On Oct 15, 2:58 am, "cm" wrote:
We have a two sided fireplace in our house. The house is 18 years old and
the fireplace is original to the house. We purchased the house about a year
ago. We tried several times last winter to have a fire and most of the time
smoke backed up in the house. We have tried having a window partially/fully
open and warming the chimney prior to lighting the logs, with no luck.

I measured the chimney height tonight and it is taller than it needs to be.
I was told it needs to be 2' taller than the point where it is 10' away from
the roof. There is a large rooftop A/C unit 7' feet away from the chimney
that is 2' taller than the chimney. Should I raise the chimney above the A/C
unit? What else should I look for?

Thanks,

cm

The design of a good working fireplace and chimney is an art form.

When you design a chimney, the first thing is to consider the size of
fire that will be used.
Next is to design the chimney of the right size to fit the fire, so
that as much of it as possible is inside the insulated exterior of
your property. A warm chimney will work well.
Ensure it has enough fresh air to burn, without the air being pulled
through badly fitting doors and windows.

Your set up would appear to be badly designed, or perhaps someone has
changed the original setup?

When you light a fire, the first thing that needs to happen is that
the kindling produces enough heat to lift the plug of cold air that is
inside the chimney, lift it out of the top and push it to one side.
If the fire struggles to find fresh air to burn, the process will
stall, and smoke will billow into the room.
The fire needs a direct supply of fresh air from somewhere.
The best solution is a pipe direct from the outside, this will avoid
the struggle for air and eliminate drafts in the home.
The next thing is for a fire to burn well, it needs to have a warm
insulated chimney, the inside needs to warm up quickly to get rid of
the smoke. A well insulated chimney is essential, especially when you
want it to burn slowly. Having more than the minimum chimney poking
through the roof is bad. A cap on the top to stop the rain from
entering and cooling the rising air will help.