Is it safe to block the 2nd chimney while other chimney is in use?

tathspts

New Member
Jan 4, 2020
55
Montreal
I have been trying to search the internet for this and didn't really come across any information that is addressing this particular issue.

I have two fireplaces in the house. In one fireplace I just had a wood burning insert installed with stainless liner running from insert all the way up to the top.

2nd fireplace has it's own chimney flue and terracotta liner on the top. Main reason I am thinking about blocking it off is to avoid any cold air draft that might be coming in through that chimney. That cold draft can result in making the whole masonry chimney colder that it would normally be if that flue was blocked from the top. It might also help me with the draft on my insert. Other reason I would like to close it is to avoid any rain water or snow to come in and potentially deteriorate the internal of chimney.

I only want to temporarily block it with some insulation and a metal cap on the top, because I would like to keep my options open for future.

I am wondering if it will be safe for me to just block the 2nd chimney like that or I am going to cause some potential issues?

My thought process is, since flexible steel liner is running all the way up to the top from insert, there is no issue of potential smoke or gases to be let lose in the 2nd chimney. On other hand if 2nd chimney is left open at the top, it might suck in the gases being emitted by chimney in use, into the house.

Looking forward to hear from you guys on this.
 

Gearhead660

Feeling the Heat
Dec 20, 2018
368
Southern WI
Assuming these chimneys are right next to each other. If the insert has an insulated liner, I don't see any issue. There is an off chance that the other chimney could pull from exhaust gases from insert if you have a down draft situation(perfect storm). If you don't plan to use the fireplace, I would cap it, otherwise you are just losing heat up that chimney.