Using this on a forced air gas wall furnace:

(noisy POS...but it heats good)

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93983

...I get flue temps (single metal layer...no insulation) at around 100F
max...and max air temps where it exits from the furnace at around 210F.

Just before the fan quits running (after the burner has stopped) I get readings
of around 135F where the air exits.

I'd like the fan to continue running down to say 100F for more
efficiency....costs a lot less to run the fan than to run the burner? Though
when the fan stops the heat reverses flow and goes UP by convection to exit at
the top...providing some heat still.

This on/off range is built in to the adjustable fan switch...so not much I could
do but replace it with one that didn't have so great an on/off range...if
available. Range might be 27F...think it was supposed to be 20-25F or so.

This is compared to a 95% efficient furnace where the flue temps might be 80F if
that much.

I'm using an anticipator setting of 0.0....so the furnace has minimum time to
heat up.

BTU setting at 30K rather than 60K BTU. Does fine until ambient temp gets below
0F...when it might be feasible to turn it up to 60K...but probably won't.

Fan switch is currently set at 108F from 105F due to having the fan come on when
the burner wasn't running in the spring on a warm day. This setting is what is
shown on the switch..which is not necessarily where it activates.