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Firing Up Burnham Boiler (Gas) with Honeywell L40006A Temp Limiter

Hydronic System: I replaced my 50-year old electro-mechanical relays with a Taco SR-504 (very popular SR 24-V series Relay Switch). My Burnham boiler (gas) is controlled by aquastat Honeywell L40006A (very popular). My battery-operated thermostats are activating zone motor pumps, but the boiler will NOT fire up. The Honeywell L40006A is connected to the Taco "Isolated End Switch" TT.

Do I need a device between the Honeywell L40006A acquastat and the Taco Relay Switch ?? Thanks.


    HVACNUT Member Posts: 5,890
    edited September 2020
    The isolated end switch should be XX, not TT, located to the left of the low volt thermostat terminals. 
    Regardless, XX does not got to the aquastat directly. 
    What model boiler and can you post a pic of the boiler's relay? XX should go to R and W, or R and G if it's a fan center control. 
    How was it before?
  • Tony_Manero
    Tony_Manero Member Posts: 3
    You're right; the Taco Isolated End Switch is XX, to TT on boiler. It's a Burnham P11-W-22AN boiler 300,000 BTU Input. The old wiring connections (R and W) were untouched. The Boiler control box is also untouched (see photo).

    Reading the Honeywell L4006A aquastat, and Burnham label , I am thinking the connections s/b 120V, not 24V ? But I don't want to fry any electro-mechanical parts or switches.
  • JohnNY
    JohnNY Member Posts: 3,244
    You're showing a picture of the gas valve but referring to the "Boiler control box". You need to, as you've said, connect the XX on the relay to the thermostat connections on the boiler.
    Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
    Consulting & Troubleshooting
    Heating in NYC or NJ.
  • unclejohn
    unclejohn Member Posts: 1,833
    Is the pilot lit.
  • Tony_Manero
    Tony_Manero Member Posts: 3
    edited September 2020
    Maybe this will help another DIY.

    So, I called a HVAC tech guy who took about 45 minutes to figure it out. He probed the Taco isolated End Switch terminals and found no voltage across. He then probed the Taco 24V transformer (via the com terminal) found it had 24V. He determined that the Boiler controller needed a constant 24V across R & W. He attached the isolated End Switch to the Taco 24V output (via the com) and then patched to the R wire that went to the acquastat which switches the boiler on/off (with the hi-temp acquastat protecting the boiler/system).

    Basically, the isolated End Switch was not constantly powered. Nothing in the Taco instructions guided the use of the 24V com to get voltage to a controller.