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Aquastat Planned Location Question

Hi All!
For my hydronic system, there is a strap-on aquastat. (Honeywell L6006C). It's never been accurate, usually running low. (I'd set the temperature to 160-F if I wanted 180-F). Now it is backwards. The switch opens when the water is 132-F.

I've tried cleaning pipe & sensor, using two different kinds of heat transfer paste and no paste. It still opens at 132-F.

I'd like to change to an immersion probe type, but am not certain that I picked a good spot for it. I attached a quick sketch of the local piping and, if you all don't mind, would you please review it and let me know if this will be an acceptable spot for the probe? It's pointed out in dark blue ink.
(There aren't any other choices without major re-piping.)

Thanks Very Much! I appreciate the guidance.



  • PGB1
    PGB1 Member Posts: 81
    I forgot to put in the drawing that the auqastat's probe will be in one of the branches of the cross, horizontally. Sorry for the goof up. Paul
    EBEBRATT-Ed Member Posts: 15,619

    If the aquastat is a high limit that runs the burner it needs to be in the boiler water or at the very least on the hot water supply immediately leaving the boiler..........and that isn't the best.

    Why not post a few pictures of the boiler so we can take a look. Is this an old coal converted "snowman"?
  • PGB1
    PGB1 Member Posts: 81
    Thanks Eberatt-Ed for taking time to reply.

    I can't figure out a place closer to the boiler to put the probe for an aquastat. The low water cut-out is hogging up the boiler pipe exit tee. The return pipe is in the corner & I'm nervous about breaking in there for the LWC because of the older-than-I-am piping.

    A couple of photos are attached. I apologize for lousy photos. I can't get father away with the camera because there is a permanent workbench mighty close.

    The brown wire dangling from the pipe at the high limit (black) is the thermocouple I put there this morning. It's for my thermometer & not permanent. The Honeywell in question is the grey one & the low water cut-out is the larger, black box at the boiler pipe exit.

    Thanks Again,

  • PGB1
    PGB1 Member Posts: 81
    I came up with Plan B (Drawing is attached)

    A) The new high limit will be installed where the low water cut-out is now installed.
    B) The low water cut-out will be moved up the supply pipe & installed into a 4-way cross.
    C) The cross will have a boiler drain on the branch opposite the probe.
    The drain will have two purposes: A) Provide the extra 1/8" the probe depth needs & B) Facilitate future
    system servicing.

    There is no tapping in the boiler's water jacket for a high limit probe. The original factory one was mechanical & rather interesting.

    The main supply pipe has a manifold of sorts on it. There is a T& P gauge on it and a 3/8" F-NPT tapping. From that tapping, a compression tube went to the mechanical high limit. When the temperature rose, the pressure increased. When the pressure was above the set point (indicated as temperature), the mechanical "switch" would close and stop gas flow to the pilot. Unfortunately this switch is not operating & wqas bypassed when I bought the house.

    As scary as it is, as-found, the pilot had no safety. If the 8" tall flame went out, gas continued to flow. To make matters even more dangerous, there was no device (thermocouple or otherwise) to stop gas flow to the burners if the pilot was out & there was a heat call. The gas valve & thermostat were supplied with 120 volts on 24 gauge single conductor wire. No fuse.

    I was young & broke when I bought the house, but before I moved in I made some changes with parts I could scrounge up. It's not ideal, but safer. I installed a standard pilot burner, a Baso thermocouple operated pilot safety valve and a 24 volt main gas valve (basically a simple gas solenoid).

    If the pilot is extinguished, flow to the pilot & main gas valve are stopped. I also put in the still-existing high limit, water temperature control & low water cut-out.

    Oddly, upon buying the house, the insurance company sent contractors out to check the electrical, heat & water heating before issuing a policy. They approved it all, including the electrical which had the meter & main (knife) switch on an open bus in the kitchen. One fuse protected the entire house, no matter the branch circuit size.

    Hopefully this summer the boiler will be replaced. I have not chosen what type to install. It was the plan for the last two summers, too but unexpected expenses & covid's impact on my income killed the plan.

    Thanks for reviewing my revised plan. It's got the high limit as close to the water jacket as possible.
    PS: I apologize for the dusty boiler cabinet. I did my pre-season cleaning & section brushing, but managed to forget to clean the cabinet & water heater top. What a schulb!