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secondary high limit wiring

Mitch_4 Member Posts: 955
Kill the burner, but leave the circ operate. If the secondary trips due to excessive heat, you want it (the heat) to get dissipated, and that will not happen if the circ shuts down. Doing it the other way you said makes NO sense to me



  • S Ebels
    S Ebels Member Posts: 2,322
    Secondary hi limit wiring?? (SE)

    How do you guys wire the secondary high limit?

    I run across a lot of old systems that have the secondary limit wired in such a way that it kills the circ as well as the burner. My logic tells me that if the boiler is hot enough to trip the limit, it would be wise to allow the circ to run in order to carry that heat away. I have seen some that would get hot enough to trip the limit and then sit there perking because the circ is off also. What's the right way on a normal sized residential boiler?
  • Steve it depends on how the system is wired


    If the primary limit is wired on 120 volts it is wired to shut down the transformer circuit for the burner only. I then will wire the secondary limit on the 24 volt side and set the temperature higher than primary limit.

    Another example:

    On oil your limit is wired to shut down the oil burner only and leave the circulator circuits powered. I would simply wire it in the same circuit with the primary limit and set it at a higher temp. C'mon some of you oil guys I am sure you have other ideas.

    I prefer LWCO's to shut off the 120 volt power instead of 24 volt to the burner cicuit and the circulator however.

    On powerpile gas systems wire the primary limit into the burner circuit and the secondary limit to break through the millivolt circuit to the power unit (when it opens the pilot goes out). Set primary limit at 180* and secondary between 220* to 230*.

    On L8124 relay applications the built in relay limit is wired as factory specified depending on the model of relay L8124A, B, C, E and etc. In that case I wire the secondary limit into the burner circuit again setting it higher than primary limit.

    In all those cases the circulator will continue to run. There may be more I just pulled those out of my head without looking up all my different wiring diagrams I have for differnt system. I have well over 5,000 different diagrams of differnt systems.
  • Mitch_4
    Mitch_4 Member Posts: 955
    That's why

    I limited the answer to keep the circ going and why..I figure the possible wiring combinations would be virtually unlimited, and I would leave tht to the pro on site as how its applied. But in all cases I cannot see why you would kill power to the circulator.


  • Dale
    Dale Member Posts: 1,317
    Safest way

    Best way is to do it like the manufactures recommend which is to drop the burner and leave the circ running if called for by the tstat or aquastat. You need to dump the excess heat. Just like a warm air furnace where the blower must be kept running is the high limit opens. I do things a bit different than Timmie in that I try to keep as much as possible in the 24 volt side on gas, just one less place some person can get zapped.
  • J.C.A._3
    J.C.A._3 Member Posts: 2,981

    The 2 ways you described are the way the instructions would read for oil. I was shown by an insurance inspector quite a few years ago. It is now required in all commercial applications in Ma.

    Steve, I've toyed with the idea of using a 6006 (R-W-B) Honeywell with a manual reset, and having the contacts that MAKE on temperature rise turn on something to alert someone by a bell, light or maybe a phone call to an alarm company.
    Kind of a back up for the backup, if you will. Chris
  • Bill Nye_2
    Bill Nye_2 Member Posts: 538
    let me add

    Let me add this, on oil when you have a pre/post - purge control make sure the secondary high limit kills power to the post purge. In case the oil valve sticks .

    On Carlin you have a black wire and a red w/white stripe. One goes to high limit and one goes to L1. You want to break the L1 with the secondary High limit.

    If I remember correctly the HW has the black and the red opposite of Carlin. Just to lead to confusion I suppose.
  • Rich W
    Rich W Member Posts: 175

    The limit should never shut down the circ. I do wire the LWCO to shut down burner and circ. Why run dry?
  • Dale we have

    major cities in the NE that require limits (unless factory wired otherwise) to be wired on the 120 volt side. Then in the event of a short across a switch on the secondary the primary will be killed by the 120 limit switch. Just thought you might want to know that.

    By the way I am looking forward to seeing you on the 1st of Feb. Will you be coming in on the 31st of Jan. If so give me a call when you get in. Let me know what hotel you will be staying at.
  • Dale
    Dale Member Posts: 1,317
    City requirement

    Thanks Tim. That's interesting, a specific requirement like that, must have been a fire some time in the past that got that in the code. Yes, looking forward to a visit to your place, should be fun. I'll be in on the 31st if the snow and airports cooperate.
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