I currently use my UG-155 as a back up heating system. My primary heat comes from my outdoor wood boiler. The OWB circulates water through the UG at 180°. I have set the HEATING limits on the UG to 140° as a back up in case the fire goes out. The DHW limit is set to 140°. However, when the DHW tank calls for heat, the boiler fires to 190° until the DHW requirement is reached. I want to set the boiler to not fire for hot water so I can stop using propane since Ive got the wood boiler.

You can mess those furnace controllers up if you get settings wrong, but all the info is in the installers manuals if you are confident of your understanding of the furnace. DON'T do it if you are unsure. Call a technician. I completely read the manuals before I did any adjusting of settings on the Ultra 240 (IIRC) that I am responsible for.

EPM

Look in your manual, the older Ultras used a reference point of plus 50 degrees for the DHW setting, i.e. factory is 190 but shows up as 140 on the setpoint. The manual references do not change unless you are sure that the boilermate or other device is capable of that temperature. You should be able to find it in the control supplement that came with the installation manual, otherwise that manual is available online at Residential & Commercial Boilers, HVAC Equipment | Weil-McLain (http://www.weil-mclain.com/)