Heed furnace technician’s venting advice
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,