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Back to your original question--I've been on vacation--you got more than you asked for, eh? Great stuff. Taking it back to the simple question, though, for comfort I think you'd want to use first stage on all the boilers before going to second stage. Depending upon the controls of course, to bring on second stage most use either timing-only (not best for comfort because you may have to wait too long) or time plus degrees away from set point (called proportional plus integral or P + I). Either way you have to wait a bit for second stage to kick in, which isn't good for comfort. And no matter how much talk about efficiency and energy savings, the purpose of heating is comfort. Immediate gratification preferred! :)