I have a problem with double chimney fireplaces; i.e. one fireplace in the living room, the other in sitting room, both sharing same chimney. When I light either one, the smoke fills the non-lit room. I thought there should be two seperate flues up the chimney or is the smoke being blown back down the non-lit flue? I've tried blocking up one with electrci fire, it works somewhat but I'd like to get to root of problem. Any ideas?
First - check that you have 2 chimney pots. Then light a few papers in each fire a few minutes apart and check if the smoke is coming out of different pots.
I think you have 2 flues but have a bad downdraft. Most likely will be sorted out with a suitable cowl available from your local builders suppliers