Wood Stove install - Through wall to clay flue thimble

[ZScott]

Hello -

I am planning on installing a Jøtul F 55 V2 Carrabassett in the basement of my home. My primary question is regarding the correct way (and part names) to connect my new stove to an existing clay flue which connects to my existing masonry chimney. I'd like to use double wall stove pipe.

A little background: The home was built in 1984, and we recently took possession this year (so I have no information on original construction, etc.). I recently discovered during a cleaning/inspection of my fireplace & chimney that the existing chimney (currently serves the fireplace) has a second flue. After a opening the basement wall (and following some original builder pencil outlines/markings) I was able to locate a clay thimble connecting to the secondary flue in my chimney. This flue was very obviously never used, and there are no existing connections. I would ideally like to connect to this flue when installing my new stove. I'm in the process of renovating the basement to build a hearth and clear the space needed, my only confusion is the actual connection to the flue. I can find a lot of information on passing through a wall, as well as passing through masonry walls to connect to the chimney, but not this combination that I have of a combustible wall to a masonry chimney.

The framed off opening measures about 17.25" x 17.25". The clearance from the thimble to the frame varies between 4.5" and 5.5" (all under 6"). The clay thimble itself is just over 8" Inner Diameter and measures approximately 17 7/8" in length. The frame is standard 2x4 studs, and there is what seems to be a styrofoam panel between the studs and the cinderblock base of the chimney (which is what I cut through to cut out the clay thimble). The chimney does not have any modern/metal liners; from running my phone camera inside I can see what appears to be the clay flue/liner all the way up the chimney.

A secondary observation that I have is that I cannot locate any cleanout doors to the chimney (unless those are covered as well), and I'm not sure if that will be an issue when I proceed with the installation of the wood stove. I can continue to check for cleanout doors, but that would require cutting out that styrofoam panel on the inside of the basement, or tearing off stone veneer on the outside of the chimney (and I'm doubtful they would have veneered over a cleanout door).

I also noticed that the base of the secondary flue terminates at the cinderblock foundation of the chimney, and the tops of the cinderblocks open (i.e.: debris, soot, etc., can just fall into the block below. I'm not sure if this was an oversite and should be closed off, or if this is either non-factor or an intentional design.

Any help is greatly appreciated. Apologies for any miss-used terminology, I'm new to most of this.

Thanks.
-Zach

 

[begreen]

Welcome. It's good you have asked these questions. The thimble does not have proper clearances. The stove manual state's what is required regarding this type of thimble for a wall pass-thru. There may be a cleanout (or not) below this thimble.


What is the clay liner ID in this chimney? If the chimney doesn't have proper clearances from combustibles, the safest approach is to install an insulated 6" stainless liner in the chimney with an insulated thimble passing through the wall.

 

[ZScott]

Thank you! I appreciate the speedy reply.

I did my best to take a rough measurement of the ID of the chimney. It is rectangular in shape and measures approximately 6.5" x 12.5" (give or take, I can only measure through the thimble right now, as I cannot reach the chimney top).

If I'm reading the diagram correctly, the thimble should have a 12" clearance from the studs on all sides. Is that correct? If so, is there any type of connection to reduce that distance? If not, I presume the best option then is to reframe the thimble to give that 12" minimum clearance. If I did that, it looks like the requirement is also that that opening is lined with some type of masonry/brick surrounding the thimble?

I need to renovate that corner anyway (I'm also planning on putting up a stone veneer wall), so I'm completely open to reframing if needed.

And once I have the correct dimensions, I'm still a little confused on what the correct adapter is to allow me to connect to the thimble. I presume there is some type of adapter that would feed into that thimble, to which I would then hook up the chimney pipe?

Edit: I forgot to mention that the chimney is constructed of come type of cement block, in case it is relevant. The base is definitely your standard cinderblock, as I can see that looking down through/inspecting from the outside. The distance measuring from the end of the thimble (the start of the inside of the chimney) to the wall studs is approximately 20".

Thank you in advance for the advice!

 

[begreen]

The stove needs a flue area of about 28 sq in. If the stove is connected to a 6.5" x 12.5" ID flue, that is almost a 3x increase to 81.5 sq in. This is strongly recommended against because it will slow down and cool the exhaust gases from the stove. This can result in weak draft, a cranky stove, hard starts, etc. It has the potention for significant creosote accumulation. Referring again to the manual:

The cross-sectional area of the flue of a chimney with one or more walls exposed to the outside below the roofline may be no more than two times the cross-sectional area of the stove flue collar.

If the chimney liner is too large to accommodate the stove, an approved relining system must be installed to resize the flue.

 

[ZScott]

Thank you, this makes more sense now. What was misleading me was in section 2.1:
The fireclay flue liner must have a nominal size of 8” X 8”, and should not be larger than 8”X 12”. A round fireclay liner must have a minimum inside diameter of 6”and maximum inside diameter of 8”.A larger chimney should be relined with an appropriate code approved liner

I'm presuming my flue actually measures around 6"x12", which was falling into the aforementioned requirements. However, I missed the following section regarding the exposed walls/cross-sectional requirements. So based on this, my rectangular flue definitely requires a liner per the manufacture specifications.

I will need to coordinate the installation of the liner with other work I am having done soon.

I have one last question regarding "Figure 5. Masonry Wall Pass-through" in the manual (your initial reference). I read through that section and cannot determine for certain if I need to line that opening with brick as the manual states. The reason I am questioning this is that the chimney sits against the foundation in the basement. The cinderblocks used in the foundation have a width of 12", leaving 6" (given the 18" thimble) for the wall of the chimney and the chimney liner. They ran the thimble flush with the inner wall of the foundation.

The manual states:
The bricks must be solid brick with a minimum of 3 inches thick (nominal 4”).Refractory mortar must be used at the junction of the chimney and the pass-through liner. The pass-through liner must not penetrate the chimney liner beyond the inner surface of the chimney liner.

It seems like this installation meets all of these requirements, the only difference being there may not be an air gap between the chimney and the foundation wall, but there is and air gap between the studs and the foundation wall. If this does fulfill the requirements, I would assume all I need to do to bring this portion up to code is to expand the distance from thimble to studs to a minimum of 12".

So my questions are:

1. Do I still need to line the framed opening with brick?
2. If so, do I then need to extend the thimble to be flush with the brick (as in the diagram)?

Thanks again for all of the help.

 

[begreen]

They make insulated pass-thrus for this purpose. This article discusses some options :

Chimney info Thread 'CHIMNEY - PASSING A CHIMNEY THROUGH THE WALL'

Oct 17, 2015

DON’T Just “Do it” - Placing a wood stove into an existing masonry chimney could be hazardous to your health and house. Read this to learn more.

One of our biggest problems occurs when a customer enters the shop and states “I have an existing woodstove into a masonry chimney - I’d like to replace the stove with a more modern one”.

In the old days, we thought this was a gravy job…these days I wonder if I should send them packing….why? because:

1. 90%+ or more of existing masonry wall-pass throughs are wrong

2. It is often difficult confirming or repairing these...

• webfish
• chimney
• Replies: 0
• Forum: Articles

• 

Insulated Chimney Thimble for Thimble and Wall Pass Thru Installations - Rockford Chimney

An Insulated Chimney Thimble protects the wall from overheating where the flue pipe passes through. Includes installation instructions. Free shipping.

www.rockfordchimneysupply.com

 

[ZScott]

Thank you for all of your help. This all makes a lot more sense now, and I definitely would have missed the flue size issue if you had not pointed it out. That article (and the reference link to the insulated wall pass thru) is extremely helpful (and my existing pass thru proves the point that most are incorrect and a hazard...). I'll weigh my options from here, fortunately they are pretty open given my remodel plans. Glad this was caught early, and thanks again!