I have a 3 storey victorian house with a stack with six chimneys in it (all are capped with pepperpots or some other cowling). In order to find out which fireplace was attached to which chimmey I put a smoke pellet in each fireplace. In one test on the ground floor fireplace, smoke came back down the flue into the upstairs bedroom fireplace. There is also some smoke leakage through the pointing which need replacing. Question 1. Obviously smoke will not be able to leave the chimney properly because of the pepperpot but does the fact that it came down the flue into the bedroom mean, catergorically that the flue has failed somehow and is leaking into the adjacent flue (because i dont think it was coming out of the pepperpot and down into the adjacent chimney? Or Question 2. Do flues ever join before going out a chimney (thus smoke was merely finding the easiest escape route)?

some chimney stacks are open with seperate pots on the top making them look like 4 seperate ones, which is why we smoke them, but if you smoke a stack like this you get smoke coming out of 2 pepper pots proving they are linked and cant be used for gas, smoke wont go up hit the pepperpot and come back down into another room, as you say it will take the easiest route when it is at the pepper pots, there is very little volume of pellet (5m3) so if it is coming out somewhere else you have a major break in the stack somewhere, which will require major work prob best resolution is to get the chimney lined by an appropriate liner and sized to suit the appliance you intend to get fitted

The pepperpot is in reality a ventilator which allows some air circulation but keeps the rain out.
It is in fact possible for two flues to share the one exit although its not recommended.....the single course of brick separating them is called the 'feather' and it was common for it to collapse while sweeping.
Obviously the flues in your house are compromised, and must not be brought into use without a thorough inspection - and probably a liner would be insisted upon.
John

thanks guys appreciate your help.

You write that you have a Victorian period home, that is somewhere between one to two hundred years old, how old?
At that time, nearly every room had a fireplace, count the fireplaces/rooms and chimney pots, very often homes as late as the end of Victoria's reign had shared chimney's.
Due to the effect of the passing wind, it is quite possible for smoke to come down another chimney without this meaning a problem, even when there is no wind, an area of high pressure can mean that the smoke is cooled to the point where it falls, rather than lifts.

what kind of wood do you use guys?

sounds like you chinmey bridges are shot!