How Barometric Dampers Function, Why Use a Barometric Damper

 
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Post by Lightning » Fri. Aug. 22, 2014 9:10 pm

Yes that all makes sense above the baro. For the sake of simplicity and applying it to what ctyankee was asking though, I believe (if I read between the lines correctly) he was ultimately asking what the result would be if you were to lower (weaken) the "draft pressure" in the firebox.. :)


 
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Post by McGiever » Fri. Aug. 22, 2014 9:45 pm

McGiever wrote:
Ctyankee wrote:If a stove is operating at a constant temperature and a barometric damper is allowed to work and lowers the draft through the stove, does the stove decrease in temperature?

If so, then do you have to open the air vents to increase the burn rate to increase the temperature?
A baro NEVER lowers the draft.(through the stove)

It can only maintain it at an even rate.

Even draft = even temp.
With his question he implies that the baro does lower the draft through the stove...to which I then stated it can NEVER happen...so it appears to be a poor question.

I was surprised when a couple of you went off on tangents and had a solution for this poor question. :roll:
Last edited by McGiever on Fri. Aug. 22, 2014 9:53 pm, edited 1 time in total.

 
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Post by Ctyankee » Fri. Aug. 22, 2014 9:52 pm

Lightning correctly understood my question. My insert has an internal damper and I read my internal stove pressure vs. room pressure with the manometer under the grates. I had the internal damper enlarged to lower the variance to -.07 which is the reading on warm days (during the winter its usually -.09 to -.12). I can try to put a baro in to further lower the value but wanted to understand it operation and effect.

 
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Post by McGiever » Fri. Aug. 22, 2014 10:16 pm

So the question is... What if a baro were to be added because there is NO baro now, and what effect will there be to the air settings/adjustments after the baro is added?

 
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Post by Cap » Sat. Aug. 23, 2014 7:12 am

McGiever wrote:What weaker pressure?

A baro can NEVER lower the draft.
Why can't it? It's an inexpensive very crude device that can get caked up with crud and stop working correctly if it ever worked correctly in the first place!. And it doesn't seal 100% tight.

I'd perfer an Johnson controls manaomter & electric 24v actuator!

 
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Post by Cap » Sat. Aug. 23, 2014 7:22 am

McGiever wrote:So the question is... What if a baro were to be added because there is NO baro now, and what effect will there be to the air settings/adjustments after the baro is added?
You will have to re-learn your damper adjustments.

 
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Post by titleist1 » Sat. Aug. 23, 2014 7:27 am

We haven't even got into the burn season yet and we're back at this again?! :bang:

And I still can't understand why Smitty drinks whiskey with a straw! :o
Last edited by titleist1 on Sat. Aug. 23, 2014 7:56 am, edited 1 time in total.


 
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Post by Lightning » Sat. Aug. 23, 2014 7:27 am

Naa, 30 seconds with a tooth brush to sweep off fly ash every couple weeks is all I've ever needed to do.. as far as the seal is concerned, it really doesn't need to. My Fields Control RC has worked great for 3 seasons. Can an actuator respond quick enough when its gusty outside?
Last edited by Lightning on Sat. Aug. 23, 2014 7:29 am, edited 1 time in total.

 
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Post by Lightning » Sat. Aug. 23, 2014 7:28 am

titleist1 wrote:We haven't even got into the burn season yet and we're back at this again?! :bang:
Anticipation is stewing..

 
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Post by blrman07 » Sat. Aug. 23, 2014 7:30 am

I should have my rear handed to me ( and that will probably happen ) for jumping in here but I can't take it any longer. Depending on the situation,,,,, everybody is right,,,,, because it depends. How's that for pandering. Gotta love sting for that line!!!

Caveat...All the following comments relate only to hand fired fossil fuel heating devices. I am an expert in my own mind.
That said,,,, do try this at home, mileage may vary, this is a closed course with paid actors portraying fictional acts blah blah blah blah. The following applies regardless of the heating device you use but I am specifically addressing solid fossil fuel devices.

Chimney draft is a term used to describe the difference between air pressure at the bottom of a chimney vrs the top of the chimney or to be more specific the outside air pressure. If you have air pressure differential you will get air flow. The direction of that air flow depends on which end of the chimney has the higher air pressure. For our purposes we want less air pressure at the top of the chimney than the bottom so all products of combustion from burning fossil fuels will be carried to the outside air and increase global warming. (Sorry I had a moment there....)

A chimney is nothing more than a tall tube that will draft (allow air movement) without anything attached to it as long as there is an air pressure difference between the top and the bottom. If you have an open 6 inch thimble sitting a set height from the base you will move a set volume of air depending on the chimneys construction AND the outside temperature. Simply put,,,,Heat rises---Cold falls. Sun heating any type of chimney will cause the air in it to heat up and rise with nothing attached to the chimney. It will try to pull in cooler air from somewhere, anywhere, it doesn't care where, and that air will heat up as it rises in the chimney. This is CHIMNEY DRAFT. This can be measured by putting a manometer in different spots on the chimney. Chimney draft is a function of air temperature and air density. Chimney draft will vary over a "day" based on these two parameters. Air pressure differential will change depending on where in the chimney you measure it at. I can stop chimney draft completely by closing off all inlets to the chimney by putting a cover over the thimble(s).

No inflow=no outflow=no air flow =no chimney draft.

I take the same chimney and I put a 6 inch unrestricted stove pipe in the same thimble. I now allow air flow in so air flow will start from bottom to top or top to bottom depending on air density. It will be under the same parameters based on construction AND temperature which affects air density.

Again heat rises, cold falls.

Now I take the same chimney with the 6 inch unrestricted stove pipe in the same thimble and I connect a heating device to said stove pipe. All air inlets on this heating device are closed. I have now added 100% restriction to the end of the stove pipe which stops air flow into the chimney. If all the air inlets are closed I have done the same thing as putting a chimney cap on the thimble described earlier. You will have zero chimney draft.

No inflow=no outflow=no air flow=zero chimney draft

Now use the same chimney with the same stove pipe with the same heating device and I open any air inlet spot on the heating device. I have now allowed introduction of air to the chimney. As long as the air in the chimney is warmer than the outside air I will achieve upward chimney draft IF I allow air into the chimney through those air inlets. That draft will be limited to the amount of air I allow through those air inlet ports. I now assume partial control of the chimney draft by opening and closing the air inlets.

air in=air out=air flow=measurable chimney draft

I now build a fire in the heating device and I changed the air temperature in the chimney to a much hotter air at the base. Heat rises and cold falls. Fire needs air. The fire in the heating device will cause an increase in chimney draft that I control by regulating how much air goes into the heating device which controls the intensity of the fire which regulates the air temp leaving the heating device and going to the chimney. The air inlets control how much air gets to the fire to sustain it. Hotter fire, hotter air at the bottom of the chimney, air rising faster, cause the chimney to try and get air from somewhere. A good tight chimney will only pull air from where you want it to and that should be through the air inlets on your heating device.

Air in for fire=hot air flow out=measurable chimney draft

Nothing else is added to this system and you regulate your fire by controlling the amount of air it gets through the air inlet ports. Steady state is what your trying for but you won't get 100% because outside air temperature is always changing. A rise or drop in outside air temperature WILL have an effect on the steady state you set up requiring adjustments of air flow through the inlet ports until you hit that steady state again.

Now assume my heating device requires a set amount of air going through it to maintain a fire good enough to generate 50k BTU's per hour max. Assume the chimney was built to move an amount of air to support a 150K BTU's per hour heating device. I only need a certain amount of air flow to hit max of 50k on my heating device but my chimney can "draft" so much that it pulls too much air for the smaller heating device. I want some way of limiting the max amount of air my chimney is going to pull. I shut down the air inlets but I can't shut them enough to control the chimney draft and still get my max BTU's. I have to introduce something else to the equation.

I install an MPD which is a hand operated variable restriction device in our connection to the chimney. I set my air inlets on the heating device to the fire intensity that I want and then fine tune the air flow into the chimney with my restriction device,,, the MPD. In order to get my desired BTU's I am now using two devices to regulate my air flow to the fire. One before the fire and one after the fire. The air inlet ports and the MPD. One regulates how much air flows into the heating device for fire control and the second (MPD) acts as a restriction in the stove pipe controlling my air flow INTO the chimney. Both devices used together will control the amount of air getting to the fire and both devices will control the amount of air getting to the chimney.

Some geographic locations add outside influences such as intense storms, rapid weather/temperature changes, varying wind speeds across the top of the chimney. These and many more outside influences will affect the pressure differential between the top of the chimney and the bottom of the chimney faster than you could realize it and adjust your air inlet and your MPD manually. Rapid air movement across the top of a chimney will reduce the air pressure at the top of a chimney causing the air to rise faster in the chimney. This is an increase in chimney draft that could pull air at a higher velocity through the fire causing your fire to burn hotter than desired. You want someway to control these random actions that you may experience.

Enter the Barometric damper. You are trying for a steady state of air flowing through the heating device, into the stove pipe, through the MPD and into the chimney. Because of rapid changes due to outside influences the chimney tries to pull more air. The barometric damper will allow that air to flow through the baro opening instead of through the air inlets on your heating device. You now have three things controlling your heating device. Air inlets, MPD, and a barometric damper. The air inlets and the MPD are manual devices and the barometric damper is automatic.

Depending on your heating device, chimney and atmospheric conditions you may need an MPD to help limit the amount of air getting to the chimney and/or a baro to help control the speed of mother natures effect on your chimney draft OR you may need neither.

IT ALL DEPENDS on your individual setup, location, equipment, chimney type and construction. To recap, a chimney will pull a set amount of air through it if all things remain steady state. Since things don't remain in a steady state we have to control the amount of air a chimney will pull through it by using a combination or (or not) of air inlet ports, an MPD, and/or a baro. All things given it will pull air from somewhere and all we are doing is controlling from where and how much.

I have now exhausted myself typing this out and I think I hurt my liver. I await responses. Be gentle I have a headache now.

Rev. Larry
New Beginning Church
Ashland Pa.

 
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Post by Lightning » Sat. Aug. 23, 2014 7:51 am

Very good Rev.. I'm onboard with all of that.. :)

The only part I might question is the statements about chimney pressure.. I would think the negative pressure on a sealed thimble chimney would be greatest at the bottom and then get closer and closer to ambient pressure as you reached the top where then it would be equal. But who's gonna drop a probe line down a chimney to find out? :lol:

 
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Post by Lightning » Sat. Aug. 23, 2014 8:49 am

This is where our chimney expert Berlin would chime in and say.... With an active chimney in use, there would be a neutral pressure plane somewhere in the top section of the chimney.. below the NPP, there would be negative pressure with the strongest at the bottom and above the NPP, pressure would be positive with the strongest at the top.. :lol:

 
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Post by McGiever » Sat. Aug. 23, 2014 9:14 am

Rev Larry, Most eloquent description... :bighug: :clap:

Some will get it...some will not. :?

 
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Post by Ctyankee » Sat. Aug. 23, 2014 9:33 am

Thank you Rev. for your enlightened and clearly written explanation. I have a better understanding now of the overall operation and the factors involved.

In my situation, I figured that the high manometer reading indicated that too much super hot stove air was leaving the firebox and being wasted up the chimney. Therefore, a more efficient burn would require me to put in a barometric damper.

With that operating, then I would need to open the air inlets to obtain enough O2 since less would be pulled though the stove but I would get more heat transfer at a given stove temperature. So maybe I could run the stove at a lower temperature.

The BTU loss from room air leaving the house would be much smaller than the loss from the stove air being pulled up the chimney before I had the baro.

Do I have this right?

 
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Post by warminmn » Sat. Aug. 23, 2014 10:10 am

Very nicely put Rev. Especially the parts about may/may not and it depends.

No two situations are the same. What works for one wont for the next person. Gotta try them all to see what works for you.


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