Heat Physics

Sunny Boy, right again, since I installed the mano I'm guessing a savings of over 5 lbs per day simply because I know just where to set the mpd for optimum draft. Too early to tell just yet because of the extreme weather; but should have a pretty good idea in a couple of weeks. Like you said it's like getting the mano for free. Jim

If you had installed a baro and adjusted it to a recommended draft, and then experimented up and down maybe a little bit, you would not have had to go through those gyrations. AND it will keep the draft at the set point if outside conditions change. Something your MPD WILL NOT do unless you watch your barometer and wind gauge(24/7) and know the effects of atmospheric conditions on your chimney/home with computer like precision. YOU must be present at all times and have your hand on the MPD while monitoring conditions to equal a baro. And then you will not be as quick to adjust it. Yes, they work. But not like some seem to think they do. They are not able to adjust to changing outside conditions. IF they are adjusted with a 10mph breeze and an outside temp of *10 and a good fire going, what is going to happen when the temp rises to *40, the breeze goes dead calm, and the fire is loosing temp? YOUR CO detectors might be squealing! I say might because there seems to be enough room for error in a good install to still remove gases from the home. Good thing.
This argument has been had many times in here, it is nothing new.

Kevin

Sorry, I haven't kept up with some great posts. I'll try not to write a book here, and bore everyone to tears. Lots of good info above. Some of it repetitive from other threads, but a few things for me to ponder over.
1st off, I've mentioned before, I "do" have a baro, "and" an MPD. And I have set the baro to the manufacturers spec of -.04. When the weather is 20 something and higher, the heat exchanger temp will go to 400* and keep us warm. When the temps are lo-teens and less, this little stove has a hard time keeping our living space warm. But I don't want to get into a discussion on required BTUs per sq/ft, cause I know I'm below my requirements. ("supplemental stove")
But back to my original question or dilemma. (And trying not to be redundant). It seems there is still some conflict of opinions, as to whether a closed MPD is retaining heat in the stove or not. This is the way I'm viewing it, so please correct me if I'm wrong. The manometer measures a flow, or airspeed/velocity. But not amplitude or quantity. If our stoves are at full speed and we were to measure the amount of "exhaust" or heat coming off the top of our chimneys, I'm willing to bet a closed MPD will reduce the amount of exhaust flow out the stack. By doing this, it must mean that some of the heat is still "trapped" in the stove, which should equate to more heat being radiated. In the case of "only" a baro installation, altho the draft will be consistent, it still is a wide open 6" hole all the way to the chimney cap. Sure you can adjust it to a higher pressure for a hotter burn, but isn't it still blowing out the chimney at an alarming rate? (Compared to a closed MPD?) Once again, what I'm trying to do is get my stove temp. up higher than a medium heat of 400*. It'll go 500, but not easily. I can get a bed of coals to be bright yellow to almost "white hot", but still the heat exchanger temp only creeps, if at all. I didn't want to drag this out, and it appears I did anyway. Sorry. My next question is, "what color is HOT?". But I'll save that for another thread.

OK I think I understand.. Lets take this situation. We have a stove with a barometric damper set at -.04" on the mano.. We have another stove with a manual damper set at -.04" on the mano.. All other variables the same. Same model stove, same combustion air setting.. Which stove has more air moving thru it? Which stove is allowing more hot gases to escape up the chimney??

Answer is that they are both the same...

Negative pressure dictates air volume thru the combustion air openings.
Combustion air in = Combusted air out
Air is the vehicle the heat is riding on.

Manual damper and baro both limit flow of air thru the stove.
The baro just does it better..

joeq wrote: In the case of "only" a baro installation, altho the draft will be consistent, it still is a wide open 6" hole all the way to the chimney cap. Sure you can adjust it to a higher pressure for a hotter burn, but isn't it still blowing out the chimney at an alarming rate? (Compared to a closed MPD?

No, no and no..

I'm willing to bet a closed MPD will reduce the amount of exhaust flow out the stack. By doing this, it must mean that some of the heat is still "trapped" in the stove, which should equate to more heat being radiated.

AND if you close the MPD, it will lower the draft in order to "trap" the heat and the stove will soon not produce the same heat.

Kevin

Here's a purely speculative thought,....if the vents supplying under fire combustion air are open say, a quarter inch and the space around the MPD flapper is a quarter inch pulse any holes in the flapper itself, other than disrupting laminar flow, how does closing a MPD affect draft in a coal stove? It seems that even when closed, the MPD allows as much or more gas past than the combustion air can draw in???

Then why don't we use 3 inch chimneys for lower drafts or size the pipe to the size of the air ports coming in? Because it must PULL or some would say PUSH easily. Why do people put oversize exhaust pipes on cars and trucks to increase horsepower? Why are headers better? why use a K&N air filter? What happens when you open the MPD in the carburetor(butterfly valve) and what happens when you close it? And on and on.

Kevin

I think the confusion is that it's easy to view the manual damper as a blockage and flue gases are piling up against it. That's really not the case. The chimney is still pulling the flue gases past it, otherwise the stove would go positive pressure and leak fumes. They both weaken the negative pressure in the stove by different methods.

I think the combustion process increases the volume to be exhausted. Water gains volume by 700 times, I believe when it becomes steam. That is how steam produces HP and hot water does not.

Kevin

I think the confusion is that it's easy to view the manual damper as a blockage and flue gases are piling up against it. That's really not the case. The chimney is still pulling the flue gases past it, otherwise the stove would go positive pressure and leak fumes. They both weaken the negative pressure in the stove by different methods

If that is the case, your car engine with a carburetor would not slow down when you let of the gas.

Kevin

I suppose simply heating the combustion air expands it's volume substantially, also as does lowering its pressure in the draft. Hmmm,..... PVnRT anyone? Ideal gas equation if I remember correctly.

Where is Yanche? I bet he could recommend a book that would enlighten us all, If we could understand it!

kevin

Carbon12 wrote:I suppose simply heating the combustion air expands it's volume substantially, also as does lowering its pressure in the draft. Hmmm,..... PVnRT anyone? Ideal gas equation if I remember correctly.

Yes the gas expands so it has more volume and less mass. Otherwise it couldn't rise.

Less mass per volume unit that is.

KLook wrote:

I think the confusion is that it's easy to view the manual damper as a blockage and flue gases are piling up against it. That's really not the case. The chimney is still pulling the flue gases past it, otherwise the stove would go positive pressure and leak fumes. They both weaken the negative pressure in the stove by different methods

If that is the case, your car engine with a carburetor would not slow down when you let of the gas.

Kevin

Kevin,
Respectfully, your analogy is off a bit. Take it from one who works on motors for a living for over 35 years.

Yes, it would slow the motor, even if the throttle plate had holes in it like an MPD. The motor would still slow down because of the change in restriction of flow rate going from wide-open-throttle to whatever the hole size in the throttle plate would be. It just wouldn't slow the motor as much as you'd expect from a throttle plate with no holes.

And the MPD is in the exhaust, not the intake. The affect on the thinner, lighter exhaust gases is different than for the denser air going into a carb. If you've ever had to trouble shoot a motor with a partially plugged muffler, you'd understand how such small holes in an exhaust system will still let a motor run - just not produce any kind of speed.

And, one of the tell-tail signs of a partially plugged exhaust system is the motor and the exhaust system are all overheating.

No matter what you do with the carb's throttle, the plugged muffler wins control every time !

Paul

Yeah, I can accept that Paul, I have seen plugged exhausts. Happens in chainsaws also with the build up in the muffler of exhaust port. Still, If it can't get out, it can't get in. It can't be magic. If a wide open MPD equals a *500 fire, then shutting it down will equal less then that. It make take a little time, but it will happen. Briefly, you may see a temp spike as the heat is prevented from flowing out the pipe, but very soon the fire will quite down and loose temp.

Kevin