I have a building with two apartments, each having independent heating systems.
The first has a Burnham Alpine 80 w/ Alliance 50SL water heater. The second has an Alpine 105 w/ AL 50SL water heater. Both have the IWH connected as part of the near-boiler piping as per the Burnham manual (fig 32A/B in the manual), and with the circulators as specified in the Burnham manual. (The one difference I see is the check valve on the DHW piping, which was installed on the return rather than after the DHW circulator as shown in the manual).
The Alpine 80 runs fine, but the 105 short-cycles on a DHW call unless the modulation is turned down to ~2000RPM -- about 41%. I'm assuming this is a flowrate issue, but both systems appear to be piped the same.
Also, for both systems, during a heating call when the boiler circulator is running, it seems that water is also circulating through the IWH tank even though the DHW circulator is not running. Is this typical? It seems that this would allow the temperature of the DHW to exceed the setpoint.
Each pump should have a check valve on the discharge side and IWH a mixing valve on the DHW, not code but prudent.
Unless I misunderstood the 80 is working OK the 105 is having a problem but the check valve on the 105 is in a different place. If this is correct, move the check valve on the 105 Since you already have check valves you don't need them on the pumps which was another choice at the time of the installation.
Only the 105 is short-cycling, but the check valves are installed the same on both boilers -- on the return.