhaust vent.
  • 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.
  • 90%+ gas furnaces — have 2 plastic pipes.
  • 1 pipe brings in outdoor air — for the combustion-air (used by the burners).
  • The 2nd pipe vents the furnace’s exhaust outdoors.
  • Note: Plastic venting is used — because the exhaust from a 90%+ efficient gas furnace is quite cool (and won’t melt the plastic).

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

https://www.google.com/search?q=residential+gas+furnace+installed&sxsrf=AOaemvLXkSM4_mNHvAZdW5IP2wBjF6iD0g:1630432911413&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjS1o7069vyAhXFWM0KHViCCVgQ_AUoAnoECAEQBA&biw=1097&bih=554#imgrc=GFLcQARs57EmzM

90% + High-Efficiency Gas Furnace Venting

hi-e gas furnace

Image Source; CanStockPhoto

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

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

If The Current Exhaust Venting Is Combined. 

When a new gas furnace is installed

The Existing Vent Pipe Is Too Large For Only The Water Heater. 

Gas furnaces with efficiency <78% — sent a lot of wasted heat up the flue.  The flue pipe was larger (as shown below) — to accommodate the large volume of exhaust.

old gas furnace

Shown: Older Gas Furnace <78% Efficient.

When a low-efficiency (<78%) furnace is replaced with any new gas furnace — the water heater’s vent pipe must be replaced with a smaller vent pipe that the water heater requires.   This ensures the water heater’s exhaust gases will rise up the vent pipe

The Water Heater’s Exhaust Vent Pipe Must Be The Same Size As (the top of) The Water Heater’s Draft Hood. 

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

Click On This Link To See Where A Gas Water Heater’s Draft Hood Is Located:Gas Water Heater Draft Hood Location

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.

SUMMARY: This article discussed Gas Water Heater Required Venting Changes — when installing any new gas furnace (standard or high efficiency) in a home built before 1987 — IF the home has a shared exhaust vent — serving both the gas furnace & gas water heater (most do).  

Al’s Plumbing, 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 HVAC).

Click On This Link To See Who Manufactures Each Brand Of Central A/C: 6 Companies Make 24 A/C Brands

Al’s also provides full-service plumbing maintenance, repairs, and replacements for every plumbing component in your home.  Al’s sells and installs Rheem Professional Series gas & electric water heaters, and tankless water heaters.  Al’s is near your home in Plano, Garland, and Richardson.  We service all homes in southern Collin County with no additional travel charges.

Call Al’s today to discuss any concerns about your Plumbing or HVAC System.  We’ll arrange an appointment at your convenience.

footer-logo