Condensation 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.

Causes of Condensation & Corrosion Damage at High Efficiency Condensing Boilers & Furnaces

Does Condensation Damage occur with Older Heating Equipment?

The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss. Sketch at page top is courtesy of Carson Dunlop Associates, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

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

Higher-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.

See HIGH EFFICIENCY BOILERS/FURNACES

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.

See SAFETY RECALLS CHIMNEYS VENTS HEATERS

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

at CHIMNEY WET TIME & CORROSION.

Also see CHIMNEY DAMAGE by LEAKS & FROST.

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.]

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.

• Q&A on Heat Exchanger & Flue Damage from Condensation in Heating Boilers or Furnaces - PDF version, use your browser's back button to return to this page. Original article, Solar Age Magazine, September 1985, adapted and updated for InspectAPedia.com November 2010.

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

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: hole in condensing furnace flue - plan to seal

(Dec 31, 2014) mr zippy said:
have a condenser glow worm sxi boiler 10 years old. Develope a small hole, after condenser trap block causing build up of water in exchange heater. will a 1200oc sealant repair the job until warmer weather,

Reply:

Zippo,

The question involves safety worries. I would give the manufacturer a call to ask this question.

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Continue reading at HIGH EFFICIENCY BOILERS/FURNACES or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• CONDENSING BOILERS/FURNACES
• CONDENSING BOILER/FURNACE CONDENSATE DRAIN
• SAFETY RECALLS CHIMNEYS VENTS HEATERS
• DIRECT VENT WIND PROBLEMS

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CONDENSING BOILERS/FURNACES DAMAGE at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to CHIMNEYS & FLUES

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