Hmmm.. A baro and MPD kinda have the same objective. They both reduce the ability of the chimney to draw the flue gases out of the stove. That said, they perform this function by using a different technique. A manual pipe damper slows the flue gases by restricting the opening that they must escape thru.. That part is easy... A baro reduces the flow of flue gases by reducing the negative draft pressure in the flue pipe. This is possible by a weighted door that opens after a desired negative flue pressure has been exceeded..
The list of pros and cons with a MPD vrs a Baro is quite extensive and very controversial..
To name a few, a manual damper can't take spikes from wind quite as well as a baro can since a baro "auto regulates" draft just by its design. Same thing happens when it warms up and cools down outside. Ditto this as the chimney warms and cools. A MPD can't maintain an equal draft pressure at one setting - it needs to be manually changed when temperure changes outside. A baro takes some of the maintenance out of the 12 hour burn by automatically regulating the draft for you.
A baro also steals air in the stove room and sends it up the chimney. This air must be replaced, and that happens thru air infiltration from outside. Meaning, that air is being replaced by air thats coming in every creek and crevasse possible.. Under doors, leaky window sills, cracks in the mortar of the foundation, even a mouse hole
I feel there is some misconception about closing manual dampers and setting up baros as a way to "hold" heat in the stove because we think of heat thats rising under its own power of convection. Its a given that heat rises but there is a little more going on here. The heated exhaust can't rise UNLESS there is something (in this case more heated exhaust) to replace it. Same concept as mentioned above
Its all about physics... An airtight combustion chamber (good seals and gaskets on the doors) can only send out what comes in.. In other words, think of air in volume.. If a gallon of air goes in under the grates to feed the fire, then only a gallon of air + coal exhaust and some heat is going to go up the chimney. It doesn't matter if that gallon of air is under a .01" WC or a .03" WC..
Lets say a gallon of air passes thru the combustion air opening in 5 seconds at .01" WC... Now, lets increase the negative draft pressure to a .03" WC and at the same time, we will reduce the combustion air setting so that the same amount (one gallon of air) passes thru it in the same amount of time (5 seconds). (IF this isn't reduced, more air will come thru the same size opening) What happened in 5 seconds in both these circumstances? A gallon of air entered the combustion air feed and a gallon of air + coal exhaust and some heat went up the chimney at two different pressures..
I wanted to edit in a clarification -
What all this means is, the determining factor on heat going up the chimney is dictated by the amount of primary and over the fire air that is going into the combustion chamber. The size of the hole its exiting (MPD) or the pressure that its exiting at (Baro) don't have any real bearing on holding heat in the stove OTHER THAN they both control how much air is going in by holding down the overall negative pressure in the firebox....
So there ya have it, I hope you enjoyed my take on MPDs Baros and controlling heat lost up the chimney