Nate's Note: If you have a Manual J load calculation run on your home, compare it to this number. This number will likely be considerably lower.  In older/leakier homes you may want to size between the numbers. If you have a newer/tighter home (post 1980 basically) you may want to size closer to what this tool says. Having either backup resistance or a furnace is a good option for safety, both are almost certain to provide more heat than needed even on extremely cold days. 

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