I installed the Man-O-Meter on the stoker before the baro. Set it to limit the draft to .04.
It opens a little with a full fire in the stoker. If the oil burner on the water heater fires then the draft increases considerably in the flue and the baro for the coal stove will open a little more. It's a chimney on the outside of the house, fieldstone and very thick, so it takes a while to warn it up and it's more sensitive to outside temperature then a chimney run up through a structure.
Remember the draft will increase with the amount of heat fed into the chimney, therefore by having to other appliances connected to the flue I lose temperature into the chimney.
Both oil burning units have 6" pipe connected to them.
For all intents and purposes I can disconnect the pipe to the hot air furnace
since I don't use it. I still need the breeching connected to the water heater for high demand situations as it will fire.