5" Flexible Chimney Liner On A Unit That Requires 6"

  • Active since 1995, Hearth.com is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.


New Member
Dec 11, 2023
Hello All,

New to the forum. However, in searching around I know similar questions have been asked in the past so please be patient with me.

I have a 100 year old house with a 28' chimney (7 feet straight, then 1 foot 30 degree turn before continuing 21 more feet straight) coming up from a solid brick and mortar fireplace. An opportunity has come up to purchase a 30' stainless steel insulated 5" flexible liner and a Spartherm 600 (max 45,000BTU) fireplace insert unit from a friend. In reading the manual insert it requires that the insert be installed with a 6" liner, not a 5" like my friend has been using with apparently no issue.

Looking for real-world experience or insight into how you would imagine the unit would perform. I understand the issues regarding insurance already, so thanks in advance, but I'm really just wondering how the unit would hypothetically perform given my set-up and any real/apparent danger I would be putting myself in.

Thanks in advance,