RMH for Dummies! Please help guide me through my first build!

Hi Stacie;  
Looks good , you are getting the hang of cobbing.
Your metal supports are fine , they won't get in the way at all.
I would have brought your rabbit wire further up the side of the barrel, but with your supports you should be fine.
Yes , that is definitely an ugly barrel !  Can't complain about the cost though!  

And then I spoke to soon on the cob!  Straw cob is a different animal.
The thing looks like a hedgehog...is there supposed to be straw bits sticking out every which way?
I got the "straw" locally.  It looked decent outside, when I cut it opened it seems more a mix of coarse hay and straw, with some very long (18" +) strands. I thought I would try it since it is here and I bought it, BUT am wondering if I should pick up a bale of straw at the feed store. Thoughts?

Plus side, I did get the manifold covered!

You make a hedgehog first. Then you cover with a finishing layer of fine sand cob with no straw.

Actually, i think i would preffer hay to straw in cob. Note, i made cob twice in my life, and in small quantities.

Hi,
I’m enjoying following your build.
Do you have access to the last 4 DVD set permies made? The innovators gathering DVD shows Erica making a (very rough and not worth getting for just this one thing) square manifold. She brings in the last two courses back to an octagonal shape to support the barrel. I know it would be a bit if a job but you may want to consider redoing the last two courses to support the barrel properly.
Thanks
Dan

Ha Stacie;  
Yes ,straw cob looks sort of like that.  Your supposed to cut that straw down into short pieces 3-4" long.  Makes it a little easier to work with,but it is just another step in a long process.
Maybe you could enlist the older kiddos into cutting the straw for you.
I mean, after all this heater is for their room...
Your bale may be a mix of hay and straw, nothing wrong with that. A bale from the feed store will be all straw ... but it will still have lots of long strands as well. It will also cost 6-8 bucks ! That's crazy to a guy that remembers buck a bale days.! Keep what you have, cut it down. Your manifold looks great !
Besides your finish coat will cover all that up anyway.

On the subject of straw/hay and I have no experience of hay.
A local yoga retreat to me built a “straw bale studio” but they did not understand the difference between browns and greens. They used lucerne (alfalfa?) instead. All was well for a few months until the moisture from the cob activated the lucerne causing it to compost and the walls collapsed.
I don’t know if hay will do the same. I certainly would not use it in case. I’m going to try sugarcane as we have good access to it, it’s cheap and it’s very finely chopped compared to straw. Zaytuna farm, where I did my pdc, used sugarcane bales to build their buildings in leu of straw

Hi Dan;
Welcome to permies!
Straw being hollow is the preferred insulator to use in a cob rmh ,Hay will work as well, as it creates an air void.  Down under you would have a much better idea what is the best local product to use, sugar cane may be it.
Unlike a cob building , No worries with a cobbed rmh having any moisture left to compost anything. After 5-6 weeks any building moisture is evaporated away.

thomas rubino wrote:Hi Dan;
Welcome to permies!
Straw being hollow is the preferred insulator to use in a cob rmh ,Hay will work as well, as it creates an air void.  Down under you would have a much better idea what is the best local product to use, sugar cane may be it.
Unlike a cob building , No worries with a cobbed rmh having any moisture left to compost anything. After 5-6 weeks any building moisture is evaporated away.

Thanks for replying. I did suspect that would be the case with drying out as Ernie and Erica promote using dung (also a green). I may use horse manure as that is locally available for free.
Thanks
Dan

Daniel Hatfield II wrote:Hi,
I’m enjoying following your build.
Do you have access to the last 4 DVD set permies made? The innovators gathering DVD shows Erica making a (very rough and not worth getting for just this one thing) square manifold. She brings in the last two courses back to an octagonal shape to support the barrel. I know it would be a bit if a job but you may want to consider redoing the last two courses to support the barrel properly.
Thanks
Dan

Hi Dan!

 I do not have the videos.  I have caught snippets of some on you tube.  Probably should have invested and studied beforehand.  Hind site is my best teacher.
\
Did she have the barrel set inside the brick?  I am planning on snugging up a ton of cob around it and partially up  (I think 1/3 is acceptable).

Staci

thomas rubino wrote:Ha Stacie;  
Yes ,straw cob looks sort of like that.  Your supposed to cut that straw down into short pieces 3-4" long.  Makes it a little easier to work with,but it is just another step in a long process.
Maybe you could enlist the older kiddos into cutting the straw for you.
I mean, after all this heater is for their room...
Your bale may be a mix of hay and straw, nothing wrong with that. A bale from the feed store will be all straw ... but it will still have lots of long strands as well. It will also cost 6-8 bucks ! That's crazy to a guy that remembers buck a bale days.! Keep what you have, cut it down. Your manifold looks great !
Besides your finish coat will cover all that up anyway.

Thanks, Thomas!
I got these bales for $5.  Feed store straw is over $10 out here- crazy!  Quality hay is nearly $20...I remember it being $10 many moons ago when I had horses.
I can try to pick out the coarser pieces and start snipping.  A new one for the kids' chore list!

Hi Stacie;
Here's a couple photos of how my 55 barrel sits.
I didn't bother looking for long pieces , just grab a small bundle and snip... your bunny can use the chaf.
OMG $10 for a bale of straw !!! $20 for hay !!!   I knew there was a good reason to be inland !!!  It sure is beautiful over there and I love visiting the ocean .... BUT I wouldn't want/ couldn't afford  to live there  :)

thomas rubino wrote:Hi Stacie;
Here's a couple photos of how my 55 barrel sits.
)

 Move out from Ohio 20-some years ago. Definitely high cost of living. We live out in the sticks, but still...

Do you think I need to add more support to/around the barrel, or will cobbing the hell  out of it be okay?

From what I can see from the picture on page 3 , you have enough support to keep the barrel steady, that is the most important.  I might have used cut brick to plug the gaps before cobbing but no matter. You will be fine with a thick cob base thinning out as it rises up 1/3 or so of your barrel.

One thing about your build , or any rmh build.   Cob comes apart EASY ! Crumble it up add water and your good to build again.
I know you are building this as perfect as you can. At some point in the future you may want to change something (maybe your barrel or riser)  It will be a piece of cake!  Tap apart , rehydrate your cob , make your changes and mud it back together again. You do not need to source new cob !!!  How cool is that ! Try that with concrete :)
If you look at my photos you can see the imprint in the cob manifold where the back of the barrel sits .  That rebuild took a few hours on a warmer middle of winter day...  As I said... Its EASY.

Hi,
 Still trucking with the cob.  Cut straw works better.
I am almost through my fourth bag of fire clay-yikes.

1)How much straw do I add?

2) How much cob do I need on the manifold.  Is it about 5" all around?

3) (see pic) Is this the right torch?  I tried it yesterday and found that it did virtually nothing.  Any thoughts/suggestions?

4) Manifold Clean out: I did not think ahead on this, or read anything prior, so of course, I goofed. When I put in the pipe and set it in, I did not allow it to stick out far enough to leave room for thick cobbing AND lid (as lid overlap is about an inch).  There is bout 1" for cob.  I realize that this is not good.  Not sure how to approach this one.

Thank you.

Hi Stacie;
#1)  Well that's hard to say ,It's purpose is to create air pockets so depending how big a batch, a few handful's, not to much or it might not have enough structural strength to stay air pockets.  
#2) 5" is good, more would be better . And don't forget your adding a finish coat of a few inches as well. Lets say you can't have enough, but if you have a spot that needs to be thinner ... let it be thinner . You will know and keep an eye  on it for any potential issues.
#3) Well my bad there! :(   That is exactly the torch I suggested. Apparently baked on enamel requires more heat than that can produce.  I never tried to use one  as I have an oxy acetylene torch.
So choices might be , finding someone with an oxy torch as that Will turn it cherry red in seconds.  A  grinder with wire wheel would get most of it but is very dangerous. Trying it as is ??? I'm suggesting Not recommending here.
I do not know what temp that baked on paint will withstand. I do know that rim should not get hotter than 4-5 hundred or less . (Just temp checked mine at 225 with the barrel top at 600) Just a thought..
#4)   Just taper your cob down to the cap. Is your cap insulated ? It needs to be.  That cap is just going to be releasing a tad more heat than the manifold, its OK . You want that heat in the room anyway and it will not be so hot that a toddler or a berki clad brick layer might burn themselves on :)
#5) I guess i'm a glutton for punishment but curiosity has me ... how much is fireclay in your area ??
 
EDIT)   Just looking at your cap photo.   Add a handle to the middle of the cap then cob out flush to the cap.   When you need to inspect, just wiggle that cap it will pop right off , save crumbles add water ... reapply ... cob is wonderful stuff  

thomas rubino wrote:Hi Stacie;

#4)   Just taper your cob down to the cap. Is your cap insulated ? It needs to be.  That cap is just going to be releasing a tad more heat than the manifold, its OK . You want that heat in the room anyway and it will not be so hot that a toddler or a berki clad brick layer might burn themselves on :)
#5) I guess i'm a glutton for punishment but curiosity has me ... how much is fireclay in your area ??
 
EDIT)   Just looking at your cap photo.   Add a handle to the middle of the cap then cob out flush to the cap.   When you need to inspect, just wiggle that cap it will pop right off , save crumbles add water ... reapply ... cob is wonderful stuff  

Hi Thomas,

 50Lb bag fire clay runs about $8.

  I will pick up a couple of handles at the hardware store and dig out my husband's drill.   I need to get some tile grout anyways, for the heat shield.  Can I cut a section of rock wool insulation and shove it inside the cap? May also pick up some paint for the barrel (toxins aside).
I'll ruminate on the rim.
Thanks!
Staci

Hooray!   At least the cost of fire clay is in the ball park!
Anything will work for a handle, I used a shed deer antler. second hand store might be a good place to check as well.
Rock wool in the cap is perfect.
I used high heat engine paint on mine.  

Staci Kopcha wrote:

thomas rubino wrote:Hi Stacie;

#4)   Just taper your cob down to the cap. Is your cap insulated ? It needs to be.  That cap is just going to be releasing a tad more heat than the manifold, its OK . You want that heat in the room anyway and it will not be so hot that a toddler or a berki clad brick layer might burn themselves on :)
#5) I guess i'm a glutton for punishment but curiosity has me ... how much is fireclay in your area ??
 
EDIT)   Just looking at your cap photo.   Add a handle to the middle of the cap then cob out flush to the cap.   When you need to inspect, just wiggle that cap it will pop right off , save crumbles add water ... reapply ... cob is wonderful stuff  

Hi Thomas,

 50Lb bag fire clay runs about $8.

  I will pick up a couple of handles at the hardware store and dig out my husband's drill.   I need to get some tile grout anyways, for the heat shield.  Can I cut a section of rock wool insulation and shove it inside the cap? May also pick up some paint for the barrel (toxins aside).
I'll ruminate on the rim.
Thanks!
Staci

Hi Staci,

map pro may do the job where propane failed.
I use this gas for soldering plumbing fittings where propane is totally useless with the type of solder I use.
https://www.homedepot.com/p/Bernzomatic-14-1-oz-Map-Pro-Cylinder-332477/203226566

Thanks
Dan

  I will pick up a couple of handles at the hardware store and dig out my husband's drill.   I need to get some tile grout anyways, for the heat shield.  Can I cut a section of rock wool insulation and shove it inside the cap? May also pick up some paint for the barrel (toxins aside).
I'll ruminate on the rim.
Thanks!
Staci

Staci, I quoted the high heat paint and could not justify the cost, I estimated priming and painting the barrel to be 50 to 60 dollars.   I ended up using old coconut oil on mine.

Okay, got the heat shield up- another learning curve with mortar for large tiles. Even messier than cob.
Tested draft with a candle- looks good!
Have another (!) piece ordered of stove pipe adapter in order to complete the chimney hook up.

**Today:  MOLD growing all over the manifold.  This was my first layer of straw cob.  Do I just cob over and proceed as before? Should I suck it up and buy the feed store straw?
Right now, going to add another lasagna layer to the bench and then another of straw cob to the manifold.  Then I am out of clay...they are not open until Monday.

That's looking really good Stacie!  Even with the mold. Just carry on, that mold cant grow without air and SOON , that mass is going to start drying out and warming up...that mold doesn't stand a chance!

Life is one long learning curve, woe the day when we stop learning something new. It would take the point away.

Big difference with that tile grout is how hard it is on the hands,  clay is much more body friendly than concrete & lime.

Clay,  nice to use uniform dry clay.  BUT ... Your living in or near the Washington rain forest.  There really isn't a creek or a forest service road near by ? Clay should be free , you know maybe some shovel work involved but no dollars... I only use the fireclay when i'm mortaring.

Super awesome that the draft is flowing already !   This is a direct result of you following existing plans carefully.  A properly designed and built RMH will work as intended every time.  

Got more clay today.  (Unfortunately it is mostly glacial til here- very sandy, rocky.  Ground is filled with large rocks, referred to as "Puyallup Potatoes" (the name of the city).  Makes post hole digging difficult! There is clay around streams and such, but since we are in salmon country, there are strict laws.)

Putting another hedgehoggy layer over the manifold today.  It looks like by the time I am finished cobbing, the line from the barrel to the fuel feed will be flush.  Should have set the barrel further back.
Does the fuel feed need as much cob as the manifold?
There is rust on my barrel at every point the cob is touching it and whenever a spray of water gets on it.  Should have oiled it prior??

Shoulda, shoulda, shoulda....

Hi Stacie;
Lets start with easy.  Wipe an oily rag (cooking oil) on the areas getting rust spots.  Or paint it ...  but I would have done that already, before cobbing it in. So oil it is. Remember Eric's post about oil smoking off and do not use very much.

When you only have a twelve inch burn tunnel roof its hard not to have the barrel cob come up to the feed tube. Don't worry about it .
No the feed tube is cooler, not as much needed there.

Should have known there would be rules about the salmon.  Do you remember Charley Tuna ?   When I work at the Columbia river dams, we say its for "Seymour salmon" :)

If its not too daunting, stop in at the forest service station and ask if any roads have a clay bog and if you could legally fill a few 5 gal buckets up.Tell them what your building they just might be super interested in it, and more willing to help you.  
 If it were me I would just "go fishing" and fill a few buckets anyway ...but hey I'm  a rule breaker from way back...

Any foundation excavation going on in the neighborhood might have a free bucket load of clay for you.  If you see a track hoe stop and ask .

EDIT)  Time to start a finish coat at the core.

Hi, was sick in bed for two days. Attempting to get back to it today.
Today:  apply gasket to lid and seal barrel.

Thomas- you previously said to seal the small spout with "stove refractory"...on my way to the hardware store shortly to get the last of my chimney piece, so could pick that up as well.
Is it refractory cement? Mortar?  Could I use the gasket adhesive that came with the gasket kit?  Silicone?

Thanks!

For the base of the screw top, I added some stove gasket and also some rockwool insulation, as there was still paint.

I found some "high heat mortar" DAP, $4.99 rated to 2000 degrees.
"draft, smoke, fireblock".

Can I put this around the edges of the black stove pipe as well?
(inside the bench, I used foil tape)

Rutland stove cement. Same thing you would use on a regular box wood stove. Comes in a plastic tub of different sizes, the smallest one would be more than you will need.  (High heat DAP is fine)

Do not use the DAP on your pipes .   Use more foil tape.  The DAP will harden in your pipe and at the first small movement it will fall out.

It is perfect for sealing a cap you will never open again.

At risk of presenting myself to be even more dense, I am going to ask a chimney question.  (I have looked on you tube and google, etc.).
I have a single wall telescoping  stove pipe running from a 90 degree to the ceiling.  I thought I would have to have another bit of stove pipe, but now it fits.
My question is on securing the sections and then being air tight.  In the You Tube, it shows someone putting screws into the top ( pipe into the ceiling support box adapter).
There are no screw holes or screws, but I can manage to generate some...is this how it is done?
Is there a stronger attachment at the base to the elbow?  Just seems wobbly to me. And there is a small gap in the telescoping pipe.
Sorry, type A personality w/ no building experience...
Thank you .

Staci Kopcha wrote:At risk of presenting myself to be even more dense, I am going to ask a chimney question.  (I have looked on you tube and google, etc.).
I have a single wall telescoping  stove pipe running from a 90 degree to the ceiling.  I thought I would have to have another bit of stove pipe, but now it fits.
My question is on securing the sections and then being air tight.  In the You Tube, it shows someone putting screws into the top ( pipe into the ceiling support box adapter).
There are no screw holes or screws, but I can manage to generate some...is this how it is done?
Is there a stronger attachment at the base to the elbow?  Just seems wobbly to me. And there is a small gap in the telescoping pipe.
Sorry, type A personality w/ no building experience...
Thank you .

Hi Stacy,
Try youtubing a few chimney install vids. This is just normal chimney installation from here on. The flat thing is just secured onto the ceiling with drywall screws as it holds no weight. The ceiling box needs to be secured from above. It's triple walled at that point. You will need to get up into the attic. Get 2 pieces of RHS aluminium that span from one ceiling stud? *(i can't remember the name of this piece of wood right now) to another running snug against the outside wall of the stove pipe (either side). Secure the aluminium to the wood on both sides and put one screw in the outer wall of the stove pipe on either side. That will hold it up.

Honestly, at this point, it may be worth shelling out for a contractor to do this part. I've had 2 leaky roofs from pretty skilled people (that had not done many chimneys). It's a half day job.
You have done great to get this far alone.

Daniel Hatfield II wrote:

Staci Kopcha wrote:At risk of presenting myself to be even more dense, I am going to ask a chimney question.  (I have looked on you tube and google, etc.).
I have a single wall telescoping  stove pipe running from a 90 degree to the ceiling.  I thought I would have to have another bit of stove pipe, but now it fits.
My question is on securing the sections and then being air tight.  In the You Tube, it shows someone putting screws into the top ( pipe into the ceiling support box adapter).
There are no screw holes or screws, but I can manage to generate some...is this how it is done?
Is there a stronger attachment at the base to the elbow?  Just seems wobbly to me. And there is a small gap in the telescoping pipe.
Sorry, type A personality w/ no building experience...
Thank you .

Hi Stacy,
Try youtubing a few chimney install vids. This is just normal chimney installation from here on. The flat thing is just secured onto the ceiling with drywall screws as it holds no weight. The ceiling box needs to be secured from above. It's triple walled at that point. You will need to get up into the attic. Get 2 pieces of RHS aluminium that span from one ceiling stud? *(i can't remember the name of this piece of wood right now) to another running snug against the outside wall of the stove pipe (either side). Secure the aluminium to the wood on both sides and put one screw in the outer wall of the stove pipe on either side. That will hold it up.
Honestly, at this point, it may be worth shelling out for a contractor to do this part. I've had 2 leaky roofs from pretty skilled people (that had not done many chimneys). It's a half day job.
You have done great to get this far alone.

securing the single wall pipe can be done with a pop rivet. Mine are not sealed. At this point, I think everything should be sucked up and not have any chance of leaking through tiny holes. At least that's how they have been done in my last 2 chimeny installs by proffesionals. The (third) last one I did myself with help from a contracter friend.

My husband helped with the seating of it and put some screws in. Hopefully that will hold.
Trying to get its first fire, but the draft is so strong it keeps blowing out the starts. :O

   


      FIRST FIRE !!!  whoo hoo !!!   take video or at least lots of photos.    

I think the word Daniel was wanting is , floor joists or ceiling joists if you will .

Drill holes, and use screws to hold pipe.  Still wrap with foil afterwards .  Plumbers tape can be used to secure non vertical pipe from above.

How FUN your lighting it off !

FIRST BURN!!

1)  YEAH!!!

2) Was VERY difficult to get it started  I primed with a candle and then followed the nice picture tutorial in Ernie and Erica's book.  Wood was dry, also tried news paper, straw , pine cones.  My husband is a good fire starter.  He worked on it close to 20 minutes and  finally got it to light (by shoving it into the burn tunnel) .  It seemed like the draft was so strong it kept putting out any young flame.  (Tried putting bricks over the top as a damper) Is that possible?

3) Smelled very smoky outside.  I was anticipating steam and super clean burn...does it get there eventually?

4) top of barrel is warping a bit in the middle (kind of ballooning )...normal?

5) Even though it said smoke point was 500 degrees, the avocado oil coating still smoked

6) I took lots of temp (see pic.)

7) I think the paint-ring is stinky, it also got tacky.  I will have to look further for paint removal options.

8) :) :) :) :)

#1)   CONGRATULATIONS !!!
#2)Strong draft is a good thing. You won't need a candle, unless your flame wants to burn up the feed tube! Yes adding bricks at the feed tube is common but not for too strong a draft. You add them to help start a lazy burn or to close off the draft after your fire has gone out.
#3) Always have smoke at start up and at burn out , it will get to steam.
#4) Totally , I talked about it in an old post how the lid can bow up and back down again. This is why you set your top gap at a 2.5" min.
#5) They lied ...
#6) +++++
#7)Burn barrel or just a fire out back , won't take long.
#8) Your hooked now... A new Rocket Scientist is born ! Soon you will be talking to perfect strangers...  telling them about rmh's ... you will be taking out your phone to show ... not pictures of your children but pictures of your rmh. you will be looking at wood sources like pallets in a whole new light, (like how can I get them in my car) face it your life has changed forever (for the better)  !
#9)  Your wood looks to large. Split ,split split. 2x2 ,even 1x1 , that bigger round I saw in your photo is what you add after the fire is roaring.
#10) I place 2-3 thin pieces of wood directly in the mouth of the burn tunnel , basically blocking most of the draft. Handful of kindling small... in the bottom. piece of paper crumpled on top, more small kindling on that. Use a long reach barbQ lighter or another piece of paper. light it outside of the feed tube , then bring it in lit.   Have more thin wood ready. Beware messing with the fire to much as its starting or it will go out.
#11) Have a party !!!

Great news. If you find the draft to be too strong, Paul reduced his wood feed by I think 35% and it did not have a detrimental effect (he had the same problem off too much draft). He also added extras bends in the pipe to slow it down at the same time.
Thanks
Dan

Daniel Hatfield II wrote:Great news. If you find the draft to be too strong, Paul reduced his wood feed by I think 35% and it did not have a detrimental effect (he had the same problem off too much draft). He also added extras bends in the pipe to slow it down at the same time.
Thanks
Dan

'

Not much reading on too much draft!  I did read on adding extra ducting in the bench. Fuel feed height reduction seems a bit easier for my build, thank you for the suggestion.  I will give it another few test fires  (follow Thomas' suggestions) and see if I have consistent results/concerns and then move forward with changes if need be.  (hoping no need be...)   I will update.
Thank you!

Stacy, on J tubes, i like to start it with 90° alcohol. Half a glass on twigs, and throw  match in the feed tube. But, don't do this when it is hot. Even lukewarm.

Stacy, following this:

Staci Kopcha wrote:

petra sips wrote:My Rocket has a strong draft .. as a Rocket should, but when there is a lot of wind I have the impression that the draft is so strong that the heat would not have enough time if it where  to heat the mass .. ?
Any ideas on this .. ?

Hi Petra,
 I know this is an old feed, but I thought I would try.  Did you ever resolve this problem?
I am building a 6" system, did my first fire last night and have some early concerns of too much draft.

Thank you,
Staci

Well, i have a solution for you. You replace part of your vertical chimney, with a bell, with a plunger tube inside.  ;D That's the best way, imho, to reduce the draft. And if well made, it can even be adjusted as you go.

Could you re post the detail of your horizontal pipes lengths please?  Best way to see if you have too much draft, is temp checking in the chimney. If you are over 80C° at the end of the first burn, on a dry heater. Let's say, one meter above the mass in the chimney.

Well, i have a solution for you. You replace part of your vertical chimney, with a bell, with a plunger tube inside.   That's the best way, imho, to reduce the draft. And if well made, it can even be adjusted as you go.

Could you re post the detail of your horizontal pipes lengths please?  Best way to see if you have too much draft, is temp checking in the chimney. If you are over 80C° at the end of the first burn, on a dry heater. Let's say, one meter above the mass in the chimney.

Hi Satamax,


 Plunger tube and bell...you'll have to dumb it down for me, or I will have to read more.   I am  currently killing brain cells attempting to strip paint off of the rim, so I may not have many left
Trying to get the rim done now, and will try another burn this evening.  I can take additional temperature readings then.

The stats on my build are:

Bench: 25 feet, includes 1- 180 degrees and 2- 90 degrees.
Fire feed: 16" height
burn tunnel:  23" long
stove pipe from bench to ceiling: 80"
triple wall chimney above the roof (almost no pitch) 36"
Height above the barrel: a bit shy of 2.5"
I was careful to maintain a constant CSA for 6" system.

Thoughts:  Bench too short.  I am not sure how to add more into it, as my space is limited.
               My manifold is enormous...is too big a possibility?

I will post more after a second burn, brain cell count dependent.

Hey Staci, If draft is too strong, there is a solution. Its called a barometric draft control.   It's basically a weighted door that gets installed in the flue pipe going to the chimney.  If the draft gets too strong, the door pulls open and lets cool air in, slowing the fire.
Example draft control

I've seen them installed on waste oil furnaces so if the draft can't pull too much oil into the furnace and cause a runaway condition