Safe venting may require use of chimney liners
Draft diverters for gas fired appliances are an asset to the venting process. However, people making claims of being able to eliminate the need to line masonry chimneys, serving 80% AFUE furnaces and boilers, have not been paying attention to history lessons. This is quite a common myth scenario; people tend to overlook the facts.
For the most part, myths are fanciful, harmless tales. The draft diverter tale is quite another matter. Property and people are being put at risk with this attempt to eliminate costs relating to safe venting of residential heating appliances.
The people promoting the use of the draft diverter as chimney vent kits are bending a recent AGA research report to suit their needs. This research concluded these draft diverters will make the new 80% appliances vent in a fashion similar to the pre-1992 draft hood equipped appliances. This allows the new appliances to be considered natural draft units. Unfortunately for these "eliminate liners" people, if they had paid closer attention to the aforementioned history lessons, they would know our industry began to focus upon masonry chimney condensation problems in the early '80s! Yes, when far less efficient gas fired appliances were all produced with draft diverters. Today's appliances emit a much cooler flue gas, reducing venting effectiveness through the cold, oversized masonry chimneys.
This is a key point: masonry chimneys are usually at least three times the size of the proper size vent and they are massive heat sponges - two nasty combinations for venting gas fired appliances.
The marketing schemes and aggressive advertising of proponents of these "draft diverter vent kits" exhibit a lack of courage of their own convictions. This is exemplified when their installation manuals qualify the use of this product with severe limitations as to when the draft diverter actually is capable of eliminating UL listed chimney liner systems. A major oversight of these people is the clearly stated point (even in their own installation instructions), "A masonry chimney must be built to code standards which includes a properly sized clay tile liner." Has anyone seen a chimney meeting these standards? They are extremely rare.
Venting basicsHere are a few considerations to provide a proper and safe venting system for Category 1 appliances:
The pre-1992 draft hood appliances discharged flue gas into the masonry chimneys at temperatures in the 5008F range. Even at these temperatures, condensing of the flue gas was a common occurrence. It is worth noting that the new mid-efficiency appliances are discharging flue gas into these same chimneys at temperatures 2008F lower than their draft hood predecessors. The condensation mixed with existing chimney contaminants result in the formation of destructive acids. This effect is further compounded by condensation migrating through the masonry chimneys, contributing to the freeze-thaw cycle. When exposed to these elements, the demise of masonry chimneys is purely a matter of time. A few regions of the United States and Canada recognized the destructive results of the flue gas condensation forming within the masonry chimneys. They sought manufacturing assistance to develop cost effective methods of downsizing these grossly oversized heat sponges. By 1980, a cost effective, easily installed, flexible vent system was available.
According to code and manufacturers' instructions, under no circumstances are we allowed to vent a gas appliance into an unlined chimney. However, a tile-lined masonry chimney can be used as a vent provided:
- All four walls of the chimney are within the building structure OR all requirements in chapter 10 and table 10 of the NFPA 54 are met.
- The chimney has a properly sized clay tile liner.
- The chimney is constructed in accordance with the NFPA 211 standard.
Every enclosed masonry chimney eventually emerges through the dwelling roof and becomes exposed to cold outdoor air and the resultant ravaging effects of flue gas condensation.
Clay tile liners are installed by brick masons with no history or experience in the sizing of a clay tile liner for gas fired appliances. These liners were designed as a vent medium for much hotter, less efficient heating apparatuses than we enjoy today. They make a very inadequate substitute for UL listed systems designed for the sole purpose of venting gas appliances.
If masonry chimneys were built in accordance with NFPA 211, they would at least stand a fighting chance. It may be there are a handful of properly built chimneys in all of North America, but it's taking a real risk to be the one installing a vent connector into one of these few.
While it may be considered very capitalistic to suggest this, what ever happened to value-added sales by offering the best possible, complete and safe home heating system to the homeowner? With the foregoing in mind, it shouldn't take a rocket scientist to realize this trade has the option of selling the customer a product which meets this criteria, or having the brick masons pick up the sale after the chimney deteriorates. The bottom line: claims of eliminating the need for ever having to line a chimney again are increasing contractor liability and transferring this trade's profits into the hands of another. Provide your customers with a proven, reliable UL-listed vent system when venting any gas appliance into a masonry chimney.