Stovepipe Damper

tcalo wrote: ↑

Sat. Nov. 26, 2022 9:54 pm

My thought is by closing the mpd it slows the exhaust gases which gives the flue pipe more time to absorb and shed the heat into the room rather than be lost up the chimney. I can visually see this through my mano and temp readings.

Also, as the exhaust gas slows, it can come closer to the pipe walls where the walls produce flow drag. By slowly sliding a mano probe into the pipe you will see slight changes in the reading, first at the pipe wall where the exhaust is slowest, then just clear of the pipe wall, and finally in mid-stream of the exhaust. Those changes in mano reading indicate changes in exhaust gas speed (once again, Bernoulli's principal on display).

And the closer and slower the exhaust is to the pipe walls then the more heat is extracted before it gets to the chimney. Ideally you want as low a mano reading near the pipe walls as possible, no matter what the primary damper setting is.

The old timers knew this that's why they used things like MPD's and check dampers to control draft after the firebed and thus control heat output. Some old cookbooks would even say to close the MPD more to help heat the oven when needing to bake at higher temperatures. The oven is heated by exhaust gases traveling through the flues surrounding the oven. Slowing those gases with an MPD transfers more heat into the oven walls without overfiring and risking heat damage to firebox parts.

Paul

I respect what's been said but just not onboard with all of it... and I'm not out to change anybody's mind just would like to explain why I think what I think.. Bernoulli's Principle is why a chimney draws harder when it's windy outside. Wind hits the side of the chimney and is forced around it as well as upward and over the top. As it's forced upward over the top the inertia of the air wants to continue in that direction (upward), but it can't because the crosswind striking that upward moving air bends it over the top. The result is that a low pressure area forms right over the top of the chimney which helps it draw... and I'll get to the drawing part later. From what I've seen, it takes more than just a gentle breeze to see a fluctuation on the manometer.

Now let's apply that to flue gases moving past the mano probe. Combustion air enters the stove thru a tiny inlet as compared to a massive 6 inch diameter pipe outlet, relatively speaking. How fast velocity wise does this combustion air enter? Hard to say. How much would it need to slow down to exit thru a 6 inch pipe? I don't know. My point being is that upon leaving the stove I can't believe it would be sailing by the tip of the mano probe to see a measurable influence of Bernoulli's Principle acting upon it. I tried this by moving the mano probe closer to the edge of the pipe then closer to the center. It didn't reveal a measurable difference, that I could see. Even if I take the mano probe out of the pipe and blow a stream of air across it, it still takes a pretty quick stream of air for it to register a difference due to air velocity by way of Bernoulli's Principal. Is air exiting the stove THAT fast?? I can't believe it does, unless you have the load door wide open. How fast are gases coming out of the chimney? Just by observation they just seem to slowly roll and tumble. So my belief is that the mano is showing true pressure difference unaffected by the velocity of the flue gases or a measurable effect of it.

And on to the drawing. Does a chimney actually draw, as in suck or pull gases up thru it? Nope.. How could it? Gravity only pulls in one direction, which is down, no magic here... So what force is causing the exhaust to go up then?

It's interesting that when I'm in the swimming pool and I try to hold a beach ball under water, there is a massive upward force against my hands. We've all done that. How can that even happen when gravity only pulls down? We know its buoyancy that causes that. The water in the pool is pushing the ball upward against my hands because that bubble of air in the beach ball is less dense than the water.

And this is exactly what happens in a stove. The bubble of hot air in the stove is less dense and has buoyancy as compared to the cooler air outside of it.. The cooler air entering the stove is actually PUSHING the exhaust up the chimney because what else could be sending exhaust up the chimney, gravity only pulls down, no magic here... SO... this is why, its my belief that, if you have absolute control (key word being absolute) of air entering the stove then an MPD does nothing to hold heat in a stove.

To expand on that word I used earlier.. "absolute". I don't have any experience with antique stoves or cook stoves. But from what I understand they have a lot of places for unregulated air to enter them. They have bolted together seems, many mica windows, cook tops that aren't sealed, the list goes on. Even in modern air tight stoves, there are unregulated air washes, places for primary air to bypass the fuel bed and become secondary air, ect... Here is where I agree that an MPD would be beneficial.. because by closing it, the negative pressure in the stove is reduced, which in turn slows this unregulated leaking air into the stove, which does what? Pushes the exhaust (and heat with it) up the chimney.... we need control of that. As mentioned earlier air is 79% nitrogen which just gets heated and carries that heat out with it. We need to use as much of the available oxygen as possible for combustion. There's only 20%. Use it wisely..

That air entering the stove gets turned into a hot combustion gas, which is of far greater volume. That is why the cross section of a stove pipe is much greater than the opening size needed to feed air to the fire.

And remember that hot gases DO NOT rise, they are pushed up by heavier gases such as the much colder room air. Same as the colder air outside and hot air inside, is what pushes a hot air balloon up.

The much hotter exhaust gases inside the stove and chimney system are far, far lighter than the air outside the system. That outside higher pressure is what is creating the force that makes "draft".

The greater the difference in temp inside verses outside the stove, the stronger the draft. Proof of that is why stoves burn so well on really cold days and why they are so sluggish on warm days.

As for back drafts in chimney systems they can happen when there is air turbulence around the chimney top. And that can be caused by a chimney that is not high enough above the roof, or by tall trees, hills, or other obstructions upwind of the chimney.


As for MPD only needed on antique stoves, with new stove or old, try closing the MPD and see what happens to the mano readings. If there is a change in mano readings lower, it is because the MPD has slowed the exhaust gases.

Paul

Another easy test to show that an MPD has a greater control over the chimney draft than the primary damper does.

I already mentioned how opening and closing the MPD will make changes in mano readings indicating that it's use to control the exhaust gas velocity can help keep heat in rather than let it race up the chimney.

Now, take the MPD out of the test by opening it fully. Then open and close the primary air and see how much change that makes in the mano reading.

Paul

Sunny Boy wrote: ↑

Mon. Nov. 28, 2022 8:58 pm

Now, take the MPD out of the test by opening it fully. Then open and close the primary air and see how much change that makes in the mano reading.

I can't do this test but I'll take a stab at it. I'm guessing that the MPD will have a much greater impact on the draft reading than opening and closing the primary air source.. Is that right?

Sunny Boy wrote: ↑

Mon. Nov. 28, 2022 6:09 pm

If there is a change in mano readings lower, it is because the MPD has slowed the exhaust gases.

I agree.. and by restricting the exhaust gases what happens to the negative pressure in the stove? It weakens.. and with a weaker negative pressure, the incoming volume of primary combustion air is lowered. That makes sense right? This is why I always saw my hand fed furnace cool down if I tried to restrict the draft with an MPD.

Sunny Boy wrote: ↑

Mon. Nov. 28, 2022 6:09 pm

And remember that hot gases DO NOT rise, they are pushed up by heavier gases such as the much colder room air.

I agree... So.. with all that being said, wouldn't it make sense that IF hot gases do not rise unless they are being pushed, THEN couldn't you just as easily control that rising hot air in the chimney by regulating the combustion air entering the stove? I think we're digging towards the same result

...and this is why I think that tightly built stoves wouldn't yield a measurable benefit of using an MPD. Because the MPD doesn't really hold the heat in the stove.. having good control of the primary and secondary combustion air will automatically keep the heat in the stove, cuz like you said, it can't rise unless its being pushed... and I mean this in the nicest of way, I don't want to offend anyone who likes to use an MPD.. TO me, an MPD is more of a band aid for a leaky stove, because at the pipe you can restrict the outgoing flow, which weakens negative pressure in the stove, which slows down undesirable air entering the stove where it can't be controlled.

In an attempt to find common ground, I would be willing to agree that an MPD has the side effect of helping keep heat in a stove under particular circumstances, I suppose lol...

Lightning wrote: ↑

Tue. Nov. 29, 2022 4:31 pm

I can't do this test but I'll take a stab at it. I'm guessing that the MPD will have a much greater impact on the draft reading than opening and closing the primary air source.. Is that right?


I agree.. and by restricting the exhaust gases what happens to the negative pressure in the stove? It weakens.. and with a weaker negative pressure, the incoming volume of primary combustion air is lowered. That makes sense right? This is why I always saw my hand fed furnace cool down if I tried to restrict the draft with an MPD.


I agree... So.. with all that being said, wouldn't it make sense that IF hot gases do not rise unless they are being pushed, THEN couldn't you just as easily control that rising hot air in the chimney by regulating the combustion air entering the stove? I think we're digging towards the same result

...and this is why I think that tightly built stoves wouldn't yield a measurable benefit of using an MPD. Because the MPD doesn't really hold the heat in the stove.. having good control of the primary and secondary combustion air will automatically keep the heat in the stove, cuz like you said, it can't rise unless its being pushed... and I mean this in the nicest of way, I don't want to offend anyone who likes to use an MPD.. TO me, an MPD is more of a band aid for a leaky stove, because at the pipe you can restrict the outgoing flow, which weakens negative pressure in the stove, which slows down undesirable air entering the stove where it can't be controlled.

Primary damper changes hardly makes the mano move. The MPD is capable of making huge changes in the mano readings.

No, the fact that changes in the primary damper does not make much, if any, change in stove pipe mano readings shows it cannot control exhaust speed- tight stove or not. And the fact that high mano readings with the MPD wide open show that the exhaust is racing up the chimney. The faster the exhaust goes up the chimney the less time it has to transfer heat before being lost.

If you close an MPD too much, yeah it will slow a stove down. Using an MPD is not a fully open or fully closed device any more than the primary damper is. Both have points that are optimal for heat output. And each stove/chimney system is different, so it takes experimenting and writing down the numbers to see a pattern form.

The trick is to experiment with MPD settings and find that point at which it lowers the mano readings as much as possible WITHOUT lowering the heat output you want.

Want more heat, you have to know how much to open both the primary to make that heat, plus the MPD to keep the mano readings lowest at that primary damper setting.

It can be done.

Paul

I do believe that an MPD will set up turbulence at its location when partially closed which will contribute to heat exchange at that location. Other than that, I agree with both Lee and Paul, but keep in mind that the primary objective is to control stove temperature which both controlling primary air in a tight stove or with an MPD in a looser stove, both do.

How much change in pipe mano readings (exhaust gas velocity) can changes in the primary damper openings make in a "tight stove" ?

As much change in mano readings as an MPD would with the same stove ?

Paul

With the wind here today my “built-in” stove damper and the mpd full closed never made a noticeable difference in seeing large manometer fluctuations, and I didn’t expect it to. Manometer went from 0”WC to 1”+WC and everywhere in between and beyond. It was howwwwwling here.

One thing is for certain flap stayed closed most of the day today … and keep in mind no ash door vents and spinners were just cracked from 12 o’clock to 11 o’clock.

What few times I could tell the wind wasn’t pulling on her she was running at -.035”WC. Normally I’m at -.005” to -.02”.

A baro might have helped some today, but the thermostat seemed to make up for it by remaining closed … a lot. House temperature stayed spot on too … even into this evening as temps are dropping and wind still blowing, the IAT is staying spot on 72F IAT.

All interesting points. What I can say for sure is my primary air has very little immediate impact on mano readings. I’m sure over time the draft will change in relation to the primary air settings and coal bed temps. However my mano reading changes instantly with my mpd adjustments. I also find there’s quite a swing in readings in relation to the mpd position. There is a sweet spot as mentioned earlier. An idle stove with a low mano reading won’t produce much heat. Just as a roaring stove with a high mano reading would be sending most of the heat out the chimney. It’s a balance!!

Yup, Tommy.

And anyone who has ever operated a direct/indirect flue type stove gets to hear firsthand what using some restriction in the exhaust can do. That roaring fire sound after a refueling is quickly quieted by closing the direct/indirect damper and slowing the exhaust velocity.

Restricting the exhaust flow, and not the primary air, is why base heaters, base burners, and back pipe Oaks are so successful at extracting heat. Not only does the longer flue path add heat-extracting surface area, but it also adds restriction to flue gas flow to reduce velocity and give more time for heat transfer into the room.

You can see a drop in the mano reading as soon as you close that D/ID damper. Open the damper to direct draft and the mano reading shoots back up.

The MPD adds additional exhaust speed control of even with those type stoves.

Do stoves have to use an MPD ? No.

But for anyone wanting to get the most heat out of their coal dollars try using an MPD and a mano. And if you just rely on primary air settings for control and don't try using both and MPD and a mano you'll never know how much heat is being wasted up the chimney.

My two stove's manos and MPDs paid for themselves just in the first couple of months and that was at the old coal prices.

Paul

The mpd is a thermal and currential brake pedal.

Well said gentlemen.

CURRENTIAL???? I'll be damned M----------------

freetown fred wrote: ↑

Thu. Dec. 01, 2022 6:14 pm

CURRENTIAL???? I'll be damned M----------------

Also had me wondering with that one.

Until,....
https://www.beckettcorp.com/support/tech-bulletin ... ing-draft/

"Currential Draft occurs when high winds or air currents across the top of a chimney create a suction in the stack and draw gases up. "

Paul