Do you have gas plumbing and a meter/service big enough to support a 199K burner?
In most houses that would require a dedicated 1.25" gas line from the tankless to as near the meter's regulator as possible. Most 40 gallon tanks have a 3/4" line.
What is the capacity of your meter, and how much BTUs is it suporting for other appliances, total?
Since you're replacing the boiler, a better bet would be to install a 40-50 gallon indirect water heater operated as the "priority zone" off the boiler. The CGa-3 puts out about 2x as much heat as the burner on a typical 40 gallon water heater- after a shower the tank will have recovered by the time you're dried off and dressed. Giving the water heater priority puts off any calls for heat from the heating zones until it's satisfied, but unless you're running 5 or more back-to-back 12 minute showers with ZERO time between showers it's never going to be a comfort issue.
