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Hot Water Boiler Short Cycling after Aquastat Replacement


I'm running a SlantFin Galaxy gg-150 HEDP with automatic damper, it was installed around 2003.
I have 3 zones (Taco head/valves) (1 upstairs baseboard, 1 downstairs radiant, 1 water heater).
My aquastat is a Honeywell L8148E, I had the older version without the built-in Molex plug and replaced it with what appears to be a newer version with a Molex plug, jumped out.

My Aquastat is high limit only, with the limit set at about 180 - 185.

Note, before this the only issues I was experiencing with the system were a lack of heat at times, and eventually no heat at all due to the relay going bad on the previous aquastat.

After I replaced the controls, my heat and hot water started working great, and still do, however, iI noticed that the boiler might be short cycling. I noticed this because recently, the boiler will fire for only a few seconds and then shut off. I checked the zone heads to see if the boiler was getting calls for heat during these short spurts, and I'm seeing zero volts for all 3 zones. Can someone help me figure out if this is normal or what might be causing it?

Additionally, when I force a call for heat, my boiler gauge only shows the temp going up to about 150-160 degrees and maintains a 15-degree differential before kicking back on again. I'm thinking that the aquastat I installed might have a faulty temp bulb as I checked to make sure that the capillary tubes weren't crimped or bent before inserting into the well.

Thanks in advance for any help you can provide


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    The bulb may not be making proper contact with the well. But usually when that happens your temperature tends to overshoot.
    0 volts means power is going thru them (they are calling). However, what are the end switch wires doing? Are they properly/fully closing? I would ohm them out, and check all low voltage wiring, and especially all wiring that was 'touched' when the aquastat was replaced.
    Could also be a bad aquastat out of the box.
    Is the temperature measured with a gauge, or by the aquastat? Could be a bad gauge. Newer, better aquastats will have a temperature display.
    I don't know what 'force a call for heat' means, but if you jumped out something on the aquastat, I hope you manually open your zone valves.

    There was an error rendering this rich post.

    EBEBRATT-Ed Member Posts: 15,847
    They also have heat conductive grease or paste you can put on the aquastat bulb which may help
  • newagedawn
    newagedawn Member Posts: 586
    theres a short in the t stat wire or the tstat itself needs to be changed
    "The bitter taste of a poor install lasts far longer than the JOY of the lowest price"