Unsafe sidewall vent (C) Carson Dunlop Associates Heat Exchanger & Chimney Flue Damage From Heating Equipment Condensation

  • CONDENSING BOILERS/FURNACES DAMAGE - CONTENTS: Review of damage to high efficiency heating boilers and furnaces (condensing boilers or condensing furnaces) due to condensation and corrosion. Can older furnaces or boilers also cause condensation damage when duty-cyclers are installed? Problems Connecting a Higher Efficiency Furnace or Boiler to an Old Chimney and Flue. Effect of Duty Cyclers on Heating System Efficiency & Performance. Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
  • POST a QUESTION or READ FAQs about handling condensate from high efficiency boilers or furnaces

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Condensatin in chimneys & heating flues or equipment:

This article discusses damage to high efficiency heating equipment caused by condensation and corrosion to heat exchangers or flues, and we address the question of condensation and corrosion damage with older heating equipment that uses duty cycle controls.

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Causes of Condensation & Corrosion Damage at High Efficiency Condensing Boilers & Furnaces; Condensation Damage with Older Heating Equipment?

Sketch at page top is courtesy of Carson Dunlop Associates.

The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

Topic: High Efficiency & Conventional Heating Furnace or Boiler Damage from Condensation

Question: Did the change to stainless steel heat exchangers fix the condensation damage worry with high efficiency heating furnaces and boilers? What about older heating equipment that uses a duty cycle timer?

A heating salesman mentioned that lawsuits were pending (in 1985) against some manufacturers of high-efficiency furnaces (and boilers) - that although condensation problems had been solved through the use of stainless-steel heat exchangers, there were new problems arising from the lower-temperature flue gases.

Also, are there any documented cases of condensation damage in older furnaces (or boilers) stemming from the use of duty-cyclers lowering flue gas temperature? - Jeff A. Pendl, Enervision, Cincinnati OH

Answer: Higher Efficiency Heating Systems are Designed to Handle Corrosive Condensate

Vent condensate freeze (C) Daniel FriedmanHigher-efficiency heating boilers and furnaces have cooler exhaust gases, which may condense in different parts of the system. If the entire system, including the heating unit itself and its venting components were not designed to handle the corrosive condensate, problems may occur.

The very high-efficiency condensing furnaces and boilers are designed to condense liquid in the heat exchanger. While some early models had troubles (, we are not aware of any current problems or lawsuits.


Condensing furnaces (warm air heat) or condensing boilers (hot water heat) use stainless-steel heat exchangers, plastic flue vent piping, and other system components that resist corrosion.

Watch out: if plastic vent piping for high efficiency heating equipment is not properly installed so that condensate is drained to a proper location, the system may become blocked or unsafe, as is happening in our photo, above left.

Problems Connecting a Higher Efficiency Furnace or Boiler to an Old Chimney and Flue

You are more likely to run into trouble when fitting a new, more efficient furnace or boiler to an old flue [especially with gas-fired heating equipment], or when retrofitting devices that lower flue gas temperatures.

In particular, problems are likely when the draft is already marginal due to a cold exterior chimney, extra long flue-vent-connector piping between the heater and the chimney, or other flue problems such as inadequate combustion air and very tight house construction.

See COMBUSTION AIR for TIGHT BUILDINGS and also the more technical problem discussion



Watch out: inadequate draft, backdrafting, and inadequate combustion air significantly increase the risk of production of dangerous, possibly fatal carbon monoxide gas. Be sure that your home is protected by working, properly-installed, properly located smoke detectors and CO detectors.

For more information see CARBON MONOXIDE - CO

Effect of Duty Cyclers on Heating System Efficiency & Performance

As for the particular device you mentioned, a duty cycler, researchers at the America Gas Association Laboratory (AGA) suggest that they are unlikely to cause condensation problems.

[In at least part, that's because a properly designed and installed duty cycler must assure that the heating equipment runs long enough to reach proper operating temperature and thus also to vent properly.- Ed.]

Photo of automatic flue vent damper (C) Daniel Friedman

Nor are duty cyclers likely to boost the efficiency of the heating system more than 1 or 2 percent.

Other energy savers such as heat reclaimers and automatic flue vent dampers (see Automatic Vent Dampers) like the one shown at left are more effective.

But stick with certified devices, follow the installation specifications, and consult with the manufacturer to be sure you have and are following the installation manual for the device.

Details about how to reduce heating costs are found at HEATING COST SAVINGS METHODS.

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Original article

The link to the original Q&A article in PDF form immediately below was preceded (above) by an expanded/updated online version of this article.

The preceding ext is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.


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