Differences Between Factory-Built Chimney Pipes

Factory-Built Chimney Pipe U.L. Listing Differences

Double and triple wall chimney pipes have been installed over the past few decades to alleviate the need to construct a costly brick chimney. Or, in situations where a brick chimney doesn’t exist. There are a few different styles of chimney pipe available that are U.L. listed to be safely installed in your home. Double wall insulated and double or triple wall air cooled chimney pipes are the available options for properly venting a high-heat appliance. Knowing which one to use and when is very important for the safely venting your wood stove or fireplace.

Side Note: Before we move on, it's important to note the difference between chimney pipe and stove pipe. Double wall chimney pipe is not the same as double wall black stove pipe! Chimney pipe and stove pipe have different clearances to combustibles and different U.L. listings.

While both insulated and air cooled chimney pipes are U.L. listed, they may not hold the same listing. The two listings we will be comparing are U.L. 103 HT and U.L. 127.

• U.L. 103 HT pipes are tested continually at 1000 degrees Fahrenheit plus three ten minute intervals of 2100 degrees Fahrenheit. The majority of today's insulated Class A chimney pipes are listed as U.L. 103 HT.
• U.L. 127 pipes are tested to 1000 degrees Fahrenheit continuous and comply to either a 1700 degrees Fahrenheit interval test or a 2100 degree Fahrenheit interval test. The higher temperature test is optional at the manufacturer's request. Many air cooled chimney pipes are listed as U.L. 127.

Typically, both types of chimney pipe have a 2” clearance rating from combustible materials.

The main dictator in choosing which chimney pipe to use is going to be the appliance itself. You must follow the manufacturer’s installation instructions exactly as they are written when installing a chimney for a fireplace or stove.

U.L. 103 HT Chimney Pipe Info

Almost all manufacturers of today’s EPA rated wood stoves and air-tight zero clearance fireplaces require the chimney stack to be U.L. 103 HT. This is because the flue temperatures of an air tight wood stove are much higher due to the reburn technologies such as a catalytic combustion or secondary air tubes. However, most wood stove manufacturers do not dictate which brand of chimney pipe to use.

U.L. 127 Chimney Pipe Info

If you have a traditional open air zero clearance fireplace, these types of fireplaces are not airtight and usually don't reach the flue temperatures that an air-tight stove does. Therefore, many open air zero clearance fireplace manufacturers require the U.L. 127 listing for chimney pipe. In most cases, the manufacturer of the fireplace indicates that only one brand of chimney pipe is U.L. tested with the fireplace. Different manufacturers put different collars on the fireplaces which will be unique diameters and specific to the chimney pipe brand indicated in the fireplace manual. If you were to stray from the installation instructions, a different brand of pipe may not fit and would most likely void the fireplace warranty.

Although there are clear differences between these two U.L. listings and both types have been installed in recent decades, insulated Class A chimneys are being installed more often.