Why Do Listed Chimneys Still Leak?

Most chimneys on the market are leaking. Fact.

A common reaction might be something like, “Not me; my chimneys are listed.” The costly reality is, in certain instances, testing standards for chimney joint sealants DO NOT protect you.

Even many listed chimneys leak not only condensation but also dangerous flue gases. For the contractor or specifying engineer, this means liability and financial risk. It’s happening more than you think. There’s something going on with some of the most commonly used chimney joint/sealant materials and it must be addressed.

Chimney joint sealants: leakage & test standards

Silicone Rubber, Vitonand Ceramic Cement are common sealant materials used to seal chimneys. Chimneys that use these materials are tested against ‘leakage limits’ set by listing agencies such as UL, Intertek, etc. The chimneys pass, and engineers specify those materials because the manufacturer said it’s a good idea. But somehow after installation, we often see a very different story.

chimney corrosion

Corrosion or hairline cracks at the chimney joint can cause leakage after installation - and after the manufacturer’s warranty has run out. Oftentimes the contractor or specifying engineer doesn’t even know it’s happening; yet, they take the blow for it.

Why is this happening?

If the products are listed, how can a chimney material pass a test, and then one year later start leaking? The test standards for modern chimney joint sealant materials – now more than 20 years old – are not in sync with real-life situations where condensation, high temperatures and varying pressures are at play.

There are a number of reasons why listed chimneys leak and why you can’t just always go with the manufacturer’s recommendation. Some of the facts around chimney testing standards cannot be ignored. For example:

  • Test standards concentrate on positive pressure applications only
  • It takes six months to simulate a real-life corrosion test with metal-based gas vents, but only three hours to test (and pass) the resiliency of the Silicone sealant
  • In a chimney rated for 550⁰F, the Viton sealant inside will only last 240 hours
  • Ceramic Cement is water soluble and, therefore, can’t be exposed to water
  • Adjustable and Variable Length sections can’t handle pressure and ARE NOT LISTED!

A cure for chimney leakage

The rate of chimney leakage was unacceptable, not to mention costly. So in 2014, we partnered with Underwriters Laboratories (UL) to show that a graphite chimney joint, as used in the EPS PowerStack™ chimney system, is a much more reliable design for mitigating chimney leakage and associated risk.

Our research revealed proof that graphite gaskets are superior to other joint sealant materials when exposed to real-life conditions:

Compared to Ceramic Cement,

  • Graphite reduced leakage by 86% in cold conditions
  • Graphite reduced leakage by 82% during positive pressure after 1400⁰F

Compared to Silicone,

  • Graphite reduced leakage by 67%

It’s not just the graphite sealant

Joint assembly and human error during installation play a critical role in the safety and performance of your chimney system. There are things you can do to protect your chimney application from leakage and ensure it will stand the test of time.

Remember, just because a chimney is listed and recommended by the manufacturer does NOT mean your chimney application is protected against leakage.

Get the facts about testing standards for chimneys, plus advice for maximum chimney performance, in this Technical Brief:

“Why Are So Many Listed Chimneys Leaking?”

whitepaper why listed chimneys leak