My boiler comes with an 87°flue elbow. Instructions clearly state that if no extensions are used a horizontal hole can be made ie 90°. But I still have an 87° flue elbow! Okay it is hardly close tolerance engineering but it does amount to 15mm off the horizontal difference. I can't work out if is supposed to be a 87/90 elbow or if I should just drill at 87°.
-- Mike W