Why do you need a liner inside a triple wall pipe?

[alexdrozd]

Why do you need to run a 3" or 4" liner inside the entire length of a 6" or 7" existing triple wall pipe? Is too much exhaust no good? Wouldn't a reducer work?

 

[MCPO]

Mine is vented into a 2 story high 7" HT stainless flue that used to serve my wood stove and it works excellent.
It is in the center of the house though and stays reasonably warm.
Masonry flues this size might not get warm enough and draft well. Larger ones and those on outside walls are even worse.
Your metal chimney should work but the best installation would call for a liner all the way up.
Id give yours a try first.

But a more direct answer to your question is that a flue of the correct size (matching the stove design) will retain the heat better , therefore inducing a better / consistant draft.

 

[alexdrozd]

I thought I read something about the triple walls aren't airtight possibly and you need an airtight exhaust or it won't work well. Any truth to this?