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It is my understanding that that is what the intention is, but I can't see that it would be much. I think it is more to work on your head and to pacify the tree huggers a bit. Just my 2¢
I have them an all three boilers at my location. However I have Therma-Stack Heat Reclaimers on mine. (think a radiator laid flat over the stack with water running through it to scavenge exhaust heat) I didn't have dampers and it got cold enough that it froze that exchanger solid. Not a good deal. So I have them more for that reason than anything. Pretty sure this didn't help much but that is what I see as the best reason.
Essentially you are extending the time it takes to reduce the temperature of the water that's left in the boiler. You are also saving heating the make-up air that comes into the building to go up the flue while the boiler is not operating. I would guess that you're better off to bank this money and put it toward a more efficient heating set up when the day comes. I've seen 5% savings but I don't know myself.
Be careful with the therma-stacks. If you pull to much heat from the vent you will no longer have a good "draw" to exhaust the gases up the "chimney". This will result in excess condensation forming in the boiler and lead to premature failure. Don't ask how I know...