I did not see a model/part # on those labels, but based on the data tags, age of the install (3+ years) and your sketch, I would guess it is not a mod-con, but that should be confirmed. Can you give us a Burnam p/n to look it up? Also, let's not rush to judgement just yet on the original installer. What is the operating range for the furnace now? He may have set it low and varied the output of each zone by coil density alone, rather than by coil density and temp. Also, even if the water were hotter than desired, you should not be burning that much more propane, as the stats will just cycle the pump and zone valves on/off more frequently. Do you have issues with the room temps running above the T-stat setting? If he has the Burnham running at low temps (130-150?), you can add a single mixing valve to your existing manifolds to keep things as they were, or use a mixing valve on the GARN side of the house loop. Lots of options. First, let's get your Burnham propane unit figured out. As to your updated sketch, you got it about 99% to what I was describing. You do not want or need to have a dedicated cold water feed to the primary loop. The GARN is an unpressurized system. Some folks (like me) plumb a feed line up to the manway collar due to access issues, but you can just drop a hose into the manway for filling. Water loss is minimal and can be checked during your biannual (biennial?) water testing. I am adding a vented boiler sight glass to my tank to have a visual on the water level, but that is just me.