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Hot Water Heater Venting

Old Chimneys

Millions of old homes years ago used to heat with coal. Based upon years of experience, heating contractors and masons knew what size chimneys worked for what size furnaces. Engineers also had developed formulas for large industrial chimneys and smokestacks.

When coal was pushed aside by natural gas the chimneys still functioned quite well. The temperature of the exhaust leaving the furnace was slightly reduced, but the draft was not adversely affected. Coal burns hotter than natural gas in the first place. The old furnaces were not nearly as efficient as today's. In fact, old coal furnaces possibly only delivered into the living space 50 to 55 percent of the heat they created. The rest went up the chimney.

This wasted heat kept the chimney warm, almost hot, between times of peak firing. Older gas furnaces did the same, but chimney temperatures dropped slightly. Why? Well, the furnace people determined how to extract more heat from the combustion process.

The Orphaning Process

High efficiency furnaces - those that deliver 90 percent or more of the combustion heat - are great for your checkbook (fuel savings) but this saved money and more is often being spent by many homeowners on chimney, plaster and repainting costs.

It is not uncommon for your gas hot water heater and furnace to vent into the same chimney. The chimney was originally sized for the furnace (probably coal) AND your hot water heater.

The new high efficiency furnaces often are required to vent to the exterior through PVC pipe. They often do not even use the chimney. This leaves the hot water heater by itself.

A Rain Forest Waterfall ... But Not in the Chimney!

An orphaned water heater in an oversized chimney creates a huge moisture problem. Cold winter air is heavy and is constantly trying to sneak into your house by slipping down the chimney.

When the old furnaces and hot water heaters were in the same vent, they produced ample heat and draft to combat the heavy, cold air.

Orphaned water heaters vent into colder chimneys. The exhaust from the combustion of natural gas contains massive amounts of water vapor. Because of the cold conditions inside the oversized chimney, the water vapor condenses inside the chimney. Soon it is "raining" inside your chimney. This water soaks into the brick and begins to make its appearance known by bubbles in paint, efflorescence on plaster and stains. The only solution is to install a metal liner that reduces the inner diameter of the chimney, or to create a reduced flue size by some other approved means.

Hot Water Heater Venting

Different associations are currently working hard to modify/change existing code requirements for fuel burning appliances. It is a tough assignment. Lots of tests have to be successfully completed and officials convinced of their accuracy before venting requirements will change. Until the changes are made, here is what you should do with your hot water heater:

  • The inside cross sectional area of the flue should be no greater than seven times the cross sectional area of the flue pipe that leaves the heater.
  • Never reduce the size of the vent pipe that leaves the heater.
  • Try to get as much vertical rise AT THE TOP of the heater as possible. This allows the exhaust gas to build speed before hitting an obstacle. A 90 degree bend on top of the heater is an obstacle! Make sure the mason roughs in the thimble as high as code will permit.

Author's Note: We've received other questions about the similar problems. Here's one from Roger Beauman of New Athens, IL:

"I have an old 2 1/2 story frame home build 1896. I recently replaced the furnace with a high efficiency unit. I have a gas water heater now the sole use of the chimney. During the summer, I get high levels of CO - carbon monoxide in the house if the temp is sustained at 98 or above. No issues at night, just in the heat of the day. We currently keep several CO - carbon monoxide detectors going and turn the water heater off during the day when we see elevated CO - carbon monoxide levels. Is there a fix for this?"


6 Responses to Hot Water Heater Venting

  1. I was asked recently to replace badly corroded (eaten through) gas furnace and hot water heater vent pipe.. It is single wall 4" galvanized pipe; and the 3" single wall vent pipe from the water joins the furnace vent through a wye at ceiling level; I am not an HVAC WIZ; but what might be causing the "water rotting"'of the pipe.......I have never seen this.......and Im kinda old...???
    Thanks for any input.... Mark

  2. Since putting in the new furnace (years now). I had exhaust problems in the home(brick). I got a insert for the hot water tank last year. This year the furnace guy found a disconnected wire. Now I have air suction and hot air out the vents I didn't have before that I had complained about.

    But as you can figure I have the hot water fumes being suctions up as well. I have heard about a boss exhaust basement system. Would that be better or would a electric hot water system work better. Do you need to vent the electric hot water systems. The basement has radion(sp) problems very high in fact. Informed the furnace has more power then I needed, but I was always cold. We have heavy lime deposits here in the water and told you have to have maintance on the electric hot water systems every year.

    So what is the next plan of attack if the hot water insert isn't drawing in the winter or being diverted inside the homestead.

  3. How do i vent my new hot watter tank when it doesn't vent through the roof any more
    Or how do you install it properly

  4. My three gas fired hot water heaters in the basement of a four story brownstone keep shutting down when the weather outside gets warm (80+). Why is this and what can be done about it? They share the chimney with the gas furnace, which is off in the summer.

  5. we are fixing to spray attic joist with foam insulation. how do i vent my 2-75 gallon water heaters or and i turn atmospheric type water heater into a direct vent?

  6. I had an inspection on my house that I am selling. The inspector said I needed to change my flue pipe on my hot water tank from a 3 inch to a 4 inch. Why would I need to do this since it's been like this for almost 65 years

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