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I took 3 HOBO's and put in flue gas B1 pipe very top - mid way - at breaching ( in basement) with a over sized 80% my house before change out but after sealing and high R was done, when from R-13 attic to R49 and sealed all holes walls from r-6 to R-20 and sealed with new windows. went from 97,000 BTU to 34,000 BTUs/hr at 0' I took the stat at 1' droop and put a 5 gal bucket under the hoz run. At top of flue pipe never got to 250' and stayed wet the whole time. I rotted the flue/furnace/ in 4 mo. You have to dry out the flue each hour to keep the pipe from just coming apart and fall down. If you keep flue gas ( in my case) above 265' with both hot water heater and furnace above 265' will dry out the flue and will last very very long time. I do my best to keep the flue gas temp as high as I can, Its the heat that's keeps the flue gas up the flue and out of the building, unless you use a power vent or PVC flue pipe.
+1 if flue gasses are too cool it will simply rot the pipe out. That's a big reason that double wall pipe (b vent) is used even when it's not required by code for low clearance.
I saw this a lot in the 60-70'. not so much with 95% furnaces. This will drop the flue temp some 100' then the flue will condensate - stays wet then it just rots outside in. For fun turn the stat up to 85' then use the IR photo to see heat loss. Then look at temp at top of flue if under dew point then drip. This will just rot the boiler/furnace/hot water and flue pipe
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