Heat Physics

Figured this title would bring in all the Sheldon Cooper fans. (Big bang theory).
Was curious about the theory or opinions on this. Concerning draft and/or rising heat out of our stoves. The more air flowing through the coal bed, usually dictates the intensity of the fire. More air equals more heat. (Providing enuff fuel to support it). Air in usually also requires air out. I've been skooled here that, if we close down our MPD, then more heat will be retained into the stove. (Makes sense to me). However, if closing the MPD slows down the draft, then the stove can't produce the required amount of heat to build the fire. I'm wondering about which phenomenon has a larger potential for heat output. Close down the MPD, to retain heat, but lose the higher draft, or open the MPD for a higher draft, but send the hot air up the chimney? Which is the preferable method?

It depends...

It is a balance and unique to each stove and each install. Sorry

Think throttle when closing down MPD

No open throttle = No work get done

Do I have to answer??? Don't forget, the stove doesn't produce heat.............the fire does. I thought we've been through all this??? Now the MPD is going to make the stove produce heat???

Sorry Lee, I didn't see your original thread of "almost" the same question. I guess it still is open to interpretation. I guess I'll have to continue experimenting with my settings. altho it's time consuming, and the variables that keep changing make it almost impossible to come up with something conclusive.

To start, MPD open bc you want the coal you have burning to burn as hot as possible, to catch other coal on fire. YOu lose heat up the chimney but it's necessary. Once enough has caught fire, you can close the MPD somewhat. Now the coals that are burning are not burning as hot as the first coals when it was just starting, but thats ok because more coals are burning, all,together the coals are putting out more heat even though the individual coals are putting out less (when compared to overall heat output of a few hot coals heavily drafted when you just start a fire).

Just to get this party started.....Take out the MPD and put in a Baro for ultimate automatic control and maximum heat in all draft conditions.

Kevin

I have both Kevin, and if my MPD is wide open, the baro maintains the draft to a constant -.04. Problem with my stove is, that draft flow isn't enuff to make my stove burn hotter than 400*. To make the stove hotter, I block off the baro opening, which brings up my stove temp. Now it's running as if there's no baro, "SO", (once again), the age old question, which is the best position for the MPD, to get the maximum amount of heat? OPEN? now get high draft, and heat up chimney. Closed? Lower draft, should cool down coals, but retain more heat in stove. OR, somewhere in between? Still experimenting.

But , if you really want more heat and to burn less coal; use a fan on the outside of the stove!

joeq wrote:Sorry Lee, I didn't see your original thread of "almost" the same question. I guess it still is open to interpretation. I guess I'll have to continue experimenting with my settings. altho it's time consuming, and the variables that keep changing make it almost impossible to come up with something conclusive.

That's ok partner In my own mind, there isn't any holding of heat in the stove by a manual damper, although it fights with first glance impression. Also, with a stove that is air tight, it may not even matter how strong the negative pressure gets since it can only vent out what its allowed to take in thru the combustion air openings. With your stove, (it has the glass panes that air can slip between, right?) it would be crucial to keep negative pressure in check so excessive air can't get by the glass panes. Excessive secondary air (caused by an unsealed stove) is what causes unnecessary heat loss.

From what I've seen, a particular chimney has particular characteristics.. Being able to keep the draft pressure consistent gives you the advantage of making the burn cycle more predictable and steady. And a barometric damper is very good at that...

Just my two cents worth of it all

Ndcoal wrote:To start, MPD open bc you want the coal you have burning to burn as hot as possible, to catch other coal on fire. YOu lose heat up the chimney but it's necessary. Once enough has caught fire, you can close the MPD somewhat. Now the coals that are burning are not burning as hot as the first coals when it was just starting, but thats ok because more coals are burning, all,together the coals are putting out more heat even though the individual coals are putting out less (when compared to overall heat output of a few hot coals heavily drafted when you just start a fire).

This is the right answer. Your coal "heat factor" for your particular house is not a function per se of how hot you're firing your stove or furnace, but rather how hot the bed of coals are once you get them going. That's why a full, deep bed of coals will heat much better than a full, deep bed of embers from wood burning by comparison. I learned through trial and error the above. I noticed one day the exhaust temp at the furnace flue was running somewhat "cool" but the house was really warm and toasty. Then I remembered I'd let the coal bed get really hot (went to get a cup of coffee) and when I came back down I throttled it way back. The coal bed was completely ignited - orange/red. From that point on just enough air to feed the beast but not send flue temps off the charts. Been doing same procedure ever since, just keeping my coffee in hand!

joeq wrote:I have both Kevin, and if my MPD is wide open, the baro maintains the draft to a constant -.04. Problem with my stove is, that draft flow isn't enuff to make my stove burn hotter than 400*. To make the stove hotter, I block off the baro opening, which brings up my stove temp. Now it's running as if there's no baro, "SO", (once again), the age old question, which is the best position for the MPD, to get the maximum amount of heat? OPEN? now get high draft, and heat up chimney. Closed? Lower draft, should cool down coals, but retain more heat in stove. OR, somewhere in between? Still experimenting.

Why don't you adjust the baro to maintain a higher draft reading? If you can't get the draft any higher than -.04 with the baro closed, you don't need it anyway.

joeq wrote:Figured this title would bring in all the Sheldon Cooper fans. (Big bang theory).
Was curious about the theory or opinions on this. Concerning draft and/or rising heat out of our stoves. The more air flowing through the coal bed, usually dictates the intensity of the fire. More air equals more heat. (Providing enuff fuel to support it). Air in usually also requires air out. I've been skooled here that, if we close down our MPD, then more heat will be retained into the stove. (Makes sense to me). However, if closing the MPD slows down the draft, then the stove can't produce the required amount of heat to build the fire. I'm wondering about which phenomenon has a larger potential for heat output. Close down the MPD, to retain heat, but lose the higher draft, or open the MPD for a higher draft, but send the hot air up the chimney? Which is the preferable method?

Keep in mind that MPD's don't "close off" anything. They have holes in them, so at most, all they do is just restrict draft flow to various degrees.

Then think of the stove as a heated "plenum". A plenum is just a larger space within a sealed system that allows gases to slow down as they flow through the system.

Once you get your fire burning well, if you set your primary draft to a certain opening to get a certain heat output, the stack opening is generally much larger than the primary openings. If you could measure the speeds of the heated gases, you would see they can leave the stove very quickly. By changing the stack size (restriction) with an MPD, you can change the speed that those gases are moving at. You can sorta see the affect on gas speeds with the higher, or lower readings shown on the mano.

But, by restricting the draft opening with an MPD to better match (but not over power) that primary air setting you chose, you slow the hot gasses trying to leave that "plenum". Slowing the gases down allows the heat stored in those gases more time to transfer from them through the plenum walls.

After getting your primary damper set where you want it, if your fire is then slowing down by using the MPD, your using too much MPD.

By using a set primary damper opening and then experimenting with MPD angle, and checking the stove temps that result, you should be able to find the sweet spot where the MPD will let the fire stay at the rate you want, and the stove temps should be a bit higher. It won't be a tremendous amount, unless the stack pipe is oversized for the stove. That sweet spot will produce a mano reading. Then, once you know what the mano reading is that gives the most heat output, you just set the primary air and then the MPD to get that same mano reading each time. With my stove it does it's best heat output at .04 -.05. .

What makes it not perfect is that on hand fed stoves, as the coal burns, temps don't stay constant, so the MPD is just a compromise that's better than no MPD. Then, in some systems, a properly installed baro will do a better job of keeping up with those temp changes automatically - by changing the draft pressure to regulate the stove's temp output.

Paul

Sunny Boy, IMHO, I feel you kinda said it all. I've been using MPD's for many years with wood, and most recently with a Harman boiler. Even though Harman does not recommend one I put one in thinking worst case scenario I could just leave it open. initially, with MPD open the heat was there, but the coal was gone. Now with a mano, and adjusting the MPD till I get a reading of .05 to .06 I can easily go 12 hrs between loads even in this weather. No different than cruising in a boat, you gotta find the sweet spot.
JF

J F Graham wrote:Sunny Boy, IMHO, I feel you kinda said it all. I've been using MPD's for many years with wood, and most recently with a Harman boiler. Even though Harman does not recommend one I put one in thinking worst case scenario I could just leave it open. initially, with MPD open the heat was there, but the coal was gone. Now with a mano, and adjusting the MPD till I get a reading of .05 to .06 I can easily go 12 hrs between loads even in this weather. No different than cruising in a boat, you gotta find the sweet spot.
JF

True, for best economical cursing , you find the boat's sweet spot by using a vacuum gauge and a tachometer, then swap props of different pitch until you find the one that gives the highest vac readings at the engine manufacturer's recommended cruising rpms. Then, that's the most economical set up for that hull's drag.

It does make a difference. But, like different boat hull shapes, I won't be surprised to see that difference likely will vary quite a bit with stove/stack differences.

Like the vacuum gauge and tach, manos and temp gauges can show what affect the MPD is having on stove output.

My RN girl friend reminds me that manometers are for showing low pressure of gases, and she's correct. There's a mano on the wall over every hospital bed as part of the oxygen system.

But, with the mano gauge hooked up "backwards" as we use it (tube to the "low" connection), that actually turns it into a vacuum gauge - like you would use to tune a gas engine. The mano is just way more sensitive than an automotive vacuum gauge.

I found the sweet spot for my stove/stack combo by using the same setting for the primary damper and then checking the mano readings as I experimented with different MPD angles. Each time, I change the MPD angle, I checked stove and stack temps with my IR gun.

I found I was wasting about a pound or two of coal a day by not using the MPD at it's optimum setting. To someone with a large coal furnace, or boiler, that may not sound like much, but with my small kitchen range averaging 30 pounds a day, that pound or two is a pretty good percentage saved. Using the MPD is like I just got a free day worth of coal every month. If a large stove could save a day worth of coal each month that adds up to some real money. Then the mano would be paid for in the first month, or two, just by the savings on coal.

Paul