BK 40, and thimble installation Questions - Masonry Chimney

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kai1414

Member
Sep 12, 2011
7
Maine
I just removed a homemade monster of a wood stove from the basement of a new-to-me home. Ran the woodstove all last winter, but it was an all or nothing type of affair and horribly inefficient. I'm going to be putting a BK40 in its place.

The install is in the basement on a concrete floor, with plenty of clearance to any combustibles. The connection will be to a masonry(concrete block) clay lined chimney, 7"x10" clay liner (liner has been looked at and checks out fine; chimney is nice and clean). I have visually verified that the chimney has at least 2" to all combustibles where it passes through the house. The chimney is about 31 feet from the cleanout to the chimney top, and right around 27' from the existing 8" thimble in the masonry chimney to the top. With the ash drawer base on it, I don't have the minimum 2ft of vertical rise let alone the recommended 3ft. I have around 18" of vertical rise from the stove to the thimble.

My plan was to install a new thimble right around the 36" mark (plenty of room and clearance to do it.) and I wanted to run double wall 8" stove pipe from the BK to the thimble.

I've seen some threads that suggest with the taller chimney pulling more draft, the min. 2ft might not be as critical. Any logic to that?

Would it make the most sense to install a new 8" clay thimble and some type of adapter to accept DSP? Or is there some type of DSP insulated thimble that installs directly into the concrete block masonry chimney and gets sealed to the clay liner?

I'm also trying to consider how to make the installation the most flexible in the event that I need/want to install a liner at a later date. I don't want to have to reinvent the wheel and would want to utilize whatever I have for a thimble. If I had to do a liner, I'd be looking at a 6.5 x 9.4" oval liner to fit the existing flue and as closely match 8" pipe cross sectional area.
 

stoveliker

Minister of Fire
Nov 17, 2019
6,228
Long Island NY
The chimney height is recommended to have sufficient draft. With such a tall chimney, you may already have too much draft, meaning the initial rise (2-3 ft) is not that important.
For a short vertical rise before an elbow, you may have to use a heat gun for a minute or two up through the bypass before you light off, depending on how long the horizontal run is. (Shorter is better.) If the horizontal run is long, then using 45 deg elbows is better.

Yes, double wall pipe inside. Get a key damper too, given your stack height.

Also, if that chimney is outside, it'll cool down the flue gasses a LOT. And they are already cool to start with (b/c efficient stove - not much heat pumped out the flue). That's why an insulated liner is very advisable.

Note that a liner will also be eligible for the tax credit (if in the same order as the stove, I believe..?). So it's best to get that liner now too.
 

kai1414

Member
Sep 12, 2011
7
Maine
The chimney height is recommended to have sufficient draft. With such a tall chimney, you may already have too much draft, meaning the initial rise (2-3 ft) is not that important.
For a short vertical rise before an elbow, you may have to use a heat gun for a minute or two up through the bypass before you light off, depending on how long the horizontal run is. (Shorter is better.) If the horizontal run is long, then using 45 deg elbows is better.

Yes, double wall pipe inside. Get a key damper too, given your stack height.

Also, if that chimney is outside, it'll cool down the flue gasses a LOT. And they are already cool to start with (b/c efficient stove - not much heat pumped out the flue). That's why an insulated liner is very advisable.

Note that a liner will also be eligible for the tax credit (if in the same order as the stove, I believe..?). So it's best to get that liner now too.
Appreciate the advice. The chimney is an interior chimney. The horizontal run will be as short as the allowable installation clearance. I may be doing a 6 x 8.93" rectangular liner to just be done with it and not worry about future issues. I'll look into adding a key damper. Unfortunately I don't think I have room in the existing flue to run an insulated liner, but where the chimney is an interior one, I think that will be a little less critical than if it was an exterior chimney (at least, that's what I tell myself). I had considered breaking out the clay liner to have room for an insulated liner, but I don't want to risk damaging the adjacent clay flue for the backup oil furnace which is is good shape.
 

stoveliker

Minister of Fire
Nov 17, 2019
6,228
Long Island NY
An internal chimney is not much better in terms of the need for an insulated liner than an external one.

Consider the flue gas temps versus the temps outside the flue..
400 F inside and either 70 outside (for an inside chimney) or 20 outside (for an outside chimney) . Not a big difference. 380 vs 330 F difference. Heat losses from the flue gases will be quite similar. (Even if the heat loss from the house will be different.)

Hence the advice for an insulated liner. Modern stoves put very little heat in the flue, i.e. they give most of the heat off to the home. That's what makes them efficient. It is therefore important to keep the little heat that's in the flue from escaping so that the flue stays warm enough to prevent condensation of creosote also near the top of the flue.
 

begreen

Mooderator
Staff member
Nov 18, 2005
96,346
South Puget Sound, WA
The main reason for insulating is safety. It halts the transfer of heat through the brick to the surrounding framing that may be touching the chimney. If the interior chimney structure has at least 2" clearance from any combustibles all the way up, then insulation is not required.
 

stoveliker

Minister of Fire
Nov 17, 2019
6,228
Long Island NY
That is true for many stoves. For the BK it is not only that. Yes, it is needed for safety. But the argument of the OP was that it may not be needed because it's an inside chimney. My argument was that that is faulty reasoning.

So for the BK it is truly both reasons for which the insulated liner is needed; safety and flue temps.
 
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logfarmer

Feeling the Heat
Oct 25, 2015
256
Ohio
My king only has a rise of about 18” and 18’ from stove top to chimney top and drafts great even at 60* outside temp. Your setup might suck the log right out of your hand, should draft well! But if you want to line the chimney of course that’s a good choice too.
 

BKVP

Minister of Fire
There is safety and there is performance. I tend not to want to spend money when it's not needed. However, insulated liners do in fact provide a margin of protection not afforded masonry chimneys. The OP had the chimney clay liner inspected.

Keep in mind, that brick and clay liners are also a heat sink. Being able to maintain sufficient draft is necessary for:

1) Complete Combustion-If a chimney has insufficient draft you may find pieces of wood that are not fully burnt at the end of the burn cycle

2) Combustor Operation-With optimal draft, combustors will stay active when the stove is operated in the lowest desirable heat output range. If you stall, the fire can go out. Optimal draft will also allow for higher top end Btu production.

If it were me, I'd drop the insulated liner. But try it without and if you do not have any performance issues, you just saved some money. But it doesn't mean you are getting the full potential performance of the heater.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
30,184
central pa
The number of calls we get after installing an insulated liner saying I never knew how bad my stove was running before is impressive. They make a huge difference
 
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BKVP

Minister of Fire
The number of calls we get after installing an insulated liner saying I never knew how bad my stove was running before is impressive. They make a huge difference
I wish I could hit the "Like" Button 50 times on this!
 
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neverstop

Burning Hunk
Oct 11, 2020
213
new hampshire
BK calls for an 8" diameter liner, that's ~50 sq in. 7x10" flue is 70 sq in. That means your flue is going to be oversized by 40%. I don't see any exclusions in the BK manual saying you can vent into a larger flue. They also strongly recommend that you insulate the liner. 31 ft of an uninsulated 40% oversized flue means there is a lot of distance and time for the gases to cool down on the way to the cap.
 

kai1414

Member
Sep 12, 2011
7
Maine
The main reason for insulating is safety. It halts the transfer of heat through the brick to the surrounding framing that may be touching the chimney. If the interior chimney structure has at least 2" clearance from any combustibles all the way up, then insulation is not required.
Understood on the safety aspect. The chimney has greater than 2" clearance from combustibles all the way up. I've verified it myself before travelling down this road.
 

kai1414

Member
Sep 12, 2011
7
Maine
I appreciate all of the feedback. I would very much like to have an insulated liner but did not pursue it because there are some roadblocks. With the added information I'm reconsidering if there is a way to do it.
1. Removing the existing tile is a non-starter for a variety of reasons that I won't dive into. It's just not viable.
2. I've taken more measurements of the interior of the flue and it's more accurately 6.5" x 10".

BK40 has an 8" outlet which is 50.27 sqin. The two liner sizes that have been recommended to me above have an area of 51.87sqin and 53.58 sqin. both of which are larger. That doesn't leave me with much wiggle room for an insulated liner, rectangular or otherwise. I believe my rectangular choices are 5.5" x 9.43 or 6" x 8.93. No matter what I do it will be a tight fit. I think it's just barely doable if the insulation is wrapped tightly and 1/4" thick.

Given that the chimney is fairly tall and should draft well, would it be feasible to go with a rectangular 5.5" x 8.56" which works out to 47.08 sqin? This is equivalent to a round flue that measures 7.875" (7 and7/8") diameter. I think losing an 1/8" off of the diameter is an acceptable tradeoff for being able to use an insulated liner. I'm confident that I could wrap that liner in 1/4" insulation and have it fit without much of a fight.
(For comparison sake, a 7" pipe is 38.48 sqin)

I ran an inspection camera through the chimney last night myself to examine the mortar joints and overall flue envelope to consider what I would be dealing with for a tight fit if I tried to insulate with one of the first two larger liner sizes. Overall it's not awful but there would be challenges. One or two spots where this is a bit of misalignment, but nothing severe.

In the end, I'm trying to find the best option for what I have to work with, and would rather only do this once if possible.
 
Last edited:

kai1414

Member
Sep 12, 2011
7
Maine
I fully get the clearance to combustible. However, this isn't about that point of view. Class A is typically exposed to cooler temperatures of attic or outside air. The question at hand, is what will the temperatures be inside the space between the old venting and the Class A? That is what has never been tested. The 2" you mentioned is not based upon this scenario. It could be the temps are so high it requires much greater clearances. Then, the next issue is will the Class A mfg warranty their product against failure given the potential for excessive temps.

BKVP
Appreciate the response. Perhaps I am missing something? Or perhaps the responses weren't about my particular situation, but more about insulation requirements vs non insualted?

Clay liner is intact, could be used as is (other than its larger and taller than optimum for the stove). I am choosing to add a liner now as a matter of preference and home age. The existing masonry chimney with the clay liner has the required 2"+clearances to combustibles (internal chimney) as it's been explained to me, and I've inspected it and verified it myself.

I had initially expressed my intention to go unlined due to some existing restrictions, but have listened to the input and am trying to determine how I can add an insulated liner with my existing situation (see post above).
 

BKVP

Minister of Fire
Appreciate the response. Perhaps I am missing something? Or perhaps the responses weren't about my particular situation, but more about insulation requirements vs non insualted?

Clay liner is intact, could be used as is (other than its larger and taller than optimum for the stove). I am choosing to add a liner now as a matter of preference and home age. The existing masonry chimney with the clay liner has the required 2"+clearances to combustibles (internal chimney) as it's been explained to me, and I've inspected it and verified it myself.

I had initially expressed my intention to go unlined due to some existing restrictions, but have listened to the input and am trying to determine how I can add an insulated liner with my existing situation (see post above).
cross-threaded..apologies!
 
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kai1414

Member
Sep 12, 2011
7
Maine
Ended up installing a rectangular insulated liner with same internal area as an 8" round. Tight squeeze but it's in. Thimble relocated to 24" above the stove. Couldn't achieve the 36" due to some existing ducting passing through.
 
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kennyp2339

Minister of Fire
Feb 16, 2014
6,702
07462
Honestly the truth here it seems is the OP wont know performance until after the stove is installed, best advice is to get a manometer and test the draft running the stove on high, if your .05"wc then your good to go, if your off the chart then you need a damper to slow things down, if your under.. you'll know because you'll have smoke leakage whenever you open the loading door.