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What causes the boiler to fire?

From the manual of my switching relay ARGO ARM-6P
  1. isolated switch coil is energized and contact closes
  2. X-X switch is closed, and X-X indic light turns on
  3. controller relay close, ZC-ZR indicator light turns on. ZC to ZR connection is required by factory installed jumper or through boiler squastat. ZC-ZR are 120 VAC connection.
  4. zone circ relay closes, turning on circulator
i was looking at the relay board. the wires on the X-X head to the boiler. and connects to the R and G terminal:

but the schematic from the boiler manual shows it for 24v thermostat.

a little confused. the XX isolated end switch contact is for the 24v thermostat part on the boiler? or does it even matter? as long as the contacts of XX is closed, the boiler will fire up? XX is just a contact, it doesn't dedicate the voltage.


  • Steamhead
    Steamhead Member Posts: 16,468
    That's correct. Run a 24V thermostat wire from R-G on the boiler to X-X on the ARM. The transformer on the boiler supplies the voltage to operate the circuit- X-X is just a switch.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • lost11
    lost11 Member Posts: 37
    Steamhead said:

    That's correct. Run a 24V thermostat wire from R-G on the boiler to X-X on the ARM. The transformer on the boiler supplies the voltage to operate the circuit- X-X is just a switch.

    So for something like the SR506, what's the end switch has two sets of terminals, Main and Priority. The priority says "TT" on boiler with DHW "TT" Inputs.

    What's the difference between the "TT" input on the MAIN end switch vs the DHW "TT" inputs on the PRIORITY end switch? I'm having a hard time trying to find what the abbreviation for DHW stand for.
  • Jamie Hall
    Jamie Hall Member Posts: 22,162
    DHW is Domestic Hot Water. A closed input on the priority contacts on your ARM should turn the other zones off and its zone on. They all should close X-X
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,753
    edited August 2020
    Sometimes you need to use your mind's eye to see what is happening inside the printed circuit boards. Now this is not an exact internal wiring diagram of the workings of this brand of board, there may be some additional circuitry because there is a priority circuit for the hot water DHW if used. but to dumb down the internals of the ARGO I have illustrated the basics of the XX terminals on most of the circuit boards available.
    There are many more contacts inside the relays than those i have shown in RED
    The upper half of the illustration indicates the circuit that is operated by the room thermostats or the water tank thermostat.

    The lower RED lines show the XX terminals are connected to dry contacts inside each zone relay. Any of the relay contacts will make the connection between X and X

    By wiring X and X from the Argo to the T and T on the boiler wiring harness, you have completed the thermostat circuit whenever any of the one thermostats calls for heat.

    Remember the thermostat does not create any voltage, it is just a switch (See Note 1 Below

    Note 1 a thermostat does not use any electricity to operate unless it has other functions like a clock timer or internet capability which has nothing to do with the W and R(h) terminals of the thermostat
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    Alan (California Radiant) Forbes