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?"