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Boiler Losing Heat, Bad L8148E Aquastat?

dooododoo Member Posts: 25
edited January 2017 in Gas Heating
I have a mercury-free CT87K that will call for heat when temperature is set 10degrees ABOVE room temperature. (I plan on replacing the newly installed Honeywell with a White Rodgers M80, which seems to have better reviews.) Not sure if the thermostat is the reason for the proceeding problem, but I thought I'd mention it.

The boiler and circulator will kick on until the thermostat is satisfied. The boiler temperature will continue to drop in temperature to 120degrees, until the thermostat calls for heat again.

I'm new to this type of heating system, but my understanding of the aquastat is to keep the boiler water at a certain temperature. Since I have my limit set to 180degrees, shouldn't the aquastat prevent the temperature to drop to 120degrees? In fact, this particular aquastat has a permanent factory differential of 10degrees, so it should turn the burners on when the temperature is about 170degrees, correct?

Also, the circulator only runs when the thermostat calls for heat. Is this normal? (I have another separate boiler with the same same aquastat that lets the circulator run even if the thermostat is satisfied. That boiler has the old mercury Honeywell thermostat; if it's relevant.)

Do these symptoms indicate a broken aquastat? Can the aquastat be repaired, or must I replace it? The relay seems fine. Its contacts are clean, and clicks on immediately when the thermostat calls for heat.


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,132
    The aquastat is working as designed. It is only a high limit aquastat and as such doesn't not maintain boiler temperature.
    During a call for heat the circulator will run until the call thermostat is satisfied. The burner will also run, but only up to the high limit setting, then shut off. If the boiler temp drops below the differential setting, and the thermostat is still calling for heat, the burner will fire again.