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Radiant Slabs tapped into Cast Iron System w/ Cast Iron Boiler
Looking at this system a friend is having a problem with. The original cast-iron HW system is still in place BUT two small bathroom radiant slabs have been tapped into the system.
They each have a thermostat and each thermostat runs to a relay which throws a circulator to the slab. That makes 3 circulators in total.
When the boiler is hot (the main system has been calling for heat) the tapped-in systems all work just fine. But on days when the main system is caught up, the slabs go cold as they are simply circulating unheated water through the boiler.
What would be the best(most efficient) approach to righting this wrong? My first impression would be to swap out the relays for a zone control (circ's) so each thermostat could call for heat, not just run a circulator.