Underslab heat off a baseboard zone

MikeG

<span style="font-size:12pt">I have three heating zones with separate 3 speed Grundfos circulators on ¾” copper finned tube baseboard, and an indirect DHW on separate zone with Taco 007.  Munchkin boiler, P/S, Vision 1 ODR, TACO SR506 switching relay.  Most of this is a retrofit.  The existing baseboard was originally serviced by 180 degree water from a conventional boiler.  One of the heating zones services the upstairs, and runs very little.  I would like to keep this zone but utilize the pump and part of the piping to supply some ¾” copper buried in gravel fill under a 4” concrete slab in a utility room/mud room.  This will most likely use the existing pump more than the upstairs zone.  The buried copper did not have a design on it for heat loss or length when installed years ago.  It has never been used.  There is 2” of extruded foam under and around the perimeter.  My goal is to try and heat this slab and gravel mass and see how it works.  Most likely short term, one or two seasons.  I have concerns about the longevity of copper buried in gravel.  I also want to use lower temps in the slab.  Long term I may install Pex on the existing floor and do an over pour.  If what I do now works, I can reuse the equipment in the future.  Is my approach reasonable, or what are some better options?  I would like to keep it simple.  Is there any issue of tapping off of this with everything being ¾” copper?  The existing upstairs zone has it’s own thermostat like the other two.   Can the upstairs zone and slab be controlled from two separate places, but both have control?  Both would have to be able to fire the boiler.  If the upstairs zone was running and the slab needed heat, the slab circulator would turn on.  If only the slab needed heat it would fire the boiler and the zone valve would block the flow to the upstairs.  If only the upstairs called for heat, the normally closed zone valve would open and the normally open valve would close.  An end switch would start the boiler.  What would be a wiring diagram for this if it were possible?  The down side of this plan is that three pumps could be running.  I do have the option to just treat this as just another separate zone.  I provided additional room for future zones. I have a pump isolation flange installed and just another capped stub off the supply manifold.  To provide lower temp water I would need to locate the pump after the mixing valve.   Are there issues if a pump sits idle for a period of time, what about in the off-season?  What is a method or a control that exercises pumps?  I don’t want to add a lot of other devices, but I also want to protect my investment.  Thanks in advance.</span>