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rumn8r

I think I can use my existing 12" flue in my very large (3' x 12') in-the-house chimney to exhaust 2 Vitodens WB2-44 boilers using 4" stainless flues (both in the same 12" flue). I can't use the concentric vent because they must be vented 12" apart so they won't fit. Therefore, the boilers will be using warm air from the boiler room the way it is now with my current boiler. Is this worth doing? How much efficiency is lost when the boiler is using pre-heated air? Are there any other considerations that I should know about? What about unequal air pressures on the supply and exhaust sides?