The house is not a concern because the system presently runs on it's own circ. Correct? The zone and boiler piping currently in the house does not enter into the equation unless you are going to do a direct run out to the wood boiler with no additional circ. The existing controls and zone valves will still operate as normal. You're just providing the heat from a different "burner" thereby eliminating the need for the fossil boiler to fire. All it knows is that when there is a call for heat from one of your zones, it doesn't have to fire the burner if it's already hot enough. Typically one would tie the wood boiler into the existing piping via primary secondary tees which are closely spaced (no more than 4 pipe diameters apart) and use a separate circ to move the water from the wood boiler into the existing piping. When it comes to piping long distances, it's always better err on the larger rather than smaller side of things. A samller circ costs less to operate and less to buy when it comes time to replace it. A 15-58 or 007 runs for less than half the cost of a 26-99 or a 0011. What is the total distance from the wood boiler to the point where the pex would tie into your existing piping? What is the heat loss of your house? In other words how much heat do you need to transfer?