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Wood fired boiler connected to Burnham boiler

I have a wood fired boiler that has a Honeywell dual aqua stat L4081B ( I use only one side) to kick on a circulator zone when the wood boiler gets too hot 220. The aqua stat is wired to a TacoSR501. Its worked fine for a few years. Recently, the circulator will not shut off even when changing the settings on the aqua stat. I can hear it click at the aqua stat but yet the circulator does not turn off. Could the problem be the relay in the Taco SR501? I would think if it was one of those fuses then my circulator would not turn on at all...but I can't get the thing to turn off??? Novice here so any helpful info would be appreciated. Thank you


  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,554
    edited January 2020
    do you have a multimeter to measure voltage? How is the L4081 connected to the SR501? I'm guessing with low voltage from R and W on the L4081 to T and T on the SR501

    You have contacts that are welded together somewhere. Once you find that you will be able to replace the defective control.

    If this is not your wiring setup, then post a picture of the wiring inside the control box covers. Someone here will be able to help you
    Edit: there are lights on the SR501 that will indicate that the power is on and the "Thermostat" is calling for operation. are both lights on? if yes then disconnect one of the low voltage wires connected to the T T on the SR501. if the light goes out and the circulator goes off, then the L4081 is defective. If the circulator keeps running, the SR501 is defective. Depending on the age of the SR501 you may be able to replace the clear plug-in relay (about the size of a small ice cube) newer model SR501 replace the entire control.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • retiredone
    retiredone Member Posts: 5
    edited January 2020
    I will look into your suggestions...Thank you for your feedback.
    I have the clear plug in relay on SR501. That is my wiring diagram . Both lights were on. Removed the low voltage wire as suggested and circulator shut off. I have L4081 on order now. thank you. I was hoping it was the SR501 relay ...a lot cheaper!
  • retiredone
    retiredone Member Posts: 5
    Can someone tell me when I replace the L4081 do I have to drain the boiler to put that temp probe in 'well' or do they disconnect so I can just attach to original probe like I've read about in some postings. I just don't want to create any leaks or future problems if I don't have to. The wiring seems straight forward since it is only the two low voltage wires coming out of it. Everything else is wired to the SR501. Thank you
  • hot_rod
    hot_rod Member Posts: 21,846
    edited January 2020
    The sensor should be in a brass well like this. Know that sometimes they are tough to pull out. It should have transfer grease applied when you slide it into the well. Did the control include a small packet of grease?

    In my opinion that control is horse and buggy technology :)

    Much better and less expensive ways to accomplish the task. This control could control a 120 load without the relay box, more features and a digital readout.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • retiredone
    retiredone Member Posts: 5
    The new one has yet to be delivered so I'm not sure if the grease pack will be included. I wondered why it was put in with a dual aqua stat when it only uses 1/2 of it...but not knowing too much about this I figured I was safe replacing it with what was there. Thank you for your feedback.
  • retiredone
    retiredone Member Posts: 5
    edited January 2020
    update...new L4081 installed but we found wired baked between L4081B and SR501...replaced wires but the purpose is to get the circulator to come on when the wood furnace gets hot to cool it down....seems like the Hi limit setting not kicking on circulator because even when I turn temp adjustment knob and can hear the click that it has reached the desired temp to kick on, it doesn't. The Lo limit seems to kick it on and shut it off once the temp goes down 20 degrees (which is good). I just thought the Hi limit side would be the side needed to control the circulator coming on and off. I have also replaced relay on SR501... I have the hi limit set at 220 and the Lo limit set at 180. My wiring seems the same as the old L4081B red wire to bottom of Lo limit....white wire to up by thermostat bulb...both going to SR501 T terminals R & W. Any suggestions? or can somebody tell me why it is not 'geared' to the Hi limit side of aqua stat rather than Lo limit switch on L4081?? ps...thanks to EDtheheaterman....he diagnosed it correctly right off the bat! If I had a multimeter...I would of saved myself some $$$.