Here is information from Englander's 30 manual. Link to the manual
http://www.englanderstoves.com/manuals/30-NC.pdf
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5. A 6†diameter flue is required for proper performance.
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WHY THE CORRECT FLUE SIZE IS IMPORTANT: 6â€
“Draft†is the force that moves air from the appliance up through the chimney. The amount of draft in
your chimney depends on the length of the chimney, local geography, nearby obstructions, and other
factors. Too much draft may cause excessive temperatures in the appliance. An uncontrolled burn or
a glowing red part or chimney connector can indicate excessive draft. Inadequate draft may cause
back puffing into the room and “plugging†of the chimney and/or cause the appliance to leak smoke
into the room through appliance and chimney connector joints.
Today’s solid fuel appliances are much more efficient than in the past. The units are designed to give
you controlled combustion, as well as maximum heat transfer, using less fuel to do so.
The design of your new appliance is such that the exhaust “smoke†is now at lower temperatures than
in the past, therefore requiring proper chimney size to give adequate draft. If your chimney is too
large, the heater will have a difficult time raising the temperature of the flue enough to provide
adequate draft, which can cause a "smoke back," poor burn, or both.
From the opening in the hearth it makes a 45 deg turn out and then once out of the house its another 45 deg turn up following the side of the house 25' up.
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The area of the chimney liner must also be equal to or greater than the area of the flue collar on the stove. If the area
of the flue is greater than the collar, it should never be more than two and 1/2 (2.5) times greater
The area of a 6 in flue is 28.27 inches. 2.5 times larger than that would work out to be a little less than a 9.5 in round flue.
I ran my englander for a year connected to a 7.25x7.25 clay liner. It ran OK. I really needed temps to be in the 30's or less for decent performance. After installing a 6 inch liner, I can run the stove easily at 50 degrees.
Is this stove going straight up and into this 8 inch chimney then 25 foot up? Or does it need to make a couple of 90 degree bends? I ask because bends do reduce your draft.
Regardless, if it were me and the current 8 inch chimney were safe I'd give it a try and see what happens. With well seasoned wood, I'll give you a good shot at success and you won't be taking any real risks. If it doesn't work out well, and you trust your fuel, then consider the 6 inch.
pen