520 flue size...

I finally saw a 520 in action yesterday, and I noticed it has an 8" flue outlet...can that be reduced to 7" or would I have to blast out my chimney opening to use one?

I can't recommend anything but what efm calls for....8" chimney.

I know for a fact that many use smaller chimneys because they have good draft. It is usually permissible to have a chimney 1 inch less depending upon the firing rate, but for me, it's the official "line".

I believe my chimney tile is 8x8, but the existing thimble is for a 7" smoke pipe for my oil boiler...it's the smoke pipe I would have to reduce...I imagine I could chop it out but I wouldn't want to if it wasn't necessary...

1 " will not make a differents get you self a 7 to a 8 reducer for the thimble you will be fine use 8" to the stoker

If you plan on running the boiler full steam : 10 teeth, full feed then I'd go 8", In my opinion even a 6" pipe would be fine if only running the boiler at the average 4 teeth.

I run 6" and can't wait to get a bigger chimney. Seems I get a wiff everytime the stoker starts. Sort of like it Scott