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What? Really? I've honestly never thought about it but I can't imagine that light bulbs have any meaningful contribution to the warmth in your house.
And as far as the hotwater tank goes, do you mean the heat generated when the burner is on? I mean gas is gas, your either getting "waste heat" from your hotwater tank firing or your furnace, no? And the tankless heater must have some waste heat too, would it not?
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