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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.




  • IronmanIronman Posts: 2,360Member ✭✭✭✭
    More than that required

    You'll also need to control the water temp to the slab and to protect the boiler from cold return.

    The only proper way to accomplish this is with a smart mixing vale or variable speed injection mixing. Look at Taco's I series valves or Tenmar's 356 mixing control.

    You have to use one these methods that matches the water temp to the load through outdoor reset or the slab will over-heat the zone and the boiler won't be protected. Simply using a thermostat will send too hot of water to the slab with no boiler protection. A thermostatic mixing valve also will not address these issues.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • fattyfatty Posts: 54Member
    title goes here

    Thank you Bob,

    I should have mentioned; there are thermostatic mixing valves in place, but the capillary sort. With the addition of the capillary bulb the valve actuates the temp. just fine. The amount of water actually returning to the boiler is matched to what the system draws... very little... due to the low temp. The circulators mostly just keep it all moving while it sips from the main supply. I don't think it could possibly shock the boiler.

    I will just go ahead with the circulator zone control as there don't seem to be any objections.
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