I have a 1955 farm home which has a chimney currently being used to exhaust an oil furnace. The furnace is in the basement, and on the main floor there was a thimble extending out on each side of the chimney. When I had a chimney expert out to inspect the chimneys, he suggested I brick/mortar one side up and place a thimble cover plate on the other side. In case the next owners move away from the oil furnace and want to put in a wood burning stove. So I followed his advice and blocked in one side and mortared it up. The side that still has an accessible thimble is in the kitchen and it currently has an original thimble cover plate (tension rods on back that slip into thimble) on it that was hand painted. I also have carbon monoxide detectors which have never gone off.

Problem: home inspector said I have to remove the cover plate and seal the thimble to prevent carbon monoxide leaks.

I reached out to the chimney guy that I originally worked with and he said that he knows of no building code regulations that say how to seal the thimble so the thimble plate w silicone caulk should be fine or to just brick/mortar the hole (which I don't want to do).

Question: can I use a high temp silicone caulk around the thimble cover plate so it's not permanent like brick/mortar but still air tight. Is there other methods of sealing a thimble so carbon monoxide doesn't escape but aren't permanent?

  • Is the "home inspector" also a certified "code inspector"? Only your local "code inspector", or a licensed engineer, is authorized to tell you whether your method of sealing is "approved".
    – Upnorth
    Nov 29, 2017 at 18:41

3 Answers 3


I think you can but I also think you are going to loose this fight. Do what the inspector says. Life will go easier for you. If in the future, someone wants to install a wood stove in that spot, it is not that hard to re-chink a hole. (BTW the oil furnace probably should be vented through a chimney liner. And any future wood stove will probable be required to have its own flue-way.)

  • In my experience, the term "home inspector" generally refers to a person hired by a prospective buyer of a home. He or she isn't typically required or qualified (or authorized) to tell what is or isn't "code". Yes, most fire codes do not allow installation of a solid-fueled appliance in the same flue as those using any other type of fuel.
    – Upnorth
    Nov 29, 2017 at 18:59

The "seal" you mention (like a pie-plate and a coat hanger) is quite common in old houses, where coal-fired parlor stoves may have been installed in nearly every room. You're lucky yours were not also plastered over and wallpapered, creating a potentially deadly situation (fire and CO venting).

Under some building codes, an "approved method of non-combustible sealing" is required, without further specificity. Your chimney expert should reference NFPA 211 "Standard for Chimneys, Fireplaces, Vents, and Solid Fuel–Burning Appliances", or similar code enacted in your locale, for more guidance.

Consider the fact that a proper masonry chimney is presumed to be able to safely endure extremely high temperatures for short intervals, e.g., a chimney fire lasting a few minutes at 1,800 degrees F, without necessarily burning down your house. Imagine how long your "pie-plate" seal will last at that temperature. Also worth noting that CO can penetrate brick, plaster, concrete and gypsum, over time, although most of it SHOULD vent with the vertical convection up an unobstructed flue.

Our local fire inspector has approved sealing a masonry flue opening by use of 4-inch fire-clay bricks and refractory mortar, although it may technically leave a "defect" in the masonry liner of the chimney. They also approved a properly installed metal cap, listed for the purpose, on a proper connector pipe in a metal "safety thimble" having the required clearance from combustibles (e.g., listed for 2-inch clearance from combustible structure) and installed according to the manufacturer's instructions. For instance, twist-on lock-seal, with non-combustible sealant, minimum of three screws holding it to the abandoned pipe and permanently marked as not available for future use by any solid-fuel appliances.

They have also required approved installation of a completely new, UL-listed, metal liner for an oil-fired boiler in a three-story wooden school-house that was built in the 19th century. That way we can be more certain that there aren't any "gaps" hidden in potentially dangerous places.


"home inspectors" actually have zero authority and almost zero knowledge about anything. Most of the time the furnace flue is separate from any other flues in the house. At least your oil burner technician should have made sure of that like 30 years ago. You would be better off having an oil tech or certified chimney person (WETT certified in my parts) have a look. There is a very good chance of it being completely separate. Besides chimneys draw constantly. There is little to no chance of anything coming out of it let alone CO.

  • Not all chimneys "draw constantly", especially while cold. During a cold start, there may be a large plug of cold air in a tall chimney that prevents it from drawing initially -- until the warm exhaust convects its way through. A proper installation may adjusted to "burn hotter" for a tall chimney, especially one not enclosed by the building.
    – Upnorth
    Feb 13, 2020 at 18:59

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