What is the best way for one thermostat to control two boilers so they fire at the same time. My new install has a efm 520 added on to a oil fired hot water heater that was used for in floor radiant heat (slab in large garage). The oil hot water heater was a simple set up, aquastat set for 110 degrees, a circulator, argo relay, thermostat. I piped the new efm520 into the taps on the oil hot water heater with its own circulator (switched) and have a switch to shut off the oil burner when i'm using the efm. The efm setup is primary/secondary with the loop for the hot water heater using a mixing valve to get temp down to 110. I also have a water to air heat exchanger that heats the main house which has a oil fired furnace, this has it's own circulator (on secondary closely spaced tee's) and an argo relay to control blower. On the efm i have jump the TT on the aquastat so that the primary circulator and secondary circulator for the furnace run continuously. When the thermostat from the radiant slab closes it runs the secondary loop circ which heats up water in hot water heater tank (works fine). When the house thermostat calls for heat it starts the blower (works fine). The issue i have is i am using the high limit control on the aquastat to shut off the stoker. The problem is the timer will not work because the stoker always shuts down on high limit and it takes many hours for the boiler temp to drop the ten degrees to let the timer start the stoker. This should work fine in the winter months(when demand is great) but i don't think it will work when weather warms up and the timer needs to work to keep fire lit.
My solution to this is to have the radiant thermostat turn on both the argo relay (HW heater) and the TT on the efm aquastat which would keep me off the high limit, since when the thermo opened it would also shut off the stoker. This would let the timer do it's job. I would also have the primary circulator wired to run all the time and the blower and it's secondary circulator would be control by the furnaces Argo relay.
After all this i'm back to how do i get one digital thermostat to run both the HW heater argo relay, and the TT on the efm aquastat......isolation relay? How to wire it. Thanks