Chimney - Energy Required to Maintain Draft

 
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Lightning
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Post by Lightning » Wed. Nov. 25, 2015 11:51 am

Here's something interesting that occurred to me recently. Since I'm using an outside block and mortar chimney, there are variable amounts of energy that the chimney requires to maintain draft during a 24 hour period. It depends on the temperature of the ambient air outside and the temperature of the chimney itself. And of course stack effect in the house which can be alleviated by bringing the neutral pressure plane down to stove level by cracking a window in the basement, but lets not complicate things with that variable right now :lol: .

This particular amount of energy is in the reflection of how much the barometric damper door is open. My baro door starts to open as it passes -.03" of WC, limiting it at that negative pressure.. That said, if my baro door is closed at -.03, this implies that just the right amount of heat energy is going up the stack to maintain draft. Excess ("wasted") heat energy would cause the draft to increase, thus opening the baro door. Less energy would cause the draft to fall under -.03.

I've discovered that secondary air can influence draft by cutting or adding heated air mass to the chimney system, that's no secret. I've preached and proven it quite conclusively with warm weather burning and maintaining draft. What is particularly interesting is that the secondary air (with a mature fire) can be tweaked to maintain a closed baro door while maintaining the desired negative pressure of -.03, under the right conditions. For me, these conditions occur when it is between 30-40 degrees outside, or when there is a huge swing between nighttime and daytime temperatures. This means that the heat energy going up the chimney is required to maintain draft AND that it is being used as efficiently as possible, instead of in excess. These variables are quite specific to this setup with the outside block and mortar chimney. So please, no one panic if your baro door is open with an interior chimney. An interior chimney requires much less energy from the stove to maintain draft since it is kept warm by being in the house.

Just thought I'd stir up some out-of-the-box thinking for you guys. I'm on vacation this week, thanks for putting up with me.. :lol:


 
scalabro
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Post by scalabro » Wed. Nov. 25, 2015 12:05 pm

.03 means just that and no more :D Your baro opens at that because you've set it at that.

Your chimney will draft at much lower pressure differential, but may not run your furnace correctly 8-)

AFA secondary air, it's much like the baro vs MPD debate. I know that my setup will not always increase draft by opening the secondaries. In fact I've never been able to measure draft in that small of an increment. On a BB like mine secondaries are used to purge coal gasses before ignition and perhaps an hour or so afterward.

After initial volatiles are ignited and burned off a bit opening the secondaries will always slow the stove down. At least that how my chimney and stove operate YRMV.

I'm on vaca too :D :)

 
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Post by Lightning » Wed. Nov. 25, 2015 12:32 pm

scalabro wrote:I'm on vaca too
Nice!!

You also are using a MPD which limits volume capacity so I believe what is happening with your situation is that there could be a "trading" of air flow between the primary and secondary air inlets. With a baro, the volume capacity isn't limited, instead the pressure is limited so no trading of air flow happens.. Actually a MPD limits pressure, by limiting volume capacity first. I believe its these differences in each of our setups, is influencing our results.. :)
scalabro wrote:Your chimney will draft at much lower pressure differential, but may not run your furnace correctly
Absulutely, I agree. Case in point is currently happening. The temp warmed up from 30 degrees last night and is cruising into the mid 40's right now. Baro door is closed and pressure has fallen to -.025 along with it my stove and pipe temps have fallen slightly. The heat energy required to maintain a -.03 has increased because of the changing variables outside, but I haven't made any adjustments to accommodate it, so pressure is weakening.

This energy requirement between interior and exterior chimneys appears to be pretty huge as part of an efficient coal powered heating system.

 
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Post by scalabro » Wed. Nov. 25, 2015 12:40 pm

Good points Lee.

I've often thought of installing a baro after my MPD :fear: :eek2: :shh: but it would be set at say .08 to keep wind gusts from affecting stove draft to a lesser degree. Yes, the stove has an internal draft bypass or "check" but a baro would be automatic.

Thankfully my chimney is a turbo-vac.

I like both the MPD and the Baro.....

Uh oh

 
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Post by Lightning » Wed. Nov. 25, 2015 1:09 pm

scalabro wrote:I like both the MPD and the Baro.....
In my opinion, a MPD above a baro is the perfect combination. Although it's been ruled as dangerous and reckless by many on the board, I don't refute that, but with a seasoned coal burner at the helm I believe it could be done safely and effectively.

By having a MPD above the baro, the baro is able to do it's job much more efficiently. The MPD can be tweaked just partly closed which allows the baro to do it's limiting job with much less room air. I've seen it in action. This would be perfect for a high drafting interior chimney where the stove is on the first floor. It would take much worry out of the baro "stealing" warm room air (which is partly replaced by cold air infiltration). It's the best of both worlds for both dampers, to work in harmony with each other.

This would also compensate some of the energy the chimney needs/uses for it's drafting..

 
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Post by Lightning » Wed. Nov. 25, 2015 2:42 pm

When it comes to base burners, I would think that an interior chimney would be a requirement in order for it run as efficiently as they are claimed to be.

 
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Post by scalabro » Wed. Nov. 25, 2015 2:51 pm

IDKAT ... my 40 only requires .02 running draft and 1/3 of the chimney is outside facing due east :lol:

Of course she'll only accelerate up to 550 at that draft, or about 75% as hot as I'm comfortable running it :shock:


 
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Post by Lightning » Wed. Nov. 25, 2015 3:57 pm

Having the bottom 2/3's of the chimney in the house is a huge contribution to keeping it warm. If I were to put your 40 on my chimney, I would be forced to either run it hot enough to maintain draft or open the secondary air (with MPD open) for more heated air mass to maintain draft.

Either way, more heat energy would be required with my chimney than yours. That same energy could be used to heat the house, instead of being used to maintain draft which would help fulfill the base burner's efficiency potential, right? :)

Exterior chimneys loose heat to the outdoors, that could be used to maintain draft if it were interior. That is where the loss is.

Thats it... The wife is gonna be pissed.. I need to build an interior chimney... :lol:

 
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Post by scalabro » Wed. Nov. 25, 2015 4:23 pm

Agreed.

The less heat you have to add to a chimney to change density for sufficient draft to run said stove, the more heat can potentially be transferred to the house.

And, YOU, are the perfect lab rat to test this out with a BB and a interior chimney.....go to Home Depot for materials & call Wilson immediately :lol: :lol: :lol:

I've only scratched the surface with longer barrels and thermal coatings.

Think of the mods you could dream up!

We eagerly await your arrival on easy street :P

 
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Post by oliver power » Wed. Nov. 25, 2015 5:29 pm

Lightning wrote:
scalabro wrote:I like both the MPD and the Baro.....
In my opinion, a MPD above a baro is the perfect combination. Although it's been ruled as dangerous and reckless by many on the board, I don't refute that, but with a seasoned coal burner at the helm I believe it could be done safely and effectively.

By having a MPD above the baro, the baro is able to do it's job much more efficiently. The MPD can be tweaked just partly closed which allows the baro to do it's limiting job with much less room air. I've seen it in action. This would be perfect for a high drafting interior chimney where the stove is on the first floor. It would take much worry out of the baro "stealing" warm room air (which is partly replaced by cold air infiltration). It's the best of both worlds for both dampers, to work in harmony with each other.

This would also compensate some of the energy the chimney needs/uses for it's drafting..
Well lighting, It's quite obvious why other members say the manual damper above the barometric damper is dangerous. I also see what you're saying. However, if you want to limit excess heat from going up the chimney, AND limit room air being sucked up the chimney through a barometric damper, why not simply install ONLY the manual damper. All in conversation..........

 
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Post by tsb » Wed. Nov. 25, 2015 8:23 pm

Did you guys hit the sauce a little early ?

 
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Post by Keepaeyeonit » Wed. Nov. 25, 2015 9:04 pm

No need for a interior chimney, round clay, perlite to fill the air space,and add a couple of feet done! I couldn't be happier with mine last week it was 65° I was running 250° on the stove and the pipe right before the clay was 120°, no foil on the baro and I was still pulling .04 :D .
You are right though about the energy, you need heat to maintain draft and to make heat you need energy and with a interior chimney it will stay warmer so less energy. The best is a interior one but to add one now would be a major undertaking (unless you can talk your better half into it :jawdrop: ) then maybe its only 1/2 the undertaking!!!
I like my baro because It keeps my stove where I want it and It's set and forget! I don't know much about mpd so I can't say anything about them but I'm thinking Its a love hate relationship :roll: .

 
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Post by Lightning » Wed. Nov. 25, 2015 9:05 pm

oliver power wrote:Well lighting, It's quite obvious why other members say the manual damper above the barometric damper is dangerous. I also see what you're saying. However, if you want to limit excess heat from going up the chimney, AND limit room air being sucked up the chimney through a barometric damper, why not simply install ONLY the manual damper. All in conversation..........
Right, although I was unsuccessful with backing flue gas up until it spilled out the baro,,, Even when I twisted the MPD completely closed - it still would not spill out the baro.. Soo... :idea: just sayin..

Why not install just a MPD, you ask - well that's easy, they can't automatically limit pressure as effectively as a baro, otherwise I would use one, and so would everyone else. :)
tsb wrote:Did you guys hit the sauce a little early ?
Sauce? Where's the sauce, I want some!! :lol:

 
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Post by Keepaeyeonit » Wed. Nov. 25, 2015 9:06 pm

tsb wrote:Did you guys hit the sauce a little early ?
LOL :rofl:

 
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Post by Lightning » Wed. Nov. 25, 2015 9:11 pm

Steering this back on topic, I just thought it was neat that I could control my draft somewhat by cutting or adding secondary air to the point that I only allow enough heat up the chimney to maintain a -.03, proven by the baro door being closed. Which shows that the remainder of the heat generated by the stove is used to heat the house. Nothing wasted, heat wise.


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