bksaun wrote:It breaks the draft or if you will, It stops excessive draft from pulling through the stove, thus its saving fuel, not wasting it...
I fully endorse this view.
coalkirk wrote:..we have to rely on a barometric damper [that] can adjust automatically. I know my boiler uses less coal with a baro and my stack temps are lower.
This is certainly true.
The MPD and barometric damper accomplish the same thing through different means: A MPD reduces the flow
of hot gases directly. When partially blocking the pipe, a pressure drop occurs across the damper and flow is reduced, and in turn the pressure at the stove exit is reduced.
The barometric damper reduces flow by cooling the temperature at the bottom of the chimney. This reduces the top-to-bottom chimney temperature difference, i.e. the driving force of the chimney itself. This reduces excessive pressure
and consequently the flow.
Naively, folks look at a partially open baro door and conclude that they are wasting the heat of 75F room air going up the chimney. The fact is they are trading the room temp gas for about the same amount of hot, say 300F gas going up the chimney.
Besides a slight energy saving (about 5-8% on an excessive draft day), a barometric damper has a much more important purpose: By automatically reducing excessive draft, a barometric damper prevents hopper fires
. This is true for chimneys as well as motor vented stoves.