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TACO SR501-4 / Honeywell Digital Thermostat RTH6500

TonyNY Member Posts: 7
I just installed a Honeywell Digital Thermostat RTH6500 to a TACO SR501-4. The issue I have is when the boiler kicks on to heat up to temp 170 degrees, the green light on the SR501-4 goes out and then comes back on once the boiler reaches the temp of 170 degrees. Is this normal?
When the thermostat request heat the red lights come on and goes off once the temp is reach which I know is normal.


  • No, it's not normal. The green light should always be on.

    How does the SR501 get its line voltage? From the boiler or a junction box? How many pumps are wired to the SR501?
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hourTwo btu/ per sq ft for degree difference for a slab
  • TonyNY
    TonyNY Member Posts: 7
    edited August 2019

    One pump is wired to the SR501-4. I believe It’s powered from a junction box.
    HVACNUT Member Posts: 5,025
    Is the relay new as well?
    Is seems as if terminal #1 on the SR501 is receiving power from the Lo side of the dual aquastat. The limit is 170°, so assuming the Lo is set to 150°, if the boiler temp is at 150° or below there wont be any power at the load side of the Lo. This is to help insure that during a heat demand, the circulator will shut off at the Lo setting to prevent condensing of the flue gasses AND keep temp in the boiler so you dont run out of hot water using the tankless coil.
    For constant power to the SR501, terminal #1 must be wired for constant 120v. So power the relay direct from the load side of the service switch. Remove the jumper between terminals 1 and 3. Remove the existing black wire from terminal 1 and move it to 3.
    Alan (California Radiant) Forbes
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,131
    edited August 2019
    Do you have the C wire connected properly on the relay and the tstat? Your thermostat requires one.
    Looks like you might, but I can’t see it in the pic that well.

  • TonyNY
    TonyNY Member Posts: 7
    Yes the Com off the relay is connected to “C” on the Tstat.
    HVACNUT Member Posts: 5,025
    How is the line volt wired?
    What powers terminal 1 on the relay?
  • Joe Mattiello
    Joe Mattiello Member Posts: 663
    Hi Tony
    as I said in our email correspondence, green light going out was an important observation.
    When green light is illuminated, that means you have power to the control, apparently, voltage is interrupted causing it to be inoperable.
    Need to determine where voltage source is, and why it’s intermittently cutting out.
    HVACNUT provided a good explanation of what may be happening because of type of wiring. It appears to be wired per alternate wiring diagram for tankless boiler application. the 120v is interrupted when there is a DHW demand so the boiler can focus energy to make hot water. Did you contact Taco Tech support, as I suggested?
    Joe Mattiello
    N. E. Regional Manger, Commercial Products
    Taco Comfort Solutions
    Erin Holohan Haskell
  • TonyNY
    TonyNY Member Posts: 7
    @HVACNUT see attached.
    @Joe Mattiello I have not had a moment to call as of yet.
  • TonyNY
    TonyNY Member Posts: 7
    All, after troubleshooting the issue I found that the wiring from the switch goes to a White Roger aquastat 11C15-11. As HVACNUT mentioned that this maybe the issue. @Joe Mattiello
  • TonyNY
    TonyNY Member Posts: 7
    I want to thank everyone for there help. The Fix was as @HVACNUT suggested with the confirmation from Mario and Joe Mattiello at TACO. All good now.
  • ratio
    ratio Member Posts: 3,403
    Thanks for the follow-up, @TonyNY.