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Taco SR501-4 - Honeywell 6000 series digital thermostat issues

pberger Member Posts: 6
On a brand a new radiant IBC HC series boiler install, I have a recommended, but seemingly old, single zone Taco SR501 relay that won't work with digital "stats". The Taco documentation claims an in-line resistor must be used to make them work. The HVAC tech tried this to no avail. Perhaps he was in error. This relay seems rather antiquated? Should I get a different, modern relay, one that will support my requirements and my system?

Thank you,


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    Does your 501 have a Red, White and Common terminal (maybe labeled 24v I think)? If so, hook a thermostat wire on that to whatever terminal needs it at the stat (you didnt say which stat)

    There was an error rendering this rich post.

  • Joe Mattiello
    Joe Mattiello Member Posts: 707
    Good morning PB
    Thanks for bringing your issue to the wall for support. This is how we learn more about the products. The SR501 relay is equipped with R, W, and Com/24 terminals, so you will not need the resistor to enable the thermostat. Normally the resistor is required when using a 2 wire digital thermostat, because the thermostat needs 24v to close the switch. A resistor placed across terminals W, and 24/Com will enable a trickle of voltage to the thermostat to close R, & W.
    There is no logical reason why it shouldn't work. The SR501-4 is a solid state control with on board transformer to power the sealed relay and in this case, a Honeywell series 6000 thermostat.
    Can you provide the exact model number of the series 6000 stat, and the Taco relay? A hand schematic of your existing wiring will help too. You can contact me directly at joemat@tacocomfort.com
    Joe Mattiello
    N. E. Regional Manger, Commercial Products
    Taco Comfort Solutions
  • TonyNY
    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.
  • hot_rod
    hot_rod Member Posts: 22,254
    I think the resistor needs to be used with some power stealing stats and certain versions of relay boxes. When you have a 3 wire stat and only two wires between the relay and stat.

    The stat is trying to use the white as it's common and it "leaks" voltage back that causes lights to go on or relays to chatter.

    We have measured as high as 18V coming back the white with some HW stats.

    The resistor "shed" that stray voltage, basically turning that current into heat :)

    So don't put your lips on the resistor

    How many wires between the stat and relay?

    Does your relay have a R W and C, some earlier versions did not.

    It's best to have 3 wires going between the two controls, if in fact you have a 3 wire, common required, stat.

    The Caleffi relays boxs, V2 have a "fix" populated on the circuit board, no need to add the pull down resistor.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream