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Q. I recently had a chimney sweep clean my furnace chimney for the first time in about eight or nine years. Originally we had an oil furnace, which created a lot of soot, but about 10 years ago we switched to natural gas which creates a lot of water in t he flue gas.
When the metal duct from the furnace through the wall to the chimney was disconnected, the opening at the bottom of the chimney was about half blocked with a tannish grit which had fallen down the inside of the chimney over the years.
The chimney sweep attributed this to acid from the soot combined with the water in the flue gas attacking the ceramic chimney liner. He proposed installing a flexible stainless steel liner, but said it should be a seven inch diameter liner for my furnac e BTU output and that he would have to send a rotating weight device down the chimney to break up the ceramic liner to make the passage large enough for the stainless steel liner.
I'm not comfortable with this because it seems like breaking up the old ceramic liner could cause damage to the brick chimney and I'm concerned about the durability of a stainless steel liner that's a thin enough gage to be flexible. Also, I'm wondering if he was just trying to sell me something I don't really need. What do you think?
A. I agree with the chimney sweep’s diagnosis as to what probably caused the chimney liner to deteriorate and I’m not concerned about the proposed stainless steel liner’s durability. That will be fine. I would, however, consult with the manufacturer of t he gas furnace as to flue size requirements for the model furnace you have. Manufacturer’s specifications not only should be followed but become the building code for that installation.
Modern, more efficient furnaces do not require the flue sizes required for the older less efficient furnaces. In fact, most modern furnaces have fans-- draft inducers-- to pull the flue gasses from the furnace and propel them out of the building. Often installers will attach a new device (gas) to a flue designed for another fuel source (oil) and problems arise, as you have seen.
It's quite possible that the flue size requirement for the gas device you have is less than what you have and breaking out the old terra-cotta liner may not be needed. Lining the old chimney with a stainless steel liner is a good idea and if there is roo m they can insulate around the outside of the stainless steel liner as it goes up through the old masonry chimney and that helps keep the flue gasses warm and buoyant as they exit the house.
I was called to an office building where the workers in one section of the building were complaining of health problems that only arose while they were in the building. I went to the basement and found a brand new high efficiency gas boiler. The flue was attached to a nearly three storey brick chimney that ran up the outside of the building to well over the roof. It was two feet square of masonry with an eight by eight terra-cotta flue liner. When the masonry got cold the flue gasses cooled and would n ot vent naturally out of the chimney and stacked up and fell back into the building, sickening the inhabitants. When we corrected this with proper flue gas venting the symptoms of the workers went away. Just be sure that the venting size you end up havin g is suited-- and not too big or small-- for the furnace you have.
You were wise to have that chimney cleaned and checked. It’s something that frequently gets overlooked with heating system maintenance and mismatched flue venting requirements is a common defect in older structures where modern fuel burners are attached to older chimneys. Not to mention the odd bird’s nest or deceased squirrel that can fall into and block a furnace flue or chimney creating a blockage and a potential carbon monoxide poisoning condition to set up inside the house.