This small P/S piping setup illustrates the concept pretty well. It's small, only two heating loads and two inputs , which are the OWB and the gas boiler. The heating loads are 1 low temp radiant slab and one high temp for the DHW. We set it up so the gas boiler is controlled by an aquastat which looks at the incoming water temp from the OWB. When it drops below 125-130 the gas boiler will be called on and it will run on it's own aquastat until the OWB loop is satisfied or the heating demand from either of the two loads ends.This allows the homeowner to leave for a weekend or a few days without having to worry about the outdoor part of the system freezing up. It also provides a little storage capacity in the OWB. Not super efficient but it's not used as such for long periods of time. The single zone Honeywell controls the primary loop pump via signal from the connection in the Taco zone control box. All calls for heat go through the Taco so the the primary loop circ runs whenever anything happens.