Why does the metal con­nec­tor be­tween our gas fur­nace and the chim­ney keep rot­ting?

Popular Mechanics (South Africa) - - Skills -

I’m go­ing to guess that you have a newer fur­nace. If that’s true, you’re prob­a­bly get­ting con­den­sa­tion in the flue. Older fur­naces didn’t have that is­sue. The hot flue gas they pro­duced lit­er­ally shot out of the chim­ney – too hot and mov­ing too quickly to con­dense. Al­though that method kept the flue clean, there was a glar­ing prob­lem: all of that hot gas was wasted en­ergy. So fur­nace man­u­fac­tur­ers, obey­ing man­dated fuel-ef­fi­ciency stan­dards, now build ap­pli­ances that use far less fuel. This re­sults in a smaller vol­ume of ex­haust at a cooler tem­per­a­ture, giv­ing the wa­ter vapour formed in the com­bus­tion process a greater chance of con­dens­ing in the flue. If a chim­ney is on the end of the house, with ex­po­sure to cold air on three sides, the prob­lem is even worse. The re­sult­ing con­den­sate can erode the chim­ney flue, rot the metal con­nec­tor, and even dam­age the fur­nace.

“Di­ag­no­sis re­quires a com­pe­tent con­trac­tor to check for con­den­sa­tion, com­bus­tion ef­fi­ciency and ad­e­quate com­bus­tion air,” says HVAC ex­pert Pat Porzio. If the fur­nace is de­signed for di­rect vent­ing, the so­lu­tion may be to by­pass the chim­ney, either by vent­ing out the side­wall or through an­other roof vent. In some cases, the con­trac­tor can in­stall a power vent – a blower to move the ex­haust gas out be­fore con­den­sa­tion can form. An in­su­lated chim­ney-flue liner may solve the prob­lem, too. Once a part works loose, cre­at­ing an airspace be­tween it and its mount­ing sur­face, the loose­ness acts as a force mul­ti­plier. You tighten it and it loosens even more. The cy­cle re­peats un­til the part comes com­pletely un­done or breaks. The mo­ral? Tighten some­thing as soon as it comes loose, oth­er­wise it will only get worse.

When tight­en­ing, check if the threads are stripped, if any­thing was over- or un­der-tight­ened, or if the wrong size or type of fas­tener was used. There’s al­most al­ways a good rea­son that tight things be­come loose.

The first thing to do in your re­pair is en­sure that the mount­ing screws for the brack­ets are se­curely in place. You may be tempted to try to fix a stripped-out hole by in­sert­ing a piece of cop­per wire or a glue-cov­ered golf tee, but this is not the place to do it. You need to reach a solid fas­ten­ing point. If that doesn’t work, you’ll need a thicker and larger mount­ing bracket. You might even sneak in one struc­tural screw (a hy­brid fas­tener, some­where be­tween a lag screw and a wood screw) at each bracket lo­ca­tion. Even if you have to slightly drill out the screw­mount­ing hole to ac­cept the struc­tural screw, it’d be worth it to fix this prob­lem for good. The prob­lem is saponi­fi­ca­tion. The primer and paint you ap­plied 30 years ago weath­ered away and let in mois­ture, which formed zinc ox­ide on the door’s sur­face. That ox­ide com­bines with oils that leach out of any new lay­ers of paint you ap­ply to cre­ate a crude form of soap. When you add more paint to it, the soap at­tacks the paint film and loosens it. Plus, when the Sun beats down on the damp door, any mois­ture un­der the paint turns into vapour and forces its way through the top­coat.

The so­lu­tion is to strip as much of the paint as pos­si­ble. Sand smooth any rough edges where the paint blis­tered and was scraped or brushed off. Next, ap­ply a “prep and etch” prod­uct, which is a di­luted form of phos­phoric acid. You brush it on, let it set for 15 to 30 min­utes, rinse thor­oughly, and then let the door dry. As soon as pos­si­ble af­ter that, prime the door with a high-per­for­mance 100 per cent acrylic primer for­mu­lated for sur­faces that might have an ad­he­sion prob­lem, such as vinyls, plas­tics and gal­vanised met­als. Fol­low that with two coats of ex­te­rior acrylic top­coat. PM

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