Flue liner, or not, for AFUE 80 fFurnace

I'm in the process of getting my gas furnace replaced. I have three bids from three different firms, based on single-stage AFUE 80 models from American Standard, Carrier, and Trane.
The bids are fairly close, but the Trane-based one is a few hundred dollars lower. This appears to be because Trane offers a kit of some sort which makes it possible to vent its single-stage furnace through the existing masonry chimney without dropping in a metal flue liner. In contrast, the other two bids include the installation of the flue liner.
So I'm wondering: (1) Just what this Trane device does to avoid the condensation which I'm told would otherwise occur if an AFUE 80 furnace were vented through the old chimney without a liner; and (2) whether there is a downside to going with this Trane solution.
TIA
hs AT dc-sug DOT org
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snipped-for-privacy@howles.com wrote:

I need one too, also a screened cap or hood.
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Don't post this shit to alt.hvac
On 2 Oct 2005 05:29:35 -0700, snipped-for-privacy@howles.com wrote:

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