Heed fur­nace tech­ni­cian’s vent­ing ad­vice

Winnipeg Free Press - Section F - - HOMES - ARI MARANTZ

QUES­TION: I have a mid-ef­fi­ciency gas fur­nace, which has a hor­i­zon­tal flue pipe that con­nects up to the chim­ney on a bit of a down­ward slope, from the fur­nace to the chim­ney pipe, of about and inch-and-a-half over a five-foot length. The tech­ni­cian says this is con­trary to code — it should be up­wards, as heat rises. He says I should ei­ther redo the flue con­nec­tions by bring­ing it down, which will im­pede head­room, or buy a new high-ef­fi­ciency fur­nace that does away with the pip­ing.

The fur­nace works well and is 12 years’ old, so it has five to 10 years left in it and I haven’t had a prob­lem for 10 years. Is there a real safety is­sue, or block­age more likely, be­cause of the down­ward slope? Or is the up­ward­slop­ing flue more an is­sue with oil or wood, etc, which are more likely to block up? Thanks for your help. Fer­nando Cininni

AN­SWER: The im­por­tant thing to re­mem­ber is that the tech­ni­cian who came to your home should be a well trained, li­censed gas-fit­ter, and heed­ing his ad­vice is in your best in­ter­est. While the rec­om­men­da­tions for flue mod­i­fi­ca­tions on your fur­nace may be for im­proved safety, they may also im­prove the per­for­mance of the fur­nace.

There should be no need to up­grade your mid-ef­fi­cient nat­u­ral gas fur­nace if it’s in good work­ing con­di­tion and has no ma­jor de­fects or worn out com­po­nents. Mod­i­fi­ca­tion of the vent to im­prove the an­gle or rise to your chim­ney should not be a ma­jor re­pair and should not be costly. It may re­quire pur­chas­ing some new vent pip­ing, and low­er­ing the ex­ist­ing pipe may cre­ate a slight in­con­ve­nience. But even if you have higher-qual­ity, dou­ble-walled B-vent, re­plac­ing a few com­po­nents should not be too dif­fi­cult.

It’s true that a proper vent rise was more of a con­cern for proper re­moval of com­bus­tion prod­ucts in older nat­u­ral-draft fur­naces. But it may be just as im­por­tant with your fur­nace, for dif­fer­ent rea­sons. Newer fur­naces like yours have an ex­haust fan that forces the cooler flue gasses up the chim­ney, so con­vec­tion is not as crit­i­cal as be­fore to achieve proper draft.

But this small fan may have to work much harder if the flue is ex­ces­sive in length or runs a down­ward an­gle be­fore it reaches the chim­ney. For this rea­son alone, it makes sense to raise the vent pip­ing to achieve a slight pos­i­tive slope rather than a neg­a­tive one.

One of the most com­mon is­sues with switch­ing from an older fur­nace with a vent hood to a mid-ef­fi­cient in­duced­draft model is the size and lo­ca­tion of the orig­i­nal chim­ney. If the chim­ney is too high, too far from the fur­nace, or has too large a liner, the ex­haust prod­ucts may not be able to es­cape the top of the chim­ney be­fore they cool ex­ces­sively

When this hap­pens, con­den­sa­tion is al­most cer­tain to oc­cur. In re­ally cold weather, this con­den­sa­tion will freeze and may cre­ate sig­nif­i­cant amounts of frost or ice. If this oc­curs within the chim­ney or flue, par­tial block­age can oc­cur, which can be a safety con­cern. When ice forms just as the ex­haust is leav­ing the top of the chim­ney, it can cause large ici­cles or dam­age to the chim­ney.

The so­lu­tion to this com­mon is­sue is of­ten to re­place the en­tire vent and the chim­ney liner with a dou­ble-walled Bvent, rather than a sin­gle-walled pipe. This cre­ates an­other layer of air around the warm flue gasses and acts like in­su­la­tion, so it may keep the ex­haust warm enough to pre­vent con­den­sa­tion be­fore it ex­its the chim­ney.

Hav­ing a neg­a­tive slope to the ini­tial sec­tion of the flue, no mat­ter what type of pip­ing, in­creases the chance of con­den­sa­tion be­cause it adds an­other im­ped­i­ment to a smooth flow of flue gasses. Any­thing that slows down the flow of these prod­ucts of com­bus­tion will in­crease the chance of con­den­sa­tion within the chim­ney.

The other big rea­son to cre­ate an im­proved slope of the vent pipe to the chim­ney is for mois­ture. If a small amount of con­den­sa­tion oc­curs, which is likely, this mois­ture should be al­lowed to run back down to the fur­nace where it will evap­o­rate when the fur­nace fires again. But, if there is a dip or low point in the vent­ing, this con­den­sa­tion can pool, and cor­ro­sion and dam­age to the metal vent can re­sult.

In some cases, this mois­ture can leak out on to the base­ment floor or sim­ply rust the flue pip­ing un­til a hole or crack de­vel­ops. Ei­ther way, there may be re­pairs re­quired or a safety con­cern when this hap­pens, as prod­ucts of com­bus­tion may en­ter the home.

While the neg­a­tive slope of the vent pip­ing be­tween your fur­nace and chim­ney may in­deed be a vi­o­la­tion of the build­ing codes, fix­ing the is­sue to im­prove the per­for­mance and safety of your fur­nace is the real is­sue.

I ap­plaud the heat­ing tech­ni­cian who pointed this out, pre­sum­ably dur­ing a reg­u­lar ser­vic­ing of your fur­nace. Oth­ers might have sim­ply ig­nored this de­fect, cleaned the fur­nace and left with­out pro­vid­ing you with this good ad­vice. I’d ask for the same tech­ni­cian when get­ting your vent pip­ing re­paired. Ari Marantz is the owner of Trained Eye Home In­spec­tion Ltd. and the Pres­i­dent of the Cana­dian As­so­ci­a­tion of Home & Prop­erty In­spec­tors— Man­i­toba (www.cahpi.mb.ca). Ques­tions can be e-mailed to the ad­dress be­low. Ari can be reached at (204) 291-5358 or check out his web­site at


Gas fur­naces should be ser­viced by a qual­i­fied ser­vice com­pany ev­ery two years,

says CMHC.

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