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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 (same company as American Standard) and Coleman HVAC (same company as York HVAC).

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 Richardson, Garland, & Rowlette. We service all homes in southern Collin and Denton Counties with no travel charges.

Call Al’s today to discuss any concerns or problems you have with your HVAC System or Plumbing.  We will arrange an appointment at your convenience and offer 24/7 Emergency Service

New Gas Water Heater Venting Requirements When Installing A

High Efficiency Gas Furnace In An Existing Home

This article addresses the changes required in existing water heater venting when a High-Efficiency furnace is installed in an existing home that had a standard-efficiency furnace and water heater if they shared the same exhaust venting. 

Many homes are built with the water heater and furnace sharing the same combustion-gases exhaust vent.  This practice is common with standard efficiency (80%) furnaces.  When installed correctly, this venting arrangement works with an 80% gas furnace (or lower than 80%) + gas water heater.  The photo below shows typical combined furnace + water heater venting.

 

With a replacement High-Efficiency Furnace, the existing vent pipe does not work for water heater only because the vent is too large.   The old vent pipe is too large because it served both the furnace and water heater.  High-E Furnaces have separate venting.

When a standard-efficiency furnace is replaced with a high-efficiency the existing vent pipe must be replaced with the vent size the water heater requires.   This ensures the water heater can heat the vent pipe enough to get exhaust gasses to rise out of the house.  This type of pipe is called “B Vent”.

Type B gas vent pipe has  a smaller pipe inside a larger pipe (also know as double wall pipe) for insulation and safety.  This ensures the outside pipe is cooler than the inner pipe.   The airspace between the inner & outer pipes provides insulation against heat loss (from the inner pipe) as flue gasses are rising out of the home.

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You Can See Both The Inner & Outer Pipes In The B-Vent Pipe Photo Shown Above

 

Gas Water Heaters Come In Different Sizes (gallons)

Larger Water Heaters Have Larger Draft Hoods.

The Water Heater’s Vent Pipe Must Be The Same Size As Its Draft Hood. 

The maximum dimension of the (water heater venting only) vent pipe is determined by; 1. How long the vent pipe is and 2. The size (in gallons) of the water heater.  Typically, if the existing vent (that served both furnace & water heater) is larger than 4 inches, a smaller vent pipe must be installed to serve only the water heater. Because the new vent pipe is smaller, it can often be installed inside the old vent pipe.

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Every Gas Water Heater Has A Draft Hood At The Center Of The Top.  The Larger The Water Heater, The Larger The Draft Hood.

 

high efficiency gas furnace with two white pipes visible

Image Source: Dreamstime 40163150

High-E Furnaces have separate venting (see white plastic pipes coming from top of furnace). One pipe provides combustion-air.  2nd pipe vents exhaust gases out.

 

What Happens If An Existing Vent Pipe Is Not Replaced?

With a vent pipe that is too large, the water heater does not produce enough heat to sufficiently warm the vent pipe to get water heater exhaust gases to rise out of the home.  In this case, the exhaust gases backdraft into the home.  Backdraft means the exhaust gasses move downward in the exhaust pipe.  The n the gasses end up inside the home near.   These exhaust gases contain carbon monoxide which will make occupants sick in smaller concentrations, and can kill in large enough concentrations.

How To Know A Gas Water Heater Is Venting Safely?

To Test For Proper Water Heater Venting:

  • If water heater and a gas furnace share the same vent, reduce the thermostat setting for the furnace so it does not run during the test.
  • Run enough hot water for the water heater start heating.
  • Allow water heater to run continuously for 10 minutes.
  • Be sure all windows & doors are closed & locked.
  • Close fireplace dampers.
  • Turn all kitchen and bathroom exhaust vents on
  • Turn clothes dryer on

With this test, even with everything running that pulls air from the house, a properly vented gas water heater will not backdraft.

Note: You Can See Backdrafting Using An Incense Stick.  While lighted, incense sticks produce a scented smoke.

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Click On Arrow To See A Backdrafting Water Heater (as it’s happening) In This You Tube Video.

Image Source: YouTube Embedded Video

A Final Note:

Existing vent size size increases backdraft problems if the old furnace is less than 80% efficient, as those furnaces have an even larger vent pipe.

Combined venting with older furnaces was common because enough heat from the furnace vents outside (usually 30% of more of all heat produced). This keeps the vent pipe warm enough for water heater venting, even if the furnace is not running at the same time.  In summer, the vent pipe remains warmer than in winter, making backdrafting less of a concern.

 

SUMMARY:  This article provides valuable information on the need to replace an existing shared (by furnace + water heater) vent pipe with one of a smaller dimension when installing a High-Efficiency Furnace.  This is because the vent will now be used by only the water heater.

Most homes have a shared vent (serving both the furnace + water heater) when the furnace efficiency is up to 80% (which is today’s standard efficiency gas furnace).  A High-E furnace has separate venting, which leaves the existing vent pipe to serve only the water heater.  The existing vent pipe is typically too large and must be replaced with one of a smaller dimension, so the water heater can heat the vent enough to ensure exhaust gases rise out of the home.   If this is not done, the water heater will likely backdraft — which means the exhaust gases will come into the home.

Al’s Plumbing, Heating & A/C maintains, repairs, or replaces any brand of water heater, furnace, heat pump or central air conditioner.  We also provide all plumbing services, and no job is too small or too big.  Call Al’s today to make an appointment for any plumbing or HVAC needs.

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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 (same company as American Standard) and Coleman HVAC (same company as York HVAC).

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 Murphy, Rowlette and Wylie. We service all homes in southern Collin and Denton Counties with no additional travel charges.

Call Al’s today to discuss any concerns or problems you have with your HVAC System or Plumbing.  We will arrange an appointment at your convenience and offer 24/7 Emergency Service