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L8124C operating logic

Stevegreer Member, Email Confirmation Posts: 2
I have an oil fired boiler with tankless domestic water. It's control is a honeywell L8124C aquastat. I was having some trouble with short cycling, so I began to study the logic of the L8124C. My boiler tech set it up at 170/210 with 10 degrees diff. But I was confused that my analog press/temp gauge would top out at 240 + and pressure of 25 psi. So I replaced the gauge assuming it was faulty. It wasn't !! Next I assumed the aquastat probe might be fouled. So I started to revised the settings downward until I got a high limit cutout of 210 degrees on gauge. (This turned out to be 170 degrees and a lo limit at 150) I also set the make up pressure to 15 psi. The boiler now is responding better cutting out at 210 and restarting at 200 with a constant pressure of 15 psi.
What I don't understand is if the hi limit will turn the boiler on and off as needed what is the purpose of the low limit settings. Is it to allow for a larger heating range when calling for domestic hot water?
So was I right to assume that the analog Press/Temp gauge should correlate with the temp settings of the L8124C


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    edited April 2020
    The dials aren’t very accurate, especially over time. Best to go with a digital one like the Hydrostat 3250 plus.
    170/210 is way too high.
    If you need 170 lo to make enough domestic hot water, or 210 to keep your house heated, you have other issues that should be addressed.


    There was an error rendering this rich post.

  • rick in Alaska
    rick in Alaska Member Posts: 1,457
    Most typical systems I deal with work well with a setting of 140/180. The 140 temperature is to keep the boiler from dropping down to the point where it will condense.
    Basically, with a no heat call, the boiler will drop down 10 degrees below the low limit setting, if the differential is 10 degrees, and shut off at the differential setting.
    On a call for heat, the boiler will run until it reaches the high limit setting, and then come back on when it drops down 10 degrees. This is a fixed differential.
  • Stevegreer
    Stevegreer Member, Email Confirmation Posts: 2
    thanks, very helpful!